Chemical reactions: reactants, products and energy changes

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SECTION III: CHEMICAL REACTIONS A chemical reaction occurs when the atoms of one or more substances are rearranged to produce one or more different substances. As a result of a chemical reaction, new substances with new properties are formed. • Reactants: the starting material or materials for a chemical reaction. • Products: the substance or substances produced from a chemical reaction. Sometimes one or more of the products can be classified as byproducts. In general, a chemical reaction will be represented as shown below: Reactants g Products EXAMPLE: The chemical reaction between hydrogen and nitrogen is shown below: 3H2 + N2 g 2NH3 • The hydrogen, H2, and nitrogen, N2, molecules are the reactants. • The resulting ammonia, NH3, is the product. VISUALIZING CHEMICAL REACTIONS When a chemical reaction occurs, the reactants do not just change directly into the products. It takes time and energy for chemical bonds to be broken and made, and the reactants are often rearranged in multiple different ways as the atoms shift around to become the final product. The road that atoms and molecules take to get from starting materials to products is called the reaction pathway. As the reactants change and bonds are broken and remade, the total energy of the reactants changes too. A reaction coordinate diagram plots the energy of the reaction—and all of the molecules and atoms involved— versus how far the reaction has progressed, as shown on the following page. When all of the molecules and atoms are lower in energy, they are more stable. Conversely, they are less stable when they are higher in energy. In general, chemical compounds try to move towards a state that is lower in energy because it is more stable. Think of a reaction pathway as a hiking trail going over a mountain, and think of altitude (height) as the total energy of the reactants. The altitude changes moving along the trail, and a hiker has to pass through all of the different altitudes along the trail to get from the beginning of the trail to the end. Similarly, reactants have to progress along the reaction pathway, changing energy as they go, until they reach the final products. 32 CHEMISTRY CONNECTIONS, SECTION III: Chemical Reactions • The initial flat portion is the energy of the reactants. The flat portion at the end is the energy of the products. • ∆E represents the overall change in energy. It is the difference between the starting energy and the final energy. ∆E = (Energy of the products) — (Energy of the reactants) • Ea represents the activation energy. Activation energy is the minimum amount of energy needed to start a chemical reaction. – The reactants pass through a state that is much higher in energy in order to get to the products, called the “transition state.” In the transition state, the bonds in the reactants are in the process of breaking and the bonds in the products are in the process of forming. The transition state is the top of the hill (highest in energy) in the reaction coordinate diagram. – Activation energy is the amount of energy required to get the reaction “over the hill,” or to start the reaction by overcoming the energy barrier. It is the energy needed to go from the reactants to the transition state. Ea = (Energy of the transition state) — (Energy of the reactants) – All chemical reactions need some amount of activation energy. The activation energy is needed to break existing bonds so that new bonds can be made. The activation energy (Ea) and the change in energy (∆E) above are independent of one another. The change in energy depends only on the starting energy and the final energy (the reactants and the products) and is not affected by energies along the way. You Be The Chemist Challenge® Passport to Science Exploration 33 Quick Fact A spark provides the energy needed to start a combustion reaction and set wood on fire. The thermal energy (heat) of the spark is greater than the activation energy of the combustion reaction. Reaction Coordinate Diagram Reaction Progress Energy Transition state Reactants Products Ea ∆E 34 CHEMISTRY CONNECTIONS, SECTION III: Chemical Reactions RATES OF CHEMICAL REACTIONS Different reactions occur at different rates (or speeds), and even the same reaction can occur at different rates under different conditions. Chemical kinetics is the field of science that studies the rates of chemical reactions. Kinetics answers the question: “how fast will this reaction occur?” To understand reaction rates, it is helpful to first understand what actually happens during a chemical reaction. Three things must be true for a chemical reaction to occur: 1. Particles of the reactants must collide with (or run into) each another. “Particle” here just means one unit, so a particle of a reactant could be an atom, an ion, a molecule, or a formula unit. 2. The particles must have enough energy to react—this usually means the particles must be going at least a certain speed. “Enough energy” means at least the activation energy. If the particles are not moving fast enough, they will not have enough energy to break old bonds and form new bonds when they collide. 