Chemical Reactions and Equations

A chemical reaction takes place whenever substances change into new substances with different properties. In this chapter you will learn to recognise a reaction from observable signs such as a change in state or colour, the evolution of a gas or a change in temperature, as when a magnesium ribbon burns with a dazzling white flame to form magnesium oxide, or when zinc reacts with dilute sulphuric acid to release hydrogen gas. You will also learn to write chemical equations and balance them so that each element has the same number of atoms on both sides, as the law of conservation of mass requires, using state symbols such as (s), (l), (g) and (aq). The chapter then classifies reactions as combination, decomposition (thermal, electrolytic and photolytic), displacement and double displacement, including precipitation, and explains why some reactions are exothermic and others endothermic. Finally, it introduces oxidation and reduction as the gain or loss of oxygen or hydrogen, redox reactions, and their everyday effects: corrosion of metals and rancidity of fats and oils.

Detailed Worksheet: Chemical Reactions and Equations Section A - Definitions (10 marks) 1. Define a chemical reaction. List any three observations that tell us a chemical reaction has taken place. (2 marks) 2. What is a balanced chemical equation? Why must chemical equations be balanced? (2 marks) 3. Define a combination reaction and a decomposition reaction. Give one balanced chemical equation for each. (2 marks) 4. Define oxidation and reduction in terms of the gain or loss of oxygen and hydrogen. What is a redox reaction? (2 marks) 5. Define corrosion and rancidity, giving one everyday example of each. (2 marks) Section B - Calculations and Applications (15 marks) 6. Balance the following equations and add state symbols: (i) H2 + Cl2 -> HCl (ii) Fe + H2O -> Fe3O4 + H2 (iii) BaCl2 + Al2(SO4)3 -> BaSO4 + AlCl3 (3 marks) 7. Translate the following statements into balanced chemical equations: (i) Hydrogen gas combines with nitrogen to form ammonia. (ii) Hydrogen sulphide gas burns in air to give water and sulphur dioxide. (iii) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas. (3 marks) 8. When 2.4 g of magnesium ribbon is burnt completely in oxygen, 4.0 g of magnesium oxide is formed. Write the balanced chemical equation for the reaction, calculate the mass of oxygen that combined with the magnesium, and name the law you used. (3 marks) 9. Water is decomposed according to the equation 2H2O(l) -> 2H2(g) + O2(g). (i) If 30 mL of hydrogen is collected during electrolysis, what volume of oxygen is collected at the same time? (ii) How many molecules of water must decompose to give 10 molecules of oxygen? (iii) At which electrode is hydrogen collected? (3 marks) 10. Predict the products, write the balanced chemical equation and name the type of reaction in each case: (i) zinc is added to copper sulphate solution (ii) sodium sulphate solution is mixed with barium chloride solution (iii) calcium carbonate is heated strongly. (3 marks) Section C - Diagrams (10 marks) 11. Draw a neat labelled diagram of the experimental setup for the electrolysis of water. Label the container, graphite (carbon) electrodes, test tubes, battery, switch and the gases collected. State the ratio of the volumes of the gases collected at the two electrodes. (4 marks) 12. Draw a labelled diagram to show the reaction of zinc granules with dilute sulphuric acid in a test tube and the testing of the gas evolved with a burning matchstick. Write the balanced chemical equation for the reaction. (3 marks) 13. Draw a labelled diagram showing the heating of ferrous sulphate crystals in a dry boiling tube held with a test tube holder over a burner. Write the balanced chemical equation and state the colour change observed. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A shiny brown-coloured element X, on heating in air, becomes black in colour. Identify X and the black compound formed, and write the balanced equation. Explain how the black compound can be changed back to X using hydrogen gas, write the equation, and identify the substances oxidised and reduced in this second reaction. (5 marks) 15. A student places an iron nail in copper sulphate solution in test tube 1 and a copper strip in ferrous sulphate solution in test tube 2. After 20 minutes, a change is seen only in test tube 1. Describe the changes seen in test tube 1, write the balanced equation, explain why nothing happens in test tube 2, and state what this tells us about the relative reactivity of iron and copper. (5 marks) 16. A student says, "When quicklime is added to water, heat is released, so all reactions involving calcium compounds are exothermic." Evaluate this statement by comparing the reaction of quicklime with water and the decomposition of limestone into quicklime. Write balanced equations for both, classify each as exothermic or endothermic, and explain why whitewashed walls develop a shiny finish two to three days after whitewashing. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write balanced chemical equations with state symbols wherever asked. Draw all diagrams neatly in pencil and label every part.