Chemical energetics - Exothermic and endothermic reactions (3)
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1.
A student carries out an experiment where a substance reacts with water, and the temperature of the water increases. State whether this is an exothermic or an endothermic reaction. Explain your answer using the concept of energy transfer.
This is an exothermic reaction.
The fact that the temperature of the water increases indicates that thermal energy is being transferred from the reaction to the water. In an exothermic reaction, energy is released as heat. This heat is transferred to the surroundings, in this case, the water, causing an increase in its temperature. Therefore, the reaction releases thermal energy and is classified as exothermic.
2.
The enthalpy change of reaction for the formation of ammonia from nitrogen and hydrogen is -92 kJ/mol. Using the following bond energies:
- N≡N: 945 kJ/mol
- H-H: 436 kJ/mol
- N-H: 413 kJ/mol
Calculate the enthalpy change of reaction for the formation of ammonia using bond energies.
Answer:
The reaction for the formation of ammonia is: N2(g) + 3H2(g) → 2NH3(g)
Reactants:
- N2: 1 N≡N bond x 945 kJ/mol = 945 kJ/mol
- H2: 3 H-H bonds x 436 kJ/mol = 1308 kJ/mol
Products:
- NH3: 2 NH3 molecules x 2 N-H bonds x 413 kJ/mol = 2 x 2 x 413 kJ/mol = 1652 kJ/mol
Total energy required to break bonds = 945 kJ/mol + 1308 kJ/mol = 2253 kJ/mol
Total energy released when forming bonds = 1652 kJ/mol
Enthalpy change of reaction = Energy released - Energy required to break bonds
ΔH = 1652 kJ/mol - 2253 kJ/mol = -599 kJ/mol
Therefore, the enthalpy change of reaction for the formation of ammonia is -599 kJ/mol.
3.
Consider the following reaction pathway diagram, which shows the enthalpy change for the decomposition of calcium carbonate.
(a) What type of reaction is represented by the main pathway? Explain your answer.
(b) What does the energy level of the reactants indicate about the energy required to start the reaction?
(c) Describe what happens to the enthalpy change as the reaction proceeds towards completion.
(a) The main pathway represents an endothermic reaction. This is because the energy level of the products is higher than the energy level of the reactants. Endothermic reactions absorb heat from the surroundings, resulting in a positive enthalpy change (ΔH > 0). The diagram shows an increase in energy as the reaction proceeds.
(b) The energy level of the reactants indicates the activation energy required to initiate the reaction. A higher energy level means a greater amount of energy is needed to break the bonds in the reactants and start the decomposition process.
(c) As the reaction proceeds towards completion, the enthalpy change remains positive but decreases. This is because less reactant remains, and therefore less energy is required to continue the decomposition. The energy difference between the reactants and products becomes smaller as the reaction progresses.