Chemistry C3

Chemistry C3

1. Analysis and synthesis

   • Test for positive ions
     • Most group 1 and group 2 metal ions can be identified using flame tests
     • Sodium hydroxide solution can be used to identify different metal ions, depending on the precipitate that is formed
   • Tests for negative ions
     • We identify carbonates by adding dilute acid, which produces carbon dioxide gas. The gas turns limewater cloudy
     • we identify halides by adding nitric acid, then silver nitrate solution. This produces a precipitate of silver halide (chloride = white, bromine = cream, iodide = pale yellow)
     • We identify sulfates by adding hydrochloric acid, then barium chloride solution. This produces a white precipitate of barium sulphate
   • Titrations
     • A titration is used to measure accurately how much acid and alkali react together completely
     • The point at which n acid-base reaction is complete is called the end point of the reaction
     • We use an indicator to show the endpoint of the reaction between an acid and an alkali
   • Titration calculations
     • concentrations of solutions can be measure in g/dm3or mol/dm3
     • concentrations can be calculated from the mass of solute dissolved in a known volume of solution
     • The mass of solute in any volume of solution can be calculated from its concentration
     • If the concentration of one of the solutions used in a titration is known, the results of the titration can be used to calculate the concentration of the other solution
   • Chemical analysis
     • Scientits working in environmental monitoring, medicine and forensic science all need to analyse substances
     • The results of their analysis are often matched against existing databases to identify substances (or suspects in the case of forensics)
   • Chemical equilibrium
     • In a reversible reaction the products of the reaction can react to re-from the original reactants
     • In a closed system, equlibrium is achieved when the rates of the forward and reverse reactions are equal
     • changing the reaction conditions can change the amounts of product and reactants in a reaction mixture at equilibrium
   • Altering conditions
     • Changing the pressure can affect reversible reactions involving gases at equlibrium
       • Increasing the pressure favours the reaction with the smaller number of molecule of gas formed
       • Decreasing the pressure favours the reaction with the larger number of molecules of gas fromed
     • Changing the temperature at which we carry out a reversible reaction can change the amount of products formed at equilibrium
       • Increasing the temperature favours the endothrmic reaction
       • Decreasing the temperature favours the exothermic reation
   • Making ammonia - the Haber process
     • Ammonia is an important chemical for making other chemicals, including fertilisers
     • Ammonia is made from nitrogen and hydrogen in the haber process
     • The haber process is done using conditions which are chosen to give a reasonable yield of ammonia as quickly as possible
     • Any unreacted nitrogen and hydrogen are recycled in the haber process
     • Can be repeated multiple times
   • The economics of the Haber process
     • The Haber process uses a pressure of around 200atmospheres to increaase the amount of ammonia produced
     • Although higher pressures would produce more ammonia, they would make the chemical plant too expensive to build and run
     • A temperature of about 450'c is used for the reaction. Although lower temperature would increase the yield of ammonia, it would be produced too slowly

2. Water

   • Hard water
     • Hard water contains dissolved compounds such as calcium and magnesium salts
     • The calcium and/or magnesium in hard water react with soap producing a precipitate called scum
     • Temporary hard water hard water can can produce a solid scale when it is heated, reducing the efficiency of heating systems and kettles
     • Hard water is better than soft water for developing and maintaining teeth and bones. it may also help to prevent heart disease
   • Removing hardness
     • soft water does not contain salts that produce scum or scale
     • Hard water can be softened by removing the salts that produce scum and scale
     • Temporary hardness is removed from water by heating it. Permanent hardness is not changed by heating
     • The hydrogencarbonate ions in temporary hard water decompose on heating. The carbonate ions formed react with Ca2+ (aq) and Mg2+ (aq) making precipitates
     • Both types of hard water can be softened by adding washing soda or by using an ion-exchange resin to remove calcium and magnesium ions
   • Water treatment
     • Water for drinking should contain only low levels of dissolved substances and microbes
     • Water is made fit for drinking by filtering it to remove solids and adding chlorine to kill microbes
     • We can make pre water by distillation but thi requires lare amounts of energy which makes it expensive
   • Water issues
     • There are advantages and disadvantages to any type of water treatment
     • Water can be treated by removing hardness and to remove harmful microbes and to improve dental health