3. The particles must have the proper orientation to react. Particles are always moving around randomly, so sometimes they will collide with one another. If the particles collide with enough energy and the right orientation, they react. The frequency of particles colliding with each other successfully and reacting determines the reaction rate. This is called collision theory. In general, the rate of a chemical reaction can be increased in a few different ways: 1. Concentration: A higher concentration of the reactants increases the number of particles in the same amount of space, so it is more likely that particles will run into each other. Think of particles as people. People are more likely to run into each other in a crowded room. Generally, increasing the concentration of the reactants increases the reaction rate. 2. Temperature: Remember, temperature is a measure of kinetic energy, or how fast the particles in a substance are moving. When particles are moving faster, they are more likely to collide. People running around in a room are more likely to run into each other than people walking. The particles also have more energy (on average), so when two particles collide they are more likely to have enough energy to react. 3. Catalysts: A catalyst is a substance that changes the rate of a reaction. Catalysts are most often used to increase the reaction rate (to make the reaction happen more quickly) by making it easier for the reaction to take place. The identity and amount of a catalyst is the same at the beginning and at the end of a reaction. – A catalyst usually lowers the activation energy by changing the reaction pathway to avoid the highestenergy state. – There is a lower energy barrier for the reaction to occur, so it is more likely that particles will have enough energy to react when they collide. – Think of a hiking trail. A catalyst is like a shortcut across the mountain so hikers don’t have to climb all the way to the top. Quick Fact For the average reaction occurring at room temperature, raising the temperature by 10 °C approximately doubles the reaction rate. You Be The Chemist Challenge® Passport to Science Exploration 35 The diagram below illustrates the effect of a catalyst on a reaction. With a catalyst, the “energy hill" that a reaction has to climb is much lower. Changing the activation energy changes the rate of the reaction. However, it does not change the energy of the products or reactants, as seen above, so ∆E remains the same. The rate of a reaction is independent of the overall change in energy. Quick Fact Car exhaust contains environmental pollutants. Automobiles use catalysts such as palladium and platinum to convert these pollutants into less toxic chemicals and improve air quality. Quick Fact Humans need catalysts! Your body burns fuel (in the form of food), just like a car’s engine burns fuel. Your body doesn’t require the amount of heat that a car needs in order to burn fuel, but it does require a lot of energy. There are special catalysts in the body called enzymes that help humans burn fuel at normal body temperature. Without catalysts, the activation energy of the reaction that burns fuel would be too high. Quick Fact The slowest known biological reaction would take 1 trillion years without a catalyst. However, this reaction is essential to creating our DNA. With enzymes, the reaction can occur in only 10 milliseconds. Reaction Progress Energy Reactants Products Ea ∆E With catalyst Without catalyst 36 CHEMISTRY CONNECTIONS, SECTION III: Chemical Reactions ENERGY OF CHEMICAL REACTIONS Kinetics deals with the activation energy and the energies along the reaction pathway, but does not describe the difference between the reactants and the products. All chemical reactions involve some overall change in energy, because the products have a different amount of energy than the reactants. Think of a chemical reaction as a hiking trail over a mountain again. Kinetics deals with how hard it is to climb that mountain and how high the mountain is. The difference in height between the beginning of the trail and the top of the mountain is the activation energy. The energy change of the reaction is the difference in height between the beginning of the trail and the end of the trail. The path a hiker takes to cross the mountain won’t change their altitude at the beginning or their altitude at the end. These two altitudes (the beginning and the end) are completely independent of the height of the mountain and the path that a hiker takes