Topic 6
3 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Rate of reaction = quantity of product formed (or reactant used) / time. Factors that affect rate: concentration, temperature, surface area, catalyst, pressure (gases).
Explanation
Collision theory: For a reaction to occur, particles must collide with sufficient energy (activation energy) and correct orientation. Factors and their effects: - Increase concentration: more particles per unit volume, more frequent collisions, faster rate. - Increase temperature: particles have more kinetic energy, more frequent AND more energetic collisions, faster rate. - Increase surface area (smaller pieces): more particles exposed, more frequent collisions, faster rate. - Add catalyst: provides an alternative reaction pathway with lower activation energy, more particles have sufficient energy, faster rate. Catalyst is NOT used up. - Increase pressure (gases only): particles closer together, more frequent collisions, faster rate.
Practice Questions
Marble chips (CaCO3) react with hydrochloric acid. Which change would increase the rate of reaction but NOT change the total volume of CO2 produced?
Key Idea
A reversible reaction can proceed in both directions. At equilibrium, the forward and reverse rates are equal, and concentrations remain constant.
Explanation
Dynamic equilibrium: - Forward rate = reverse rate. - Concentrations of reactants and products are constant (not necessarily equal). - The system is closed. Le Chatelier's Principle: If a change is made to a system at equilibrium, the system will shift to oppose that change. Effects on equilibrium position: - Increase concentration of reactant: shifts forward, more product formed. - Increase temperature: shifts in the endothermic direction. - Decrease temperature: shifts in the exothermic direction. - Increase pressure: shifts to the side with fewer moles of gas. - Add catalyst: NO shift in equilibrium position. Reaches equilibrium faster. No change in yield. Haber Process: N2(g) + 3H2(g) to 2NH3(g), deltaH = -92 kJ/mol Industrial conditions: 450 degrees C, 200 atm, iron catalyst. Compromise: higher pressure gives more NH3 but expensive equipment. Lower temperature gives more NH3 but rate too slow.
Worked Examples
Practice Questions
The Contact Process: 2SO2(g) + O2(g) to 2SO3(g), deltaH = -196 kJ/mol. a) State the effect of increasing pressure on the position of equilibrium. Explain your answer. [3 marks] b) State the effect of increasing temperature on the yield of SO3. [2 marks]
Key Idea
OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons. Oxidising agent is reduced; reducing agent is oxidised.
Explanation
Key definitions: - Oxidation: loss of electrons (or gain of oxygen / loss of hydrogen). - Reduction: gain of electrons (or loss of oxygen / gain of hydrogen). - Oxidising agent: causes oxidation of another substance. It is itself reduced. - Reducing agent: causes reduction of another substance. It is itself oxidised. Oxidation state rules (Extended): - Free elements: oxidation state = 0. - Simple ions: oxidation state = charge of ion. - Oxygen in compounds: usually -2. - Hydrogen in compounds: usually +1. - Sum of oxidation states in a compound = 0.
Worked Examples
Practice Questions
Fe2O3(s) + 3CO(g) to 2Fe(s) + 3CO2(g). a) Identify the substance that is oxidised and the substance that is reduced. [2 marks] b) Determine the oxidation state of iron in Fe2O3 and in Fe. [2 marks]