OCR GCSE Chemistry A (Gateway)
OCR GCSE Chemistry A C3.1 Introducing Chemical Reactions Exam Questions
Free OCR GCSE Chemistry A exam questions for C3.1 Introducing Chemical Reactions, with a mark scheme for every pack. Covers conservation of mass, formulae and balanced equations, state symbols, ionic equations, the mole, Avogadro constant, reacting masses and limiting reactants.
C3.1 Introducing Chemical Reactions exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About OCR GCSE Chemistry A C3.1 Introducing Chemical Reactions
C3.1 introduces the language and mathematics of chemical reactions. On the OCR Gateway Chemistry A course it opens Topic C3 Chemical reactions, assessed in Paper 1 (Foundation) or Paper 3 (Higher). You learn to write formulae and balanced equations with state symbols, and Higher students learn the mole, the Avogadro constant and reacting mass calculations. These skills are used throughout the rest of the course, especially in electrolysis (C3.4) and quantitative chemistry (C5.1). Examiners regularly test balancing equations, explaining apparent mass changes, ionic equations, mole calculations and identifying the limiting reactant.
What you need to know
- Conservation of mass In a chemical reaction no atoms are created or destroyed, so the total mass of reactants equals the total mass of products. An apparent mass change happens when a gas escapes or is taken in from the air in an open container.
- Formulae and equations Use formulae of common ions, such as SO4 2-, NO3 -, OH- and CO3 2-, to write compound formulae. Balance equations by changing numbers in front of formulae, never the formulae themselves, and add state symbols (s), (l), (g), (aq).
- Ionic equations (Higher) Ionic equations show only the ions that react, leaving out spectator ions. For example, neutralisation is H+(aq) + OH-(aq) -> H2O(l).
- The mole (Higher) One mole contains 6.02 x 10^23 particles (the Avogadro constant). Number of moles = mass / Mr, and the mass of one mole of a substance in grams equals its Mr.
- Reacting masses (Higher) Use the balanced equation to find the mole ratio, then convert moles to masses. Balanced equations can also be worked out from the masses of reactants and products.
- Limiting reactants (Higher) The limiting reactant is completely used up and determines the amount of product formed; the other reactant is in excess.
Common mistakes in C3.1 Introducing Chemical Reactions
- Changing subscripts to balance an equation, such as writing H2O2 instead of 2H2O. Only add numbers in front of formulae.
- Saying mass is lost when a metal carbonate is heated. Mass appears to decrease because carbon dioxide gas escapes, but total mass is conserved.
- Using Ar instead of Mr for a compound or diatomic element. Oxygen gas is O2, so its Mr is 32.
- Forgetting to use the mole ratio from the equation. Moles of product are not always equal to moles of reactant.
- Including spectator ions in an ionic equation. Only include the ions that change.
Try three C3.1 Introducing Chemical Reactions exam-style questions
1. Balance the equation: Al + O2 -> Al2O3. (1 mark)
Show the mark scheme answer
4Al + 3O2 -> 2Al2O3.
2. Calculate the mass of magnesium oxide formed when 6.0 g of magnesium burns completely. 2Mg + O2 -> 2MgO. (Ar: Mg = 24, O = 16) (3 marks)
Show the mark scheme answer
Moles Mg = 6.0 / 24 = 0.25 mol; mole ratio 2 : 2 so 0.25 mol MgO; mass = 0.25 x 40 = 10.0 g.
3. Calculate the number of molecules in 0.50 mol of carbon dioxide. (1 mark)
Show the mark scheme answer
0.50 x 6.02 x 10^23 = 3.01 x 10^23 molecules.
C3.1 Introducing Chemical Reactions FAQs
What does OCR GCSE Chemistry A C3.1 cover?
It covers conservation of mass, writing formulae and balanced equations, state symbols, and for Higher, ionic equations, moles, the Avogadro constant, reacting masses and limiting reactants.
Which paper is C3.1 examined in?
Topic C3 is assessed in Paper 1 (Foundation) or Paper 3 (Higher), and mole calculations are also used in Paper 2 and Paper 4.
What equations do I need for C3.1?
Learn moles = mass / Mr, number of particles = moles x 6.02 x 10^23, and how to use mole ratios from a balanced equation.