to get across. Some chemical reactions require the input of energy and others release energy, depending on whether the products are higher or lower in energy than the reactants. • Exothermic reactions: chemical reactions that produce energy, often in the form of heat, light, or sound. The reactants themselves lose energy, and that energy is released to the environment around the reaction. The products of an exothermic reaction are more stable (lower in energy) than the reactants. A + B g C + D + energy EXAMPLE: CH4(g) + 2O2(g) g CO2(g) + 2H2O(l) + energy In the exothermic reaction above, methane and oxygen produce carbon dioxide, water, and heat. • Endothermic reactions: chemical reactions that require the input of energy to occur. The reactants gain energy by taking energy from their surroundings, often in the form of heat or light. An endothermic reaction would feel cold to the touch because it absorbs heat from its environment. The products of an endothermic reaction are less stable (higher in energy) than the reactants. A + B + energy g C + D EXAMPLE: energy + 2Al2O3(s) g 4Al(s) + 3O2(g) In the endothermic reaction above, energy is added to bauxite (aluminum oxide) to produce aluminum metal and oxygen gas. Quick Fact A common example of an endothermic reaction is the process of photosynthesis. During photosynthesis, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. Quick Fact A common example of an exothermic reaction is burning wood in a wood stove. Wood combines with the oxygen in the air to produce carbon dioxide, water, light, and heat. You Be The Chemist Challenge® Passport to Science Exploration 37 • In the exothermic reaction above, the products are at a lower energy level than the reactants. The difference, ∆E, is the amount of energy released from the reaction. – Since ∆E = (Energy of the products) — (Energy of the reactants), ∆E is negative. It is lost by the reactants. • In the endothermic reaction above, the products are at a higher energy level than the reactants. This difference, ∆E, is the amount of energy that has to be put into the reaction. – The product energy is greater than the reactant energy, so ∆E is positive. It is gained by the reactants. • Just because a reaction requires energy in order to start does not mean that it is an endothermic reaction. The exothermic reaction above requires a certain amount of energy (Ea) to get over the energy barrier, but releases more energy during the reaction than it takes to get started. Chemists often initiate exothermic reactions, adding a small amount of energy, to produce much more energy. The reaction coordinate diagrams below show the difference between exothermic and endothermic reactions. Quick Fact Diamond and graphite are both forms of pure carbon. Graphite is more stable than diamond at normal room temperature and pressure, so the reaction of diamond turning into graphite is exothermic and should spontaneously occur. The reason that diamonds do not turn into graphite is because the activation energy for the reaction is extremely high, making it almost impossibly difficult for the reaction to occur. Think About It... What does the reaction coordinate diagram for diamond turning into graphite look like? How do the energy of the reactants, the energy of the products, and activation energy compare to one another? Exothermic Reaction Reaction Progress Energy Ea ∆E Endothermic Reaction Reaction Progress Energy Ea ∆E 38 CHEMISTRY CONNECTIONS, SECTION III: Chemical Reactions GENERAL TYPES OF CHEMICAL REACTIONS In the chemical reactions described in this section, the letters A, B, C, and D are used to represent chemical elements or compounds. SYNTHESIS (OR COMBINATION) REACTION: a chemical reaction in which smaller molecules combine to form a larger one. As shown below, two or more reactants (A and B) combine to form a product (AB). A + B g AB EXAMPLE: 2H2 (g) + O2 (g) g2H2O (l) The (g) and (l) after the compounds correspond to the phases of each of those compounds. A (g) means that compound is a gas, (l) means a liquid, and an (s) means a solid. You might also see (aq), which stands for aqueous solution – or a liquid mixture of a compound where water is the solvent. DECOMPOSITION REACTION: a chemical reaction in which a compound (AB) breaks apart into two or more products (A and B). It is the opposite of a synthesis reaction. Most decomposition reactions need an outside source of energy to take place. AB g A + B EXAMPLE: ∆ 2HgO (s) g 2Hg (l) + O2 (g) In this decomposition reaction, mercury(II) oxide (HgO) splits into liquid mercury and oxygen gas. The small triangle above the arrow means the reaction needs energy (usually in the form of heat) to take place. SINGLE DISPLACEMENT (OR REPLACEMENT) REACTION: a chemical reaction in which a reactant (A) takes the place of some part of a compound (BC). In doing so, a new compound (AC) is made, and