Question

1. Classify each of the following reactions as endothermic or exothermic.

a) CaCO3 (s) -----> CaO (s) + CO2 (g) ΔHrxn = +177 kJ
b) 2C (s) + H2 (g) + 21 kJ -------> C2H2 (g)
c) H2 (g) + F2 (g) --------> 2HF (g) ΔHrxn = -543 kJ
d) CH4 (g) + 2O2 (g) --------> CO2 (g) + 2H2O (l) + 803 kJ

Answer 1

a) Endothermic

b) Endothermic

c) Exothermic

d) Exothermic

Step-by-step explanation:

Endothermic reactions are chemical reactions in which heat is absorbed, while exothermic reactions are chemical reactions in which heat is released. Since endothermic reactions require energy (heat), heat can be found in the reactant side. For exothermic reactions, heat is found in the product side since it releases energy. In this question, heat can be found on the reactant side in reaction (b), so it is endothermic. In reaction (d), heat is in the product side so it is exothermic.

Another way to determine whether a reaction is endothermic or exothermic is to look at the value of its heat of reaction (also known as enthalpy), ΔH. It measures how much heat energy is involved during a chemical reaction, and is negative when heat is released and positive when heat is absorbed. Thus, reaction (a) is endothermic since it has a positive ΔH and reaction (c) is exothermic since it has a negative ΔH.