3. Organic chemistry

   • Structure of alcohols, carboxylic acids and esters
     • The homologous series of alcohols contain the -OH functional group
     • The homologous series of carboxylic acids contain the -COOH functional group
     • The homologous series of esters contains the -COO- functional group
   • Properties and uses of alcohols
     • Alcohols are used as solvents and fuels, and ethanol is the man alcohol in alcoholic drinks
     • Alcohols burn in the air, forming carbon dioxide and water
     • Alcohols react with sodium to form a solution and give off hydrogen gas
     • Ethanol can be oxidised to ethanoic acid, either by chemical oxidising agents or by the action of microbes. Ethanoic acid is the main acid in vinegar
   • Carboxylic acids and esters
     • solutions of carboxylic acids have their pH value less than 7. Their acidic solutions react with carbonates, gently fizzing as they release carbon dioxide gas
     • Aqueous solutions of weak acids have a higher pH value than soloutions of strong acids with the same concentration
     • Esters are made by reacting a carboxylic acid and an alcohol together with an acid catalyst
     • Esters are volatile compounds used in flavouring and perfumes
   • Organic issues
     • Alcohols, carboxylic acids and esters have many uses in which benefit society
     • However, some of these substances, such as ethanol and solvants, can be abused
     • In future, the use of biofuels such as ethanol and ester, could help society as crude oil runs out
     • However, future uses of biofuels might conflict with the need to feed the world

4. The periodic table

   • The early periodic table
     • The periodic tble of the elements developed as chemists tried to classify the elements. It arranges them in a pattern in which similar elements are grouped together
     • Newlands' table put the elements in order of tomic weigth but failed to take into accont of elements that were unknown at the time
     • Mendeleev's periodic table left gaps for the unknown elements and so provided the basis for the modern periodic table
   • The modern periodic table
     • the atomic (proton) number of an element determines its position in the periodic table
     • The number of electrons in the outermost shell (highest energy level) of an atom determines its chemical properties
     • The group number in the periodic table equals thee number of electrons in the outermost shell
     • We can explain trends in reactivity as we go down a group in terms of:
       • the distance between between the outermost electrons and the nucleus
       • The number of occupied innersheels (energy levels) in the atoms
   • Group 1 - the alkali metals
     • The elements in group 1 of the periodic table are called the alkali metals
     • Those metals all react with water to produce hydrogen and an alkaline solution containing the metal hydroxide
     • They form positive ions with a charge of 1+ in reactions to make ionic compounds. Their compounds are usually white or colourless crystals that dissolve in water producing colourless solutions
     • The reactivity of the alkali metals increases going down the group
   • The transition metals
     • Compared with the alkali metals, transition metals have much higher melting points and densities. they are also stronger and harder but less reactive
     • The transition elements do not react vigorously with oxygen or water
     • Transition elements can form ions with different charges, in compounds that are often coloured
     • Transition elements and their compounds are important industrial catalysts
   • Group 7 - the halogens
     • The halogens all form ions with a single negative charge in their ionic compounds with metals
     • The halogens form covalent compounds by sharing electrons with other non-metals
     • A more reactive halogen can displace a les reactive halogen from a solution of one of its salts
     • The reactivity of the halogens decreases going down the groups

5. Energy calculations

   • Comparing the energy released by fuels
     • when fuels and food react with oxygen, energy is released in an exothermic reaction
     • A simple calorimeter can be used to compare the energy released by different fuels or different foods in school lab
   • Energy transfer in solutions
     • We can calculate the energy for reactions in solutions by measuring temperature change and using the equation Q= mc(triangle)T
     • Neutralisation and displacement reactions are both examples of reactions that we can use this technique for
   • Energy level diagrams
     • We can show the relative difference in the energy of reactants and products on energy level diagrams
     • Catalysts provide a pathway with a lower activation energy so the rate of reaction increases
     • Bond breaking is endothermic and bond making is exothermic
   • Calculations using bond energies
     • In an exothermic reaction, the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken
     • In an endothermic reaction, the energy released when new bonds are formed is less than the energy absorbed when bonds are broken
     • We can calculate the overall energy change in a chemical reaction using bond energies
   • Fuel issues
     • Much of the world relies on fossil fuels. However, they are non-renewable and they cause pollution. Alternative fuels need to be found soon
     • Hydrogen is one alternative. It can be burned in combustion energies or used in fossil cells to power vehicles