a separate product (B) is released. A + BC g AC + B EXAMPLE: Mg (s) + 2HCl (aq) g MgCl2 (aq) + H2 (g) This displacement reaction happens when a piece of solid (s) magnesium metal is combined with some aqueous (aq) hydrochloric acid. When these reactants combine, they produce two products: a liquid solution called aqueous magnesium chloride and hydrogen gas (g). Think About It... The reaction to the left could be used for hydrogen-powered cars. There is plenty of oxygen in the air to fuel the reaction, but where would the hydrogen come from? Quick Fact If you wanted to say what was occurring in the reaction to the left, you would describe it as: “Two mercury two oxide molecules decompose into two mercury atoms plus one oxygen gas molecule.” Think About It... How would you describe the displacement reaction to the left? OXIDATION-REDUCTION (REDOX) REACTIONS An oxidation-reduction reaction or redox reaction is a chemical reaction in which at least one atom’s oxidation number changes. This change is the result of an atom or group of atoms gaining or losing electrons. All oxidation-reduction reactions involve the transfer of electrons between substances. • Oxidation: a chemical reaction that involves an increase in oxidation number. Oxidation results from an atom or group of atoms losing one or more electrons. • Reduction: a chemical reaction that involves a decrease in oxidation number. Reduction results from an atom or group of atoms gaining electrons. Oxidation and reduction reactions take place together. The electrons lost by one element are gained by another element. Some elements are oxidized while others are reduced, but the number of electrons on both sides of the equation remains the same. EXAMPLE: 2Na (s) + Cl2 (s) g 2NaCl (aq) The reaction between Na and Cl2 gas is a redox reaction. The complete reaction can be divided in half to illustrate the oxidation and reduction parts. Oxidation: The sodium metal atoms begin with an oxidation number of zero and end with an oxidation number of +1. Each sodium metal atom loses one electron, and is oxidized to a sodium cation, Na+. Na g Na+ + electron (production of an Na+ cation plus release of an electron) Reduction: Each chlorine atom in Cl2 gas begins with an oxidation number of zero and ends with an oxidation number of -1. Each chlorine atom gains one electron, and is reduced to a chloride anion, Cl–. electron + Cl g Cl– (released Na electron reacts with Cl to produce a Cl– anion) In oxidation-reduction reactions, the atom that is reduced is called the oxidizing agent, and the atom that is oxidized is called the reducing agent. In the reaction above, chlorine atoms oxidize the sodium metal atoms to sodium cations, so chlorine is the oxidizing agent. On the other hand, sodium metal atoms reduce the chlorine atoms to chloride anions, so sodium is called the reducing agent. During a redox reaction, the oxidizing agent is reduced and the reducing agent is oxidized. You Be The Chemist Challenge® Passport to Science Exploration 39 DOUBLE DISPLACEMENT (OR REPLACEMENT) REACTION: a chemical reaction between two compounds, AB and CD, in which part of one reactant trades places with part of another reactant. The elements are rearranged to form two or more different compound products, AD and CB. AB + CD g AD + CB EXAMPLE: Fe2O3 (s) + 6HNO3 (aq) g 2Fe(NO3)3 (aq) + 3H2O (l) In this double displacement reaction, iron(III) oxide combines with nitric acid. The reaction produces iron(III) nitrate and water. Quick Fact The reaction shown to the left could also be written as: AB + CD g DA + BC A molecule composed of a single D atom and a single A atom can be written as either DA or AD, but scientists have adopted a rule that says to place the more electropositive element first. The products will be written as AD and CB, if A and C represent cations. Quick Fact An easy way to remember the different redox reactions is the acronym “OIL RIG”: Oxidation Is electron Loss, Reduction Is electron Gain. 40 CHEMISTRY CONCEPTS, SECTION III: Chemical Reactions Redox reactions occur all around us. The combination of hydrogen and chlorine gas to form hydrogen chloride is a redox reaction. Likewise, when carbon dioxide and hydrogen gas interact to produce carbon monoxide and water, a redox reaction has occurred. Some of the most familiar types of redox reactions involve oxygen. For example, combustion and corrosion are types of redox reactions involving oxygen. • Combustion is a redox reaction with oxygen as a reactant that occurs rapidly and produces energy, usually in the form of heat and light. The burning of fuel is a combustion reaction. • Corrosion is a redox reaction that occurs when a metal is oxidized, usually in the presence of moist air. The rusting of iron is a corrosion process. CHAIN REACTIONS A chain reaction is a series of chemical reactions in which the products of one reaction