Test – 5.1 Chemical Energetics – Exothermic and Endothermic Reactions

Table of Contents
IGCSE Chemistry: Chemical Energetics Test
Instructions:
  • Time allowed: 90 minutes
  • Total marks: 100 (30 marks for Section A, 70 marks for Section B)
  • Answer ALL questions
  • Write your answers clearly in the spaces provided
  • Show all working for calculations

SECTION A: MULTIPLE CHOICE QUESTIONS [30 marks] #

Choose the best answer for each question. Each question is worth 1 mark.

1. Which statement correctly describes an exothermic reaction?
A) It takes in heat from the surroundings
B) It transfers heat to the surroundings
C) The temperature of the surroundings decreases
D) The products have more energy than the reactants
2. When solid ammonium chloride dissolves in water, the temperature of the solution decreases. This process is:
A) Exothermic with negative ΔH
B) Exothermic with positive ΔH
C) Endothermic with negative ΔH
D) Endothermic with positive ΔH
3. Which of the following is an example of an exothermic reaction?
A) Photosynthesis
B) Thermal decomposition of calcium carbonate
C) Combustion of methane
D) Melting of ice
4. The enthalpy change (ΔH) for an endothermic reaction is:
A) Always negative
B) Always positive
C) Always zero
D) Can be positive or negative
5. What is activation energy?
A) The energy released when products form
B) The total energy of the reactants
C) The minimum energy particles need to react when they collide
D) The difference in energy between reactants and products
6. In a reaction pathway diagram, the activation energy is shown as:
A) The horizontal distance from reactants to products
B) The vertical distance from reactants to the peak of the curve
C) The vertical distance from products to the peak of the curve
D) The vertical distance between reactants and products
7. Bond breaking is:
A) Always exothermic
B) Always endothermic
C) Sometimes exothermic, sometimes endothermic
D) Neither exothermic nor endothermic
8. Which process releases energy?
A) Breaking a C-H bond
B) Breaking an O=O bond
C) Forming an H-O bond
D) Separating water molecules
9. Look at this reaction pathway diagram:

What is the enthalpy change (ΔH) for this reaction?
A) -40 kJ/mol
B) +40 kJ/mol
C) -80 kJ/mol
D) +80 kJ/mol
10. Using the same diagram from question 9, what is the activation energy?
A) 40 kJ/mol
B) 60 kJ/mol
C) 80 kJ/mol
D) 120 kJ/mol
11. In the reaction: 2H₂ + O₂ → 2H₂O, how many bonds are broken in total?
A) 2
B) 3
C) 4
D) 6
12. A reaction has an enthalpy change of -286 kJ/mol. This means:
A) 286 kJ of energy is absorbed from the surroundings
B) 286 kJ of energy is released to the surroundings
C) The temperature stays the same
D) The reaction cannot happen
13. Which factor does NOT affect the enthalpy change of a reaction?
A) The strength of bonds broken
B) The strength of bonds formed
C) The presence of a catalyst
D) The number of bonds broken and formed
14. When calculating enthalpy change using bond energies, the formula is:
A) ΔH = Energy of bonds formed – Energy of bonds broken
B) ΔH = Energy of bonds broken – Energy of bonds formed
C) ΔH = Energy of bonds broken + Energy of bonds formed
D) ΔH = Energy of products – Energy of reactants
15. A student touches a test tube where magnesium is reacting with hydrochloric acid. The test tube feels hot. This indicates:
A) An endothermic reaction with positive ΔH
B) An endothermic reaction with negative ΔH
C) An exothermic reaction with positive ΔH
D) An exothermic reaction with negative ΔH
16. Which statement about catalysts is correct?
A) They increase the enthalpy change
B) They decrease the activation energy
C) They increase the activation energy
D) They make endothermic reactions become exothermic
17. In the reaction N₂ + 3H₂ → 2NH₃, how many N-H bonds are formed?
A) 2
B) 3
C) 4
D) 6
18. The bond energy of H-H is 435 kJ/mol. This means:
A) 435 kJ is released when one mole of H-H bonds break
B) 435 kJ is needed to break one mole of H-H bonds
C) 435 kJ is the total energy in one H₂ molecule
D) 435 kJ is released when H₂ gas forms
19. Which reaction pathway diagram represents an endothermic reaction?

