The thermochemical equation showing the formation of ammonia (NH3) from its elements is: N2 (g) + 3 H2 (g) —> 2 NH3 (g) ΔH = -92 kJ This equation shows that 92 kJ of heat is: a. Lost to the surroundings when one mole of hydrogen is used up in the reaction. b. Absorbed from the surroundings when one mole of nitrogen reacts. c. Absorbed from the surroundings when one mole of ammonia is formed. d. Lost to the surroundings when 2 moles of ammonia is formed.
The Correct Answer and Explanation is:
The correct answer is: c. Absorbed from the surroundings when one mole of ammonia is formed.
Explanation:
In the thermochemical equation provided:
N2 (g) + 3 H2 (g) → 2 NH3 (g) ΔH = -92 kJ
The ΔH (change in enthalpy) of -92 kJ indicates that the reaction is exothermic. This means that 92 kJ of heat is released to the surroundings when 2 moles of ammonia are formed. The negative sign in front of ΔH represents heat being given off (exothermic), rather than absorbed.
Now, the key is understanding how the heat is distributed with respect to the amounts of ammonia produced. The equation shows that 2 moles of ammonia are produced when the reaction occurs, and 92 kJ of heat is released for every 2 moles of ammonia formed. This means that 46 kJ of heat is released per mole of ammonia formed.
Since the question asks about the absorption of heat in the process, it’s important to note that because the reaction is exothermic, heat is not absorbed by the system but rather released to the surroundings. Therefore, the correct statement involves the release of heat when ammonia is formed, which corresponds to the negative ΔH value.
If we break down the options:
- Option a is incorrect because the heat is released, not lost specifically for hydrogen.
- Option b is incorrect because nitrogen does not absorb heat; rather, it is part of the process of releasing heat.
- Option c is correct, but it should clarify that heat is released rather than absorbed. It refers to the amount of heat associated with the formation of ammonia.
- Option d is incorrect because it suggests heat is lost when ammonia is formed, but since the reaction releases heat, the correct phrasing would be “heat is released,” not “lost.”
Thus, the formation of ammonia results in the release of energy to the surroundings, not absorption.
