Chemistry 2B

Rate of reaction
1. Slowest reaction is rusting
2. Moderate speed reaction is metal reacting with acid
3. A really fast reaction is an explosion
4. Rate of reaction depends on 4 things
  1. Temperature
  2. Concerntration
    1. or pressure for gases
  3. Catalyst
  4. Surface area of solids
Measuring Rates of reaction
1. R o R = Amount of reactant used or amount of product formed / Time
2. It is measured in three ways :
  1. 1) Precipitation
    1. When the product of the reaction is a precipitate which clouds the solution
      1. Observe a mark through the solution and time until it disappears
  2. 2) Change in mass (usually gas given off)
    1. As the gas is released the mass disappearing is easily measured on the balance
      1. The quicker the reading drops, the faster the reaction
  3. 3) The volume of gas given off (uses syringe)
    1. The more gas given off during a given time interval, the faster the reaction
Rates of reaction experiments
1. 1) Reaction of hydrochloric Acid and Marble Chips
  1. The smaller the bits of marble the faster the reaction
  2. Measure the volume of gas evolved with a gas syringe and take readings at regular intervals
  3. This experiment is often used to show the effect of breaking the solid up into small bits
2. 2)Reaction of magnesium metal with dilute HCl
  1. This reaction is good for showing the effects of increased concerntration
  2. This reaction gives off hydrogen gas, which we can measure with a mass balance, as shown
  3. The higher the concentration the quicker the reaction
More rates of reaction experiments
1. 3)Sodium Thiosulfate and HCl Produce a cloudy precipitate
  1. The higher the temp the quicker the reaction
  2. View the mark until it disappears, through the solution, and time it.
2. 4)The decomposition of hydrogen Peroxide
  1. This is a good reaction for showing different catalysts
  2. Oxygen gas is given off, which provides an ideal way to measure the rate of reaction using the good old gas syringe method
  3. This is usually slow but if you add the catalyst, manganese(IV) oxide, it speeds it up to no end. Other catalyst that work are found in potato peel and blood
Collision Theory
1. he Collision Theory: The rate of reaction simply depends on how often and how hard the reacting particles collide with each other. The basic idea is that particles have to collide in order to react.
2. Higher temperature increases collisions
  1. More energy = more vibrations = more collisions
3. Higher concentration (or pressure) increases collisions
  1. more particles = more collisions
4. Larger surface area increases collisions
  1. more area to work on so more collisions
Collision Theory and catalysts
1. A solid catalyst works by giving the reacting particles a surface to stick to. This increases the number of successful collisions, speeding the reaction up.
2. Industrial reactions
  1. Cheaper because less time is needed as there is a faster reaction
  2. Cheaper because lower temps are needed so less energy will be needed.
  3. Expensive to buy
  4. However never get used up so they use them over and over again
  5. different catalysts are needed for different reactions
  6. Catalysts can be poisoned by impurities.
Energy and transfer in reactions
1. Exothermic
  1. An exothermic reaction is one which transfers energy to the surroundings, usually in the form of heat and usually shown by a rise in tempreture
2. Endothermic
  1. An endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and is usually shown by a fall in temperature
3. Reversible reactions can be endothermic and exothermic
Acids and Alkalis
1. A Ph scale goes from 0 to 14
2. Acids and Bases
  1. An ACID is a substance with a pH of less than 7. Acids form H+ ions in water.
  2. A BASE is a substance with a pH of greater than 7.
  3. An ALKALI is a base that dissolves in water. Alkalis form OH- ions in water.
  4. Overall H+ ions make solutions acidic and OH- ions make them alkalis
Acids reacting with metals
1. Metals react with acids to give salts
2. Acid + Metal ==> Salt + Hydrogen
3. You can use the squeaky pop experiment
4. Hydrochloric acid will always produce chloride salts
5. Sulfuric acid will always produce sulfate salts
6. Nitric acid produces nitrate slats, however they can also produce nitrogen oxides.
Oxides, Hydroxides and ammonia
1. Metal Oxides and Metal Hydroxides are bases
2. Acid + Metal Oxide ===> Salt + Water
3. Acid + Metal Hydroxide ===> Salt + Water
4. The Combination of metal and acids decides the salt
5. e.g. sulfuric acid + zinc oxide ===> zinc sulfate + water
6. Ammonia can be nuetralised with HNO3, to make fertiliser
  1. When reacted with nitric acid, you get a neutral salt.
Making salts
1. You need to react soluble salts using a metal or an insoluble base
2. Making soluble sallts using an alkali
  1. Because you cant tell whether the reaction has finished you must...
    1. Add exactly the right amount of alkali to just neutralise the solution
3. Making Insoluble salts - precipitation reactions
  1. If the salt you want to make is insoluble then you can use a precipitation reaction
  2. You need to pick two solutions that contain the ions you need
    1. Mix them together
      1. When the salt has precipitated out, filter it out and then dry it, and wash it, then dry it
  3. Precipitation reactions can be used to remove poisonous ions in drinking water.
Electrolysis
1. Electrolysis means "Splitting up with electricity"
2. If you pass a electronic current through a ionic substance (aqueous or molten)
  1. The ions attract to the opposite pole anode or cathode
3. Electrolysis Reactions involve Oxidation and reduction
  1. Reduction is a loss of oxygen and the gain of electrons
  2. Oxidation is the gain of oxygen and the loss of electrons
Electrolysis of Sodium Chloride Solution
1. Reactivity affects the products formed by electrolysis
  1. Sometimes there are more than two free ions in the electrolyte
  2. At the negative electrode, if metal ions and H+ ions are present, the metal ions will stay in solution if the metal is more reactive then hydrogen. This is because the more reactive an element, the keener it is to stay as ions. So, hydrogen will be produced unless the metal is less reactive then it
  3. At the positive electrode, if OH- and halide ions (Cl-,Br-,I-) are present then molecules of chlorine, bromine or iodine will be formed. If no halide is present, then oxygen will be formed
2. The electrolysis of Sodium Chloride Solution
  1. When common salt (sodium chloride) is dissolved in water and electrolysed, it produces three useful products: hydrogen, chlorine and sodium hydroxide
  2. 1) At the negative electrode, two hydrogen ions accept two electrons to become one hydrogen molecule.
  3. 2) At the positive electrode, two chloride (Cl-) ions lose their electrons and become one chlorine molecule.
  4. 3) The sodium ions stay in solution because they're more reactive than hydrogen. Hydroxide ions from water are also left behind. This means that sodium hydroxide (NaOH) is left in the solution
3. The half equations - make sure the electrons belence
  1. Half equations show the reactions at the electrodes.
    1. You need to make sure the number of electrons is the same.
      1. The half equation for the electrolysis of sodium chloride is:
        Negative Electrode: 2H+ + 2e- ==> H2
        Positive Electrode: 2Cl- ==> Cl2 + 2e-
Extraction of aluminium and electroplating
1. Electrolysis is used to remove aluminium from it's ore
  1. This is because it is always found in compounds
2. Cryolite is used to lower the temperature (and costs)
3. Lower temperatures = cheaper
4. Aluminium forms on the negative electrode and oxygen forms on the positive electrode.

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Chemistry 2B

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	Created by James Squibb over 10 years ago