Thermochemistry
About this topic
This topic explores energy changes during chemical reactions, focusing on enthalpy changes (ΔH) as per the Zambian CBC. You will learn to differentiate between exothermic and endothermic reactions, interpret energy profile diagrams, and perform calculations involving heat changes using the formula q = mcΔT. Understanding thermochemistry explains real-world applications like fuel combustion and hand warmers. ECZ examinations often include questions on interpreting energy diagrams and simple enthalpy calculations. Competence in this topic enhances your ability to analyze the energy aspects of chemical processes, a skill assessed in both theory and practical components of your Form 3 and Form 4 assessments.
What you'll practise (22 questions)
- Study the energy profile diagram below. What is the value of the enthalpy change for the reaction? A) -20KJ B) +20KJ C) +40KJ D) -40KJ
- Which statement is correct for all exothermic reactions? A) A catalyst is needed for the reaction to take place B) Light is absorbed during the reaction C) The products of the reaction have less energy than the reactants D) They are reactions which require heat to start
- When concentrated sulphuric acid is added to cold water, the temperature of the mixture rises. Is the process endothermic or exothermic? Give a reason for your answer.
- Which letter represents the enthalpy change of reaction?
- Consider the combustion reaction below: C(s) + O₂(g) → CO₂(g) ΔH = -394 kJ/mol. How much heat is released to the surroundings when one mole of carbon is burnt completely in oxygen?
- Suggest the molar enthalpy of combustion of the fifth alkane (pentane) given the values for the first four: -890, -1600, -2220, -2880 kJ/mol.
- Explain why the amount of heat energy produced increases consecutively when 1 mole of each alkane burns completely.
- Use the following bond energies to calculate the ΔH for the reaction: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies: C-H = 412, O=O = 490, C=O = 826, O-H = 464 kJ/mol.
- Draw an energy level diagram for the combustion of methane.
- The energy level diagram for the reaction of X and Y to form XY is shown. Which statement is true about the reaction?
- The table below shows the energy released by the complete combustion of some compounds used as fuels. Which fuel produces the most energy when 1g of the compound is completely burnt?
- Which of the following is an endothermic reaction? A) The combustion of carbon B) The combustion of ethanol C) Photosynthesis D) Respiration
- The diagram below shows the energy profiles for a chemical reaction. What is the correct description of the reaction?
- Consider the bond energies below. Using the bond energies, what is the enthalpy change, ΔH, of the reaction: H₂(g) + Cl₂(g) → 2HCl(g)?
- Which statement about energy change is false?
- Propane burns in oxygen exothermically producing 2220KJ of heat according to the equation below. What is the enthalpy change (ΔH) when 22g of propane is completely burnt in oxygen?
- The energy profile diagram for the backward direction of a reversible reaction is as shown below. Which row correctly shows both the sign of the activation energy and the type of enthalpy change for the forward reaction?
- Which of the following is true about exothermic reactions?
- What is an exothermic reaction?
- What is an endothermic reaction?
- Give an example of an exothermic reaction.
- Give an example of an endothermic reaction.
Answer each question above to see the correct answer and a full explanation.