initiate further reactions of the same kind until a stable product is formed. A product in the first step becomes a reactant in the second step. A product from the second step becomes the reactant for a third reaction, and so on. Chain reactions are found in gas explosions, combustion, the formation of smog, and nuclear reactions. • Initiation reaction: the chemical reaction that starts a chain reaction. The product sets up a sequence of repeating reactions. • Propagating reactions: reactions whose products cause other reactions. • Termination: the reaction or reactions that consume the substances needed to continue the reactions. At this point, one or more of the starting materials are exhausted (used up) and the reaction stops. EXAMPLE: Overall reaction: H2 + Cl2 g 2HCl Initiation: Cl2 + light g Cl. + Cl. Propagation: Cl. + H2 g HCl + H. H. + Cl2 g HCl + Cl. Cl. + H2 g HCl + H. H. + Cl2 g HCl + Cl. Termination: Cl. + Cl. g Cl2 H. + Cl. g HCl H. + H. g H2 When chlorine and hydrogen interact in the presence of light, a chain reaction occurs. • The light absorbed by a Cl2 molecule breaks the molecule into two separate atoms, called chlorine free radicals (Cl•). “Free radical” means that it has an unpaired valence electron. • The chlorine radicals are very reactive, and react rapidly with hydrogen molecules to produce hydrogen chloride and hydrogen free radicals (H•). • The hydrogen radicals react with Cl2 molecules, producing hydrogen chloride and more chlorine radicals. Then, the chlorine radicals react further with hydrogen to continue the chain. • This continues until some other reaction uses up the free radicals of chlorine or hydrogen. In this case, a termination reaction occurs when two chlorine radicals combine with each other to form Cl2, which is a stable molecule known as chlorine gas. Quick Fact Most chain reactions occur very quickly. Sometimes, the rate of the reaction increases so much that it results in an explosion. You Be The Chemist Challenge® Passport to Science Exploration 41 REVERSIBLE REACTIONS AND EQUILIBRIUM Reversible reactions are reactions that can go forward (from reactants to products) or backward (from products to reactants), depending on the conditions of the experiment. Under certain conditions, a reversible reaction may go fully from reactants to products, using up all of the reactants. Under other conditions, the reaction may not go fully to completion and may not use up all of the reactants. If a reaction goes to completion, that means it has used up all of one or more of the reactants, so no more product can be made. Not all reactions go to completion. • Reversible reactions are usually represented in a chemical equation by a double arrow: A + B ⇌ C + D EXAMPLE: 2NaCl + CaCO3 ⇌ Na2CO3 + CaCl2 The forward reaction can be seen in saltwater lakes: 2NaCl + CaCO3 g Na2CO3 + CaCl2 Sodium chloride (salt) in the water reacts with calcium carbonate (limestone rocks). The reaction produces sodium carbonate and calcium chloride. Calcium chloride is the salty residue seen on rocks near saltwater lakes. The reverse reaction is as follows: Na2CO3 + CaCl2 g 2NaCl + CaCO3 Sodium carbonate reacts with calcium chloride to produce sodium chloride and calcium carbonate. Both reactions coexist in nature and are happening at the same time. If only the forward reaction occurred, limestone rocks would quickly dissolve in saltwater lakes— something that does not naturally occur. • In a reversible reaction, both reactants and products may be present at the same time in a state of equilibrium. Equilibrium: the state of a chemical reaction in which the forward and reverse reactions occur at the same rate. As a result, the concentrations of the reactants and the products do not change (unless the conditions change). However, the concentrations of the reactants and products do not have to be equal to each other. Equilibrium also describes how far a reversible reaction will go towards completion under given conditions. It indicates how much product a reaction will actually produce, rather than the maximum amount of product that could be made from the amount of reactants. Scientists can change the equilibrium, and the amount of product formed, by manipulating conditions of the reaction such as temperature and concentration. Quick Fact French chemist Claude Berthollet came up with the idea of reversible reactions while visiting the Natron Lakes in Egypt as part of Napoléon Bonaparte’s military expedition. He noticed that both the products (Na2CO3) and the reactants (NaCl and CaCO3) of the reaction above were present at a saltwater lake with limestone rock, and went on to develop the idea of chemical equilibrium. OBJECTIVES • Describe the law of conservation of mass. • Apply the law of conservation of mass to correctly balance equations. SECTION IV: BALANCING CHEMICAL EQUATIONS When a chemical reaction