AQA GCSE Chemistry - Paper 2 Questions & Answers

Mean rate of reaction (with reactant) - ANSWERSRate of reaction: Quantity of reactant used ---------------------------- Time taken Mean rate of reaction (with product) - ANSWERSRate of reaction: Quantity of product formed ---------------------------- Time taken Measurements for quantity of product or reactant - ANSWERSEither as mass in g or as volume in cm³ Measurements for the rate of reaction - ANSWERSEither as g/s, cm³/s, or mol/s Factors which affect rate of chemical reaction - ANSWERS- *Temperature*: a faster temperature means faster reaction - *Concentration / pressure*: a higher concentration or pressure means a faster reaction - *Surface area*: a larger surface area: volume ratio means a faster reaction - *Catalysts*: a reaction in the presence of a catalyst is faster Collision theory - ANSWERSThis states that a chemical reaction can only happen when reacting particles collide with each other with enough energy. Activation energy - ANSWERSThis is the minimum energy required for a reaction to take place. How surface area, concentration or pressure changes rate of reaction in terms of collision theory - ANSWERSThis increases the frequency of collisions, as there are more particles that are able to collide, increasing rate of reaction. How temperature changes rate of reaction in terms of collision theory - ANSWERSThis increases the frequency of collisions as well as the energy the collisions have, increasing rate of reaction. Catalysts - ANSWERS- A catalyst is a substance that changes the rate of reaction - It is not used up during the reaction, and therefore does not show up in the products or reactants - Different reactions require different catalysts, for example an enzyme - The catalyst does this by supplying a different pathway to the reaction that requires a lower activation energy, shown in the picture Reversible reactions - ANSWERSThis is a reaction in which the products can react to produce the original reactants A + B ⇌ C + D Example of a reversible reaction - ANSWERSThis reaction is reversible, meaning the reactants can be heated to get the products as well as the products can be cooled to get the reactants: Ammonium chloride ⇌ ammonia + hydrogen chloride Energy changes in reversible reactions - ANSWERSIf a reaction is endothermic in one direction, it is exothermic in the other. The same amount of energy is transferred each way. Equilibrium in a reversible reaction - ANSWERSIf the apparatus are set up to prevent the escape of reactants or products, an equilibrium can be reached, in which the rate of forward and reverse reactions are the same. This will always occur in consistent conditions. The effect of changing concentration in a reversible reaction - ANSWERSWhen the concentration of one product or reactant is changed, the system is no longer at equilibrium: - If the concentration of the reactants is increased, more products are formed until a balance is reached - If the concentration of the products is increased, more reactants will form until a balance is reached The effect of increasing temperature in a reversible reaction - ANSWERSWhen the temperature is *increased*: - More product is formed for an endothermic reaction - Less product is formed for an exothermic reaction The effect of decreasing temperature in a reversible reaction - ANSWERSWhen the temperature is *decreased*: - Less product is formed for an endothermic reaction - More product is formed for an exothermic reaction The effect of changing pressure in a reversible reaction - ANSWERSFor a gaseous reaction at equilibrium: - An increase in pressure causes the equilibrium position to shift towards the side of the smaller number of moles, meaning whichever side has less moles, more of it will be formed - A decrease in pressure causes the equilibrium position to shift towards the side with more moles, meaning whichever side has more moles, more of it will be formed Hydrocarbon - ANSWERSA hydrocarbon is a material that contains only carbon and hydrogen atoms. Alkanes - ANSWERS- These are saturated, meaning there are only single bonds between atoms - They are relatively unreactive, although they do combust - Their single bonds are quite strong - They are a homologous series of hydrocarbons (organic compounds with the same functional group and similar chemical properties) - There are trends in their physical properties Formula for alkanes - ANSWERSCn H2n+2 The first four alkanes - ANSWERS- Methane - Ethane - Propane - Butane Formula for methane - ANSWERSCH₄ Formula for ethane - ANSWERSC₂H₆ Formula for propane - ANSWERSC₃H₈ Formula for butane - ANSWERSC₄H₁₀ Crude oil - ANSWERS- A finite resource found in rocks - Mostly ancient biomass, e.g. plankton, which has been buried in mud - It is a mixture of many compounds, mostly hydrocarbons, which are mostly alkanes - They are separated by fractional distillation Why fractional distillation works - ANSWERSThe hydrocarbons in crude oil can be separated into fractions, which are chemicals that have a similar boiling point. These are molecules with a similar number of carbon atoms. Each fraction has different properties and uses, a lot of which are processed to produce fuels and feedstock for the petrochemical industry, e.g. petrol, kerosene, heavy fuel oil and liquefied petroleum gases. Process of fractional distillation - ANSWERS1. Oil is firstly heated so it turns into a gas and evaporates 2. It is piped to the bottom of the column 3. The gas rises up the column and it gradually cools 4. Different compounds in the mixture have different condensation points, so they turn back into a liquid at different places in the column Properties of molecules at the top of the column - ANSWERS- Small - Low boiling point - Very volatile (easily evaporated) - Flows easily - Ignites easily Properties of the molecules at the bottom - ANSWERS- Large - High boiling point - Not very volatile - Does not flow easily - Does not ignite easily Products of fractional distillation (from bottom upwards) - ANSWERS- *Bitumen*: used in roads and roofs - *Heavy fuel oil*: used as a fuel for ships and power stations - *Diesel*: used as a fuel for some cars and trains - *Kerosene*: used as a fuel for aircraft - *Petrol*: used as a fuel for cars - *Liquified petroleum gases*: used in domestic heating and cooking Trends in physical properties of hydrocarbons - ANSWERS- The shorter the molecule, the *less viscous* (thick, sticky) it is - The shorter the molecule, the *lower the boiling point* - The shorter the molecule, the *more flammable* Combustion in hydrocarbons - ANSWERSWhen a hydrocarbon is burnt in air, both hydrogen and carbon are oxidised, including a release of energy. When there is enough oxygen, the carbon will be completely oxidised, called *complete combustion*: Hydrocarbon + oxygen → carbon dioxide + water (g) Cracking - ANSWERSCracking is the process of breaking down hydrocarbons into smaller, more useful molecules, either by steam cracking or catalytic cracking. It involves a long chain being broken down into several smaller chains. It is a *thermal decomposition* reaction, meaning it involves heating to break the molecules apart. Process of catalytic cracking - ANSWERS1. Heat the larger hydrocarbon until it vaporises 2. Pass the vapour over a hot, powdered catalyst, e.g. *aluminium oxide* 3. The long chain molecules split apart on the surface of the specks of catalyst Process of steam cracking - ANSWERS1. Heat the larger hydrocarbon until it vaporises 2. Mix the vapour with steam (water vapour) 3. Heat to very high temperature (often around 850°C) 4. This leads to the thermal decomposition of long chain hydrocarbons to form smaller ones Products of cracking - ANSWERSThe products of cracking are alkanes and alkenes. Cracking is done because there is a high demand for fuels with small molecules. Alkenes are also used to produce polymers. Alkenes - ANSWERS- A homologous series of hydrocarbons - They have a double carbon (C=C) bond, which is their functional group - They are unsaturated because they have two fewer hydrogen atoms than the alkane with the same number of carbon - They are more reactive than alkanes Formula for alkenes - ANSWERSCn H2n The first four alkenes - ANSWERS- Ethene - Propene - Butene - Pentene Formula for ethene - ANSWERSC₂H₄ Formula for propene - ANSWERSC₃H₆ Formula for butene - ANSWERSC₄H₈