OCR GCSE Chemistry A C4.1 Predicting Chemical Reactions Exam Questions

About OCR GCSE Chemistry A C4.1 Predicting Chemical Reactions

C4.1 uses the periodic table to predict how elements react. On the OCR Gateway Chemistry A course it opens Topic C4 Predicting and identifying reactions and products, assessed in Paper 2 (Foundation) or Paper 4 (Higher), which also assume knowledge of C1 to C3. You study trends in Groups 1, 7 and 0, the properties of transition metals and the reactivity series of metals. It builds on electronic structure from C2.2 and redox from C3.3. Examiners frequently ask you to explain reactivity trends in terms of electrons and shielding, predict properties of unfamiliar group members, and write equations for halogen and metal displacement reactions.

What you need to know

• Group 1 Alkali metals react with water to form a metal hydroxide and hydrogen, for example 2Na + 2H2O -> 2NaOH + H2. Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily.
• Group 7 Halogens exist as diatomic molecules; down the group melting and boiling points increase and reactivity decreases, because it is harder to gain an electron when the outer shell is further from the nucleus. A more reactive halogen displaces a less reactive halide from solution.
• Group 0 Noble gases are unreactive because they have a full outer shell of electrons. Their boiling points increase down the group.
• Transition metals Transition metals have high melting points and densities, form coloured compounds, form ions with different charges, and are often used as catalysts, such as iron in the Haber process.
• Reactivity series Metals can be ordered by how vigorously they react with water and dilute acid. The reactivity of a metal is related to its tendency to form positive ions.
• Displacement reactions A more reactive metal displaces a less reactive metal from a solution of its salt, for example zinc + copper sulfate -> zinc sulfate + copper. These are redox reactions.

Common mistakes in C4.1 Predicting Chemical Reactions

• Saying Group 7 reactivity increases down the group like Group 1. Halogen reactivity decreases down the group.
• Explaining Group 1 trends just by 'more shells'. Mention increased distance and shielding, so weaker attraction between the nucleus and the outer electron.
• Stating that noble gases are unreactive because they have eight electrons. Helium has only two; the key point is a full outer shell.
• Writing halogen displacement products the wrong way round. The more reactive halogen ends up as the halide ion, for example Cl2 + 2KBr -> 2KCl + Br2.
• Describing transition metals as having the same properties as Group 1 metals. Transition metals are harder, denser and much less reactive with water.

Try three C4.1 Predicting Chemical Reactions exam-style questions