occurs, it can be described by a chemical equation, which uses chemical symbols and formulas to describe a reaction. A chemical equation uses chemical symbols to show the reactants and products involved. A chemical equation can also show the physical states of the reactants and products, as well as the involvement of heat, light, or radiation. Unlike mathematical equations, the two sides are separated by an arrow to show that the reactants form the products. CONSERVATION OF MASS Law of conservation of mass: matter cannot be created or destroyed, although it may be changed. According to this law, the mass of the reactants must equal the mass of the products. (Nuclear reactions are an exception.) Because all matter is made of atoms, the law implies the conservation of atoms as well. Therefore, although atoms may be rearranged, they are not lost. What does conservation of atoms mean? The number of atoms of each element on the reactants side (left side of the arrow) must equal the number of atoms of each element on the products side (right side of the arrow). When the atoms on both sides are equal, the equation is balanced. A balanced equation demonstrates conservation of atoms. It also shows the lowest whole-number ratio of products to reactants. • If a hydrogen atom goes into a reaction, it has to appear somewhere in the products of the reaction. • Likewise, if three hydrogen atoms appear on the reactant side of a chemical equation, three must appear on the product side. EXAMPLE: Methane interacts with oxygen in air as shown in the reaction below: CH4 + O2 g CO2 + H2O As written above, the equation correctly indicates that methane and oxygen combine to form carbon dioxide and water. However, this reaction violates conservation of matter. There are more oxygen atoms on the right than on the left, and more hydrogen atoms on the left than the right. The correctly balanced equation for the reaction looks like this: CH4 + 2O2 g CO2 + 2H2O 42 CHEMISTRY CONNECTIONS, SECTION IV: Balancing Chemical Equations Quick Fact Diatomic elements must always have a subscript of 2 in pure form. In the reaction shown to the left, oxygen must be present as O2 and not just O. The diatomic elements are Br, I, N, Cl, H, O, and F. HISTORY: ANTOINE LAVOISIER (1743–1794) Antoine Lavoisier proposed the first version of the law of conservation of mass. His law stated that during an ordinary chemical change, there is no noticeable increase or decrease in the quantity of matter. Lavoisier is known as the father of modern chemistry. He changed chemistry from a qualitative to a quantitative science. He recognized and named oxygen. He also discovered the role oxygen plays in combustion. You Be The Chemist Challenge® Passport to Science Exploration 43 To determine the number of atoms of one element in a chemical formula: • Multiply the number in front of the chemical formula by the subscript number on that atom in the chemical formula. The number in front of the chemical formula is called a coefficient. It indicates how many of that molecule or atom there are. The coefficient of a molecule applies to all of the elements in the molecule’s formula. • The number one is never written. CH4 stands for 1C1H4. • To make sure the equation is balanced correctly: – Write the number of each type of atom on the reactant side. – Write the number of each type of atom on the product side. – Compare the numbers. EXAMPLE: CH4 + 2O2 g CO2 + 2H2O Reactant Side of Equation Product Side of Equation C: 1 × 1 = 1 C: 1 × 1 = 1 H: 1 × 4 = 4 H: 2 × 2 = 4 O: 2 × 2 = 4 O: (1 × 2) + (2 × 1) = 4 The number of each type of atom is the same in the reactants as it is in the products, therefore the reaction is balanced. Quick Fact Balancing chemical equations is like putting a puzzle together. You may not be able to tell which pieces fit where, so you may have to try a few different ways before you find a good fit. With chemical equations, you may not be able to see which numbers will work to balance the equation, so you have to experiment! BALANCING CHEMICAL EQUATIONS 1. Write the unbalanced equation and see which elements are not balanced (not equal). C2H6 + O2 g CO2 + H2O Reactant Side of Equation Product Side of Equation C: 1 × 2 = 2 C: 1 × 1 = 1 H: 1 × 6 = 6 H: 1 × 2 = 2 O: 1 × 2 = 2 O: (1 × 2) + (1 × 1) = 3 • There is 1 fewer carbon atom, 4 fewer hydrogen atoms, and 1 more oxygen atom in the products than there are in the reactants. This equation is not balanced. 