A) Diagram showing products lower than reactants
B) Diagram showing products higher than reactants
C) Diagram showing products at same level as reactants
D) Diagram showing no activation energy
20. During respiration, glucose reacts with oxygen to produce carbon dioxide and water. This process:
A) Absorbs heat from the body
B) Releases heat to keep the body warm
C) Has no energy change
D) Only happens when the body is cold
21. A reaction has these bond energies:
Bonds broken: 1000 kJ
Bonds formed: 1200 kJ
What is the enthalpy change?
A) +200 kJ/mol
B) -200 kJ/mol
C) +2200 kJ/mol
D) -2200 kJ/mol
22. Why does a match need to be struck before it burns?
A) To decrease the activation energy
B) To provide the activation energy
C) To make the reaction endothermic
D) To break all the bonds first
23. Which diagram correctly shows the effect of a catalyst?
A) Lower peak, same start and end points
B) Higher peak, same start and end points
C) Same peak, products at lower energy
D) Same peak, products at higher energy
24. The reaction 2HI → H₂ + I₂ involves breaking 2 H-I bonds. If the H-I bond energy is 298 kJ/mol, how much energy is needed to break all bonds?
A) 149 kJ
B) 298 kJ
C) 596 kJ
D) 894 kJ
25. Which process is endothermic?
A) Neutralization of acid with alkali
B) Dissolving sodium hydroxide in water
C) Thermal decomposition of copper carbonate
D) Combustion of natural gas
26. In an experiment, citric acid reacts with sodium hydrogencarbonate. The temperature drops from 20°C to 15°C. What type of reaction is this?
A) Exothermic with energy released
B) Endothermic with energy absorbed
C) No energy change
D) Cannot determine from this information
27. The reaction CH₄ + 2O₂ → CO₂ + 2H₂O has ΔH = -890 kJ/mol. This means:
A) The reaction needs 890 kJ to start
B) 890 kJ is released when one mole of methane burns
C) 890 kJ is absorbed when one mole of methane burns
D) The products have 890 kJ more energy than reactants
28. Which bond is likely to have the highest bond energy?
A) C-C single bond
B) C=C double bond
C) C≡C triple bond
D) All have the same bond energy
29. A reaction profile shows the peak at 150 kJ, reactants at 50 kJ, and products at 30 kJ. Which statement is correct?
A) ΔH = +20 kJ, Ea = 100 kJ
B) ΔH = -20 kJ, Ea = 100 kJ
C) ΔH = +20 kJ, Ea = 120 kJ
D) ΔH = -20 kJ, Ea = 120 kJ
30. Why must all chemical reactions have an activation energy?
A) To make sure products have less energy than reactants
B) To prevent all reactions happening instantly
C) To make reactions exothermic
D) To increase the temperature

SECTION B: WRITTEN RESPONSE QUESTIONS [70 marks] #

Answer all questions in the spaces provided. Show all working for calculations.

1. Define the following terms: [4]
  1. Exothermic reaction (2 marks)

    _____________________

    _____________________
  2. Activation energy (2 marks)

    _____________________

    _____________________
2. Give two examples of exothermic reactions and two examples of endothermic reactions that happen in everyday life. [4]

Exothermic:
1. _____________________
2. _____________________

Endothermic:
1. _____________________
2. _____________________
3. A student adds calcium oxide to water in a beaker. The reaction can be shown as:
CaO + H₂O → Ca(OH)₂
  1. The student notices the beaker becomes very hot. What type of reaction is this? [1]

    _____________________
  2. What is the sign of ΔH for this reaction? Explain your answer. [2]

    _____________________

    _____________________
4. Explain why bond breaking is endothermic and bond making is exothermic. Use the idea of energy in your answer. [4]

_____________________

_____________________

_____________________

_____________________
5. The diagram shows a reaction pathway for the decomposition of hydrogen peroxide:

  1. What is the activation energy for this reaction? [1]

    _____________________
  2. What is the enthalpy change (ΔH) for this reaction? [1]

    _____________________
  3. Is this reaction exothermic or endothermic? Explain how you know. [2]

    _____________________

    _____________________
6. Photosynthesis is an important endothermic reaction in plants:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
  1. Where does the energy for this reaction come from? [1]

    _____________________
  2. Explain why this reaction is endothermic in terms of the energy of reactants and products. [2]

    _____________________

    _____________________
  3. What would be the sign of ΔH for this reaction? [1]

    _____________________
7. Draw and label a complete reaction pathway diagram for an exothermic reaction where: [4]
  • The reactants have an energy of 150 kJ/mol
  • The products have an energy of 50 kJ/mol
  • The activation energy is 80 kJ/mol
Include labels for: reactants, products, activation energy (Ea), and enthalpy change (ΔH).
8. Calculate the enthalpy change for the reaction: H₂ + Cl₂ → 2HCl [5]
Use these bond energies:
Bond Bond energy (kJ/mol)
H-H 435
Cl-Cl 240
H-Cl 430
Show all your working.
9. A student investigates the reaction between zinc and copper sulfate solution:
Zn + CuSO₄ → ZnSO₄ + Cu
The temperature rises from 18°C to 26°C.
  1. Is this reaction exothermic or endothermic? [1]

    _____________________
  2. Explain your answer to part (a) using the temperature change. [2]

    _____________________

    _____________________
  3. Predict the sign of ΔH for this reaction. [1]

    _____________________
10. The combustion of ethanol can be represented as:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
  1. State whether this reaction is exothermic or endothermic. [1]

    _____________________
  2. Explain why all combustion reactions have the same type of energy change. [2]

    _____________________

    _____________________
  3. Why does ethanol need a spark or flame to start burning? [2]

    _____________________

    _____________________
11. Calculate the enthalpy change for: CH₄ + 2O₂ → CO₂ + 2H₂O [6]
Bond energies (kJ/mol):
Bond Energy Bond Energy
C-H 410 C=O 740
O=O 498 O-H 460
Show all steps of your calculation.
12. A reaction has the following energy values:
  • Energy needed to break bonds = 850 kJ/mol
  • Energy released when bonds form = 920 kJ/mol
  1. Calculate the enthalpy change (ΔH) for this reaction. [2]

    _____________________
  2. Is the reaction exothermic or endothermic? [1]

    _____________________
  3. Sketch a reaction pathway diagram for this reaction. [3]
13. Explain the difference between enthalpy change (ΔH) and activation energy (Ea). Include what each one measures in a chemical reaction. [4]

_____________________

_____________________

_____________________

_____________________
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