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AQA GCSE chemistry c2

for gcse revision

kristof bolhuis

on 13 May 2014

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Transcript of AQA GCSE chemistry c2

chemistry c2 c2-5 salts and electrolysis c2 5.1 acids and alkalis Water is a neutral substance and has a PH of 7. When added to water acids produce hydrogen ions; H+ (aq) Acidic solutions have a PH of less than 7 Bases react with acids and nuetralise them. An alkali is a base dissolved in water. Alkaline solutions have a PH of more than 7 and produce
hydroxide ions; OH- (aq) indicators such as universal indicator have different colours
in different solutions and can show the PH of a solution. c2 5.2 making salts from metals and bases Acids react with metals that are lower in the reactivity
series than hydrogen. This is because the hydrogen ions
in the acid need to be displaced. Mixing an acid with a very reactive metal such as sodium or potassium is very dangerous. When a metal reacts with an acid it produces a salt and hydrogen. Sulfuric acid + zinc H2SO4 + Zn ZnSO4 + H2 Zinc sulfate + hydrogen metal oxides and metal hydroxides are bases when an acid reacts with a base neutralisation happens and a salt and water are produced hydrochloric acid + magnesium oxide 2HCl + MgO MgCl2 + H2O Magnesium chloride + water After a reaction between a metal and a base the mixture is then filtered leaving the salt. The salt is then left in crystalised form. C2 5.3 making salts from solutions We can make soluble salts by reacting an acid with an alkali. HCl + NaOH NaCl + H2O Hydrochloric acid + sodium hydroxide Sodium chloride + water The nuetralisation of any acid can be represented by H+ + OH- H2O When nuetralisation occurs there is no visible change so we need indicator to show when the solution is nuetralised. The solution can then be heated so the salt will crystalise and can be collected. Ammonia solution is an alkali that does not contain a metal. It reacts with an acid to produce an ammonium salt such as ammonium nitrate; NH4NO3. Ammonium salts can be used as fertlisiers. We can make an insoluble salt by mixing solutions of soluble salts that contain the specific ions needed. Example: making lead iodide. The lead iodide forms a precipitate that can be filtered out of the solution then dried after being washed in distilled water. Pb(NO3)2 + 2KI Lead nitrate + potassium iodide PbI2 + 2KNO3 Lead iodide + potassium nitrate Some pollutants such as metal ions can be removed from water by precipitation, the water is treated by adding substances that react with the metal ions to form insoluble salts. C2 5.4 electrolysis Electrolysis is the process of using electricity to break down an ionic compound. You can electrolyse a molten ionic compound or a solution containing an ionic compound. A substance that is broken down using electrolysis is called an electrolyte. In electrolysis the electrical circuit has 2 electrodes that are in contact with the electrolyte. The electrodes are made of an inert substance that will not react with the product. The ions in the electrolyte move to the electrodes where they are discharged to produce elements. Positively charged ions are attracted to the negative electrode and vise verca. Positively charged ions will form hydrogen or a metal. Negatively charged ions will form a non metal. C2 5.5 changes at the electrodes When charged ions reach the oppositely charged electrodes the ions lose or gain electrons to become neutral atoms. Gaining electrons is called reduction. Losing electrons is called oxidation. Mostly elements are formed after electrolysis but sometimes non metals form molecules such as bromine, Br2. Positive electrode: anode
Negative electrode: cathode We can represent changes at the electrodes with half equations.
Example; electrolysis of lead bromide Cathode : Pb2+ + 2e- Pb Anode : 2Br- Br2 + 2e- Water contains both hydrogen and hydroxide ions. When electrolysing a solution in water hydrogen may be produced at the negative electrode. However this only happens if the positive ions in the solution are of a metal more reactive than hydrogen. At the positive electrode oxygen will be produced from the aqeous solution. However if the solution contains a high concentration of halide ions then a halogen will be produced. C2 5.6 Extraction of aluminium Aluminium is more reactive than carbon so it must be extracted from its ore by electrolysis. Aluminium ore contains aluminium oxide which must be purified and melted for electrolysis. Aluminium oxide melts at over 2000 degrees and this would require a lot of energy. Before the melting process aluminium oxide is mixed with cryolite which is an ionic compound. This mixture melts at 850 degrees. The electrolysis reaction of aluminium is... Aluminium oxide 2Al2O3 aluminium + oxygen 4Al + 3O2 The cryolite remains in the cell and fresh aluminium oxide is added as aluminium and oxygen are produced. At the cathode aluminium ions are reduced to aluminium atoms by gaining electrons. The molten aluminium is collected from the bottom of the cell. At the anode oxide ions are oxidised to oxygen atoms by losing electrons. The oxygen atoms form oxygen molecules, O2. Cathode: Al (3+) liquid + 3e- Al liquid Anode: 2O2 (-2) aqeous 2O2 gas + 4e- Cryolite is used to lower the operating temperature of an electrolysis reaction. In this reaction the positive electrodes are made of carbon. At high temperatures the oxygen reacts with the carbon electrode to produce carbon dioxide. The carbon electrode is gradually used up and must be replaced after the reaction. C2 5.7 electrolysis of brine Brine is a solution of sodium chloride in water. Brine contains sodium ions Na+ and chloride ions Cl-. There are also hydrogen ions H+ and hydroxide ions OH-. When brine is electrolysed hydrogen is produced at the cathode and chlorine is produced at the anode. What is left is a solution of hydroxide and sodium ions; NaOH. This is a strong alkali. Anode; 2Cl- Cl2 + 2e- Cathode; 2H+ + 2e- H2 Sodium hydroxide solution can be used to make
-bleach Chlorine can be used for
-killing bacteria in water