2. Balance the equation. An easy way to start is to balance the elements that appear in only one reactant and one product (carbon and hydrogen in this reaction). Once those elements are balanced, move on to the elements that appear in multiple reactants or products (oxygen in this reaction). To test each different equation multiply the different atoms and molecules on each side by different amounts based on their coefficients. • To get the same number of carbon atoms on both sides, multiply CO2 (on the right side) by 2. This is shown by placing a 2 in front of CO2. C2H6 + O2 g 2CO2 + H2O • Be sure to multiply all atoms by the coefficient. Therefore, 2CO2 means there are 2 carbon atoms and 4 oxygen atoms. Do not change the subscripts. Changing the subscripts changes the ratio of different elements in the compound, which makes it a different molecule altogether. In this case, 1 molecule of CO2 is made of 1 carbon atom and 2 oxygen atoms. The coefficient says that the reaction produces 2 molecules of CO2. • To get the same number of hydrogen atoms on both sides, add a coefficient of 3 in front of H2O on the right side, making it 3H2O. C2H6 + O2 g 2CO2 + 3H2O • Now there are 2 carbon atoms and 6 hydrogen atoms on each side. However, the number of oxygen atoms is not the same. There are 2 on the left side and 7 on the right side (4 from CO2, 3 from H2O). • Add a coefficient of 3.5 in front of O2 on the left side, making it 3.5O2. C2H6 + 3.5O2 g 2CO2 + 3H2O This equation has the same number of each type of atom on both sides, but is not quite done. 3. A balanced equation should not contain decimals. In the equation above, the oxygen on the left is written as having a half molecule. Because there is no such thing as half an oxygen molecule, we must eliminate it from the equation. We do this by multiplying all the coefficients by two. 2C2H6 + 7O2 g 4CO2 + 6H2O This equation is the properly balanced equation for the reaction. 44 CHEMISTRY CONNECTIONS, SECTION IV: Balancing Chemical Equations Quick Fact Often the best way to balance an equation is by trial and error, and testing a few different numbers until one set of coefficients works. OBJECTIVES • Explain pH and describe substances as acidic or basic based on the pH scale. • Define and identify common acids and bases. • Identify and describe common indicators. SECTION V: ACIDS, BASES, AND pH ACIDS Substances that release hydrogen ions (H+) when dissolved in water are acids. An acidic solution has an excess of hydrogen ions (H+). • Acids often donate H+ ions to bases. • Acids dissociate (break down) into ions in solution, making them able to conduct electricity. Some acids are corrosive in nature, and have the ability to dissolve some metals. – Most acids can react with a metal to produce a metal salt and hydrogen. EXAMPLE: When magnesium comes into contact with hydrochloric acid, the acid reacts with the metal. The reaction produces magnesium chloride (a salt) and hydrogen gas. Mg (s) + 2HCl (aq) g MgCl2 (aq) + H2 (g) BASES Substances that either release hydroxide ions (OH_ ) when dissolved in water or that accept H+ ions from acids are bases. A basic solution has an excess of OH_ ions. • Bases often donate OH_ ions to acids. • Bases feel slippery to the touch and are often used to make soaps. However, strong bases such as drain cleaner can harm human skin. You Be The Chemist Challenge® Passport to Science Exploration 45 Quick Fact The word “acid” comes from the Latin term “acidus,” which means sour. Substances with a sour taste (like lemon juice) are usually acidic. • Although the term “alkali” is often used as a synonym for base, they are not the same thing. Alkalis are a type of base made of ionic salts of an alkali metal or an alkaline earth metal. All alkalis are bases, but not all bases are alkalis. EXAMPLE: Calcium carbonate (CaCO3) and soda lye (NaOH) are bases that are also alkali salts. Ammonia (NH3) is a base but not an alkali. THE pH SCALE The pH of a solution measures how acidic or basic it is. The pH scale is used to measure the acidity of a solution. • Acids release hydrogen ions (H+) when dissolved in water. Thus, the acid content of a solution is based on the concentration of hydrogen ions in the solution. When a lot of hydrogen ions are released into a solution (high concentration of hydrogen ions), that solution is very acidic. • The pH scale is the tool used to indicate the concentration of hydrogen ions in a solution. • Usually, substances range from 0 to 14 on the pH scale. pH values do not have units. – Substances with a pH value less than 7 are acids. The smaller the number on the pH scale, the more acidic a substance is. A substance with a pH of 1 is a stronger acid than a substance with a pH of 5 (though both are acidic). – Substances with a pH value greater than 7 are bases. The higher the number on the pH scale, the more basic a substance is. A substance with a pH of 13 is a stronger base than a substance with a pH of 10 (though both are basic). – Pure (distilled) water has a neutral pH of 7.0. A neutral substance is neither acidic nor basic. Water has an equal number of hydrogen (H+) ions and hydroxide (OH_ ) ions, giving it a neutral pH. H+ + OH_ ⇌ H2O – Small changes on the pH scale actually mean large changes in acidity. An increase of just one pH unit indicates that the concentration of H+ ions (and the acidity) has increased by a factor of ten. For example, the concentration of H+ ions in a solution with a pH of 5.0 is ten times that in a solution with a pH of 6.0; the solution with pH 5.0 is ten times more acidic. Similarly, a solution with a pH of 3.0 is 1,000 times more acidic than a solution with a pH of 6.0. 