Imagine you have been given a series of materials to react together and the following enthalpy change data has been recorded. Explain the terms exothermic and endothermic, and identify which of the following are exothermic and endothermic reactions. Ensure that you explain your reasoning.

1. N2 + 2O2 → 2NO2 ΔH = +68kJ mol–1

2. C3H8 + 5O2 → 3CO2 + 4H2O ΔH = -2220kJ mol–1

3. 2H2 + O2 → 2H2O ΔH = -486.3kJ mol–1

4. 2CO2 → 2CO + O2 ΔH = +566kJ mol–1

(B) You have been asked to calculate the enthalpy change in the following reaction: 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(s)
The reaction occurs in the following stages:


1. 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(l) ΔH = -732.5kJ mol–1

2. Al2O3(s) + 2Fe(l) → Al2O3(s) + 2Fe(s) ΔH = -27.6kJ mol–1

3. Al2O3(s) + 2Fe(l) → Al2O3(s) + 2Fe(s) ΔH = -91kJ mol–1

Note stages 2 and 3 look the same, but the reaction is cooling from 1700oC to 25oC and hence the enthalpy change there. Calculate the overall enthalpy change for the reaction.

To explain the terms exothermic and endothermic, we need to understand the concept of enthalpy change. Enthalpy change is the amount of heat energy released or absorbed during a chemical reaction. It is represented by the symbol ΔH.

Exothermic reactions:
Exothermic reactions release heat energy into the surroundings. This means that the products have lower energy than the reactants, resulting in a negative ΔH value. In an exothermic reaction, the surroundings feel warm because heat is being given off during the reaction.

Endothermic reactions:
Endothermic reactions absorb heat energy from the surroundings. This means that the products have higher energy than the reactants, resulting in a positive ΔH value. In an endothermic reaction, the surroundings feel cool because heat is being absorbed during the reaction.

Now let's analyze the reactions and identify which ones are exothermic and which ones are endothermic:

1. N2 + 2O2 → 2NO2 (ΔH = +68 kJ mol–1)
In this reaction, the enthalpy change is positive (+68 kJ mol–1). Since the ΔH value is positive, this reaction is endothermic. It requires the absorption of heat energy from the surroundings.

2. C3H8 + 5O2 → 3CO2 + 4H2O (ΔH = -2220 kJ mol–1)
In this reaction, the enthalpy change is negative (-2220 kJ mol–1). Since the ΔH value is negative, this reaction is exothermic. It releases heat energy into the surroundings.

3. 2H2 + O2 → 2H2O (ΔH = -486.3 kJ mol–1)
In this reaction, the enthalpy change is negative (-486.3 kJ mol–1). Since the ΔH value is negative, this reaction is exothermic. It releases heat energy into the surroundings.

4. 2CO2 → 2CO + O2 (ΔH = +566 kJ mol–1)
In this reaction, the enthalpy change is positive (+566 kJ mol–1). Since the ΔH value is positive, this reaction is endothermic. It requires the absorption of heat energy from the surroundings.

Now, let's calculate the overall enthalpy change for the given reaction: 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(s)

We have the following stages with their respective enthalpy changes:
1. 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(l) (ΔH = -732.5 kJ mol–1)
2. Al2O3(s) + 2Fe(l) → Al2O3(s) + 2Fe(s) (ΔH = -27.6 kJ mol–1)
3. Al2O3(s) + 2Fe(l) → Al2O3(s) + 2Fe(s) (ΔH = -91 kJ mol–1)

To calculate the overall enthalpy change, we sum up the individual enthalpy changes:
Overall ΔH = ΔH1 + ΔH2 + ΔH3

Overall ΔH = (-732.5) + (-27.6) + (-91) = -851.1 kJ mol–1

Therefore, the overall enthalpy change for the reaction is -851.1 kJ mol–1, which means it is exothermic and releases heat energy into the surroundings.