-making plastic Hydrogen can be used for making
-hydrochloric acid C2 5.8 electroplating Electroplating involves using electrolysis to coat an object with a thin layer of metal. Gold, silver and chromium are often used to electroplate objects. Reasons for electroplating
-to make an object look more attractive
-to protect a metal from corrosion
-to increase the hardness of an object
-to reduce costs by not using a pure metal For electroplating the object that is being electroplated is used as the negative electrode. The positive electrode is made of the plating metal. The electrolyte is a solution containing ions of the plating metal. At the anode atoms of the plating metal lose electrons and form metal ions which dissolve into the solution. At the cathode metal ions gain electrons and form metal atoms around the electrode which is made of the substance that is electroplated. example; nickel electroplating Anode; Ni (solid) Ni (2+) (aqueous) + 2e- Cathode; Ni (2+) (aqueous) + 2e- Ni (solid) C2 4 rates and energy C2 4.1 rates of reaction The rate of reaction can be masured by the speed of the reaction and the amount of product formed. Rate of reaction = amount of product time If a gas is given off then the rate of reaction can be measured by the time taken for a certain amount of gas to be given off. On a line graph measuring product against time the gradient of the line would be the rate of reaction. Other variables that could be used to find the rate of reaction include PH, concentration or colour. C2 4.2 collision theory Collision theory states that a reaction can only take place if particles collide with each other. The particles must collide with enough energy to start a reaction. the minimum energy required for a certain reaction is the activation energy. Factors that increase the number of succesful collisions include
-rise in temperature (so the particles have more energy)
-concentration (so there are more particles to collide with each other)
-pressure (only in gases but same reason as concentration)
-surface area (in solids and same reason as concentration)
-a catalyst (the activation energy will be reduced) The activation energy is low for powders becasue they have a large surface area for particles to collide. The finer the powder the greater the surface area. C2 4.3 the effect of temperature on the rate of reaction Increasing the temperature increases the spread of particles in a mixture because the particles get more energy. This means that the particles will collide more often increasing the rate of reaction. This is why a small change in temperature can have a large effect on the rate of reaction. From room temperature a rise of 10 degrees will double the rate of reaction. A decrease of 10 degrees would halve the rate of reaction. This is why we refrigerate food so the rate of reaction will be decreased and the food will stay fresh. C2 4.4 the effect of concentration and pressure on the rate of reaction. Particles in a solution move about randomly. Increasing the concentration means there are more particles in the same volume of solution. This means there will be more collisions. This will increase the rate of reaction. This is the same for pressure in gases as more molecules in the same volume means that there are more collisions and increased rate of reaction. C2 4.5 the effect of catalysts on the rate of reaction Catalysts change the rate of chemical reactions; most are used to speed up reactions. Catalysts lower the activation energy of a reaction so there are more successful collisions. Catalysts are not used up in the reaction and so are re-usable. Solid catalysts are turned into powders so they have a large surface area to make them as effective as possible. Catalysts are usually specific to one reaction. C2 4.6 catalysts Some catalysts are expensive but they can be re-used. They are often used in industrial processes to reduce the energy needed for a reaction. Catalysts can help conserve resources, reduce pollution and reduce costs. Many catalysts in industrial processes contain transition metals. Some transition metal compounds are toxic and would be damaging to the environment. Inventing new catalysts for industry is a major area of research. Possible new catalysts include... enzymes nanoparticles C2 4.7 exothermic and endothermic reactions When chemical reactions take place energy is transferred as bonds are broken and new bonds are made. Reactions that release energy into the surroudnings are called exothermic reactions. The temperature increases as the surroundings are heated up. Examples of exothermic reactions Combustion Oxidation neutralisation Endothermic reactions take in energy from the surroundings. Some cause a decrease in temperature as they require a supply of energy. Thermal decomposition reactions need a constant input of heat to keep them going. C2 4.8 energy and reversible reactions In a reversible reactions the products can form the reactant again.
The reverse reaction has an opposite energy transfer. The energy released in the exothermic direction is equal to the energy taken in by the endothermic reactions. Example of a reversible reaction; heating of copper sulfate crystals. Hydrated copper sulfate
CuSO4.5H2O Anhydrous copper sulfate + oxygen
CuSO4 + 5H2O Anhydrous copper sulfate can be mixed with water to create hydrated copper sulfate. This is an exothermic reaction. This is an endothermic reaction. C2 4.9 using energy transfers from reactions Exothermic reactions can be used to heat things. Hand warmers and self heating cans use exothermic reactions. Reactions that could be used include the oxidation of iron or the reaction with calcium oxide with water. Hand warmers use reversible reactions such as crystalisation of a salt.
This means they can be re-used. Endothermic reactions can be used to cool things. Chemical cold packs can contain ammonium nitrate and water that are kept apart. When mixed the ammonium nitrate dissolves in the water and this is an endothermic reaction and feels cold. Cold packs are often used on sports injuries.
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