Question

Calculate the amount of heat released (kJ) in the complete oxidation of 6.95 g of Al metal at standard condition.

Answer 1

The standard enthalpy change of formation of aluminum oxide (heat per mole that the substance tranfers to the environment during the process of its formation): $\Delta H^o_f=-1676\text{ kJ/mol}.$

1) Write the reaction:

$$4\text{Al}+6\text{O}_2\rightarrow2\text{Al}_2\text{O}_3.$$

We notice that 2 Al atoms are required to produce 1 molecule of aluminum oxide. One molecule of the oxide produce -1676 kJ of heat per mole.

2) How many moles of aluminum reacted?

$$n_\text{Al}=\frac{m_\text{Al}}{M_\text{Al}}=\frac{6.95}{27}=0.257\text{ mol}.$$

3) How many moles of aluminum oxide formed in the reaction? Two times less:

$$n_{\text{Al}_2\text{O}_3}=\frac{n_\text{Al}}{2}=\frac{0.257}{2}=0.129\text{ mol}.$$

4) How much heat the process generated?

$$Q=\Delta H\cdot n=-1676\cdot0.129=-216\text{ kJ}.$$