46 CHEMISTRY CONNECTIONS, SECTION V: Acids, Bases, and pH Quick Fact Negative pH: some very strong acids may have a pH lower than 0. For example, concentrated hydrochloric acid, HCl, may have a pH of zero or less than zero. Quick Fact Bases typically have a bitter taste and, like acids, can conduct electricity. Quick Fact Clean rain usually has a pH of 5.6, which is slightly acidic because of the carbon dioxide naturally present in the atmosphere. Rain measuring less than 5 on the pH scale is abnormally acidic, and is called acid rain. The table below lists some common acids and bases on the pH scale. All of the substances whose pH is less than the pH of water (pictured above water on the table) are acidic. All of the substances whose pH is greater than the pH of water are basic. *See the subsection on Indicators. STRENGTH OF ACIDS AND BASES Acids and bases may be strong or weak depending on how well an acid or base produces ions in water. Many things will affect the strength of an acid or a base. ACIDS • A strong acid produces many hydrogen ions. A weak acid produces fewer hydrogen ions. As a result, indicator paper and litmus paper reveal slightly different colors depending on the strength of the acid (see the subsection on Indicators). • The chemical equation of an acid, HA, dissociating (producing hydrogen ions) is shown below: HA (aq) ⇌ A _ (aq) + H+ (aq) • For some acids, the strength of an acid may be affected by the size of the anion produced when the hydrogen is released into water (see the section on Ions from The Core of Chemistry). Within a group, acid strength increases as atomic radius increases moving down the periodic table. You Be The Chemist Challenge® Passport to Science Exploration 47 Acids (pH < 7.0) Substance Approximate pH Approximate pH Indicator Paper Color* Sulfuric acid, battery acid 0.8–1.5 Stomach acid 1.0–2.0 Lemon juice, cola 2.3–2.5 Vinegar 2.9 Apple juice, orange juice 3.3–3.8 Coffee 5.0–5.5 Milk 6.5 Neutral Pure water 7.0 Bases (pH > 7.0) Human blood 7.4 Sea water 8.0 Baking soda solution 8.5–9.0 Milk of magnesia 10.5 Household ammonia 11.5–12.0 Bleach 12.5 Liquid drain cleaner 13.5–14.0 Think About It... Like acids, metallic substances can also conduct electricity. What do they have in common? How do acids conduct electricity differently than metals? – Larger anions are more stable, so they are more easily separated from the hydrogen ion and the corresponding acid is stronger (produces more H+ ions). Hydroiodic acid (HI (aq)) is a stronger acid than hydrofluoric acid (HF (aq)) because the iodide ion (I _ (aq)) is larger than the fluoride ion (F _ (aq)), and therefore more stable. Binary acids (made from hydrogen and one other element) increase in strength going down a group on the periodic table. • Electronegativity also affects acid strength (see the subsection on Electronegativity). A more electronegative atom more strongly attracts electrons away from hydrogen atoms in solution, so more hydrogen atoms lose an electron to become H+ cations. The more electronegative the element bonded to hydrogen in an acid, the stronger the acid. For example, hydrofluoric acid (HF(aq)) is more acidic than ammonia (NH3(aq)) because fluorine is more electronegative than nitrogen. Within a period, acid strength increases as electronegativity increases moving from left to right across the periodic table. • Strong acids include hydroiodic acid (HI), hydrobromic acid (HBr), hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). BASES • Strong bases act in a manner similar to strong acids, producing hydroxide ions instead of hydrogen ions. The equation of a base, BOH, dissociating looks like this: BOH(aq) ⇌ B+ (aq) + OH_ (aq) • Strong bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and lithium hydroxide (LiOH). • Bases can also produce OH_ ions by accepting H+ ions. In the equation below, a base, B, accepts an H+ ion from water: B(aq) + H2O(l) ⇌ HB+ (aq) + OH_ (aq) – Ammonia, NH3, is an example of a base that accepts H+ ions, as shown in the equation below: NH3(aq) + H2O(l) ⇌ NH4 + (aq) + OH_ (aq) INDICATORS Indicators are substances that change color at a specific pH. They provide a way to determine approximately how acidic or basic a solution is. Some common indicators: • Litmus paper turns red in an acidic solution and blue in a basic solution. • Phenolphthalein solution changes from colorless to pink for a pH greater than 8.2. • Bromothymol Blue (BTB) turns yellow in acidic solutions and blue in basic solutions.