Cambridge IGCSE Chemistry 6 Chemical Reactions Exam Questions

About Cambridge IGCSE Chemistry 6 Chemical Reactions

Chemical reactions in Cambridge IGCSE Chemistry 0620 groups four ideas: physical and chemical changes, rate of reaction, reversible reactions and equilibrium, and redox. You need to explain how concentration, pressure, surface area, temperature and catalysts affect rate, and interpret rate graphs from practical work. Extended candidates also use collision theory with activation energy, predict how conditions shift the position of equilibrium, and learn the conditions for the Haber and Contact processes. Redox covers oxidation and reduction in terms of oxygen, electrons and oxidation numbers. Examiners frequently set rate experiments with gas syringes or mass loss, equilibrium explanations and identification of oxidising and reducing agents.

What you need to know

• 6.1 Physical and chemical changes A chemical change forms new substances and is often hard to reverse; a physical change, such as melting or dissolving, forms no new substance.
• 6.2 Factors affecting rate Rate increases with higher concentration, higher gas pressure, larger surface area (smaller pieces), higher temperature and a catalyst. Rate is measured by gas volume (gas syringe), mass loss or time for a colour change or precipitate.
• 6.2 Collision theory (Supplement) Higher concentration or pressure gives more particles per unit volume, so more frequent collisions. Higher temperature gives more frequent collisions and a greater proportion of collisions with energy at least equal to the activation energy. A catalyst provides an alternative pathway with lower activation energy.
• 6.3 Reversible reactions Some reactions go both ways, shown with ⇌. Heating hydrated copper(II) sulfate (blue) gives anhydrous copper(II) sulfate (white); adding water reverses it. Hydrated cobalt(II) chloride is pink and the anhydrous form is blue.
• 6.3 Equilibrium (Supplement) In a closed system, equilibrium is reached when forward and reverse rates are equal and concentrations stop changing. Raising pressure shifts the position to the side with fewer gas molecules; raising temperature shifts it in the endothermic direction.
• 6.3 Haber and Contact processes Haber: N2 + 3H2 ⇌ 2NH3, 450 °C, 20 000 kPa (200 atm), iron catalyst; nitrogen from air, hydrogen from methane. Contact: 2SO2 + O2 ⇌ 2SO3, 450 °C, 200 kPa (2 atm), vanadium(V) oxide catalyst. Conditions are a compromise between rate, yield, cost and safety.
• 6.4 Redox Oxidation is gain of oxygen and reduction is loss of oxygen. Extended candidates also use electron transfer (OIL RIG) and oxidation numbers, identify oxidising and reducing agents, and use acidified potassium manganate(VII) (purple to colourless) and potassium iodide (colourless to brown) as tests.

Common mistakes in 6 Chemical Reactions

• Saying particles collide harder at higher concentration. Concentration increases the frequency of collisions, not their energy.
• Saying a catalyst gives particles more energy. It provides a pathway with a lower activation energy and is not used up.
• Saying equilibrium means equal amounts of reactants and products. It means the forward and reverse reaction rates are equal.
• Explaining surface area as more particles. A larger surface area exposes more particles to collisions, giving more frequent collisions.
• Confusing the oxidising agent with the substance oxidised. The oxidising agent is itself reduced.

Try three 6 Chemical Reactions exam-style questions