Energy and chemical change: Grade 11 Past Paper Questions

QUESTION 3. The following reversible reaction takes place in a closed container: A₂ + B₂ ⇌ 2AB. The graph below shows the changes in chemical potential energy during the course of the reaction. 3.1 Is the forward reaction endothermic or exothermic? Give a reason for the answer. 3.2 Calculate ΔH for the forward reaction. 3.3 Define activation energy. 3.4 What is the activation energy for the reverse reaction? 3.5 How does the strength of the bonds within the reactant molecules compare to the strength of the bonds within the product molecules for the forward reaction? Choose from STRONGER THAN, WEAKER THAN, or OF SIMILAR STRENGTH. Give a reason for the answer. 3.6 Redraw the above graph in your answer book, and label it A. Do not label the axes or show any given values. On the same set of axes, draw the graph that will be obtained if a catalyst is added. Label this as graph B.

1.4 Which ONE of the following pairs represents the CONJUGATE ACID and CONJUGATE BASE of HPO₄²⁻? A. PO₄³⁻ and HPO₄²⁻ B. HPO₄²⁻ and H₃PO₄ C. H₂PO₄⁻ and PO₄³⁻ D. HPO₄²⁻ and PO₄³⁻. 1.5 Which ONE of the following processes is exothermic? A. Melting ice cubes. B. Baking bread. C. Burning a candle. D. Photosynthesis. 1.6 Which ONE of the following is correct for Boyle's Law? A. P ∝ 1/V when temperature is constant. B. P ∝ V when temperature is constant. C. P ∝ 1/T when volume is constant. D. P ∝ T when volume is constant. 1.7 Which ONE of the following graphs represents the deviation of real gas behaviour (broken line) from ideal gas behaviour (solid line)?

2.3 A 29,05 g sample of a compound contains 11,62 g carbon, 1,94 g hydrogen and 15,49 g oxygen. 2.3.1 Determine the empirical formula of this compound. 2.3.2 The molecular mass of this compound is 90 g·mol⁻¹. Determine the molecular formula of this compound. QUESTION 3. The following reversible reaction takes place in a closed container: A₂ + B₂ ⇌ 2AB. The graph below shows the changes in chemical potential energy during the course of the reaction. 3.1 Is the forward reaction endothermic or exothermic? Give a reason for the answer. 3.2 Calculate ΔH for the forward reaction. 3.3 Define activation energy. 3.4 What is the activation energy for the reverse reaction? 3.5 How does the strength of the bonds within the reactant molecules compare to the strength of the bonds within the product molecules for the forward reaction? Choose from STRONGER THAN, WEAKER THAN, or OF SIMILAR STRENGTH. Give a reason for the answer.

3.6 Redraw the above graph in your answer book, and label it A. Do not label the axes or show any given values. On the same set of axes, draw the graph that will be obtained if a catalyst is added. Label this as graph B. QUESTION 4. The balanced equation below represents the ionisation of nitric acid (HNO₃) in water: HNO₃(l) + H₂O(l) ⇌ H₃O⁺(aq) + NO₃⁻(aq). 4.1.1 Define an acid in terms of the Lowry-Brønsted theory. 4.1.2 Write down the FORMULAE of the TWO bases in the equation above. 4.1.3 Is nitric acid a STRONG or a WEAK acid? Give a reason for the answer. 4.1.4 Define an ampholyte. 4.1.5 Write down the formula of an ampholyte present in the equation above. 4.2 A learner performs a titration using a standard potassium hydroxide solution of concentration 0,1 mol·dm⁻³ and sulphuric acid of unknown concentration. The balanced equation for the reaction is: H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l). 4.2.1 Define a standard solution. 4.2.2 Using the above data, determine the concentration of the H₂SO₄ solution.