Write down the given reactions and their enthalpy changes.

Here's how to determine the enthalpy change for the conversion of graphite to diamond using Hess's Law:

Step 1: Write down the given reactions and their enthalpy changes.

1. Combustion of graphite: $\text{C(graphite)} + O_2(g) \longrightarrow CO_2(g); \quad \Delta H_1 = -393 \text{ kJ/mol}$
2. Combustion of diamond: $\text{C(diamond)} + O_2(g) \longrightarrow CO_2(g); \quad \Delta H_2 = -395 \text{ kJ/mol}$

Step 2: Identify the target reaction. The reaction we want to find the enthalpy change for is: $\text{C(graphite)} \longrightarrow \text{C(diamond)}$

Step 3: Manipulate the given reactions to match the target reaction. We need $\text{C(graphite)}$ on the reactant side, so we keep Reaction 1 as it is: $\text{C(graphite)} + O_2(g) \longrightarrow CO_2(g); \quad \Delta H_1 = -393 \text{ kJ/mol}$ We need $\text{C(diamond)}$ on the product side. In Reaction 2, $\text{C(diamond)}$ is a reactant, so we reverse Reaction 2. Reversing a reaction changes the sign of its enthalpy change: $\text{CO}_2(g) \longrightarrow C(diamond) + O_2(g); \quad \Delta H_{reversed 2} = -(-395 kJ/mol) = +395 \text{ kJ/mol}$

Step 4: Add the manipulated reactions and their enthalpy changes. Adding the two manipulated reactions: $(C(graphite) + O_2(g) \longrightarrow CO_2(g)) + (CO_2(g) \longrightarrow C(diamond) + O_2(g))$ The $\text{O}_2\text{(g)}$ and $\text{CO}_2\text{(g)}$ cancel out on both sides, leaving the target reaction: $\text{C(graphite)} \longrightarrow \text{C(diamond)}$ The total enthalpy change is the sum of the enthalpy changes of the manipulated reactions: $\Delta H_{target} = \Delta H_1 + \Delta H_{reversed 2}$ $\Delta H_{target} = -393 kJ/mol + 395 \text{ kJ/mol}$ $\Delta H_{target} = +2 \text{ kJ/mol}$

The enthalpy change for the conversion of graphite to diamond is $\boxed{+2 kJ/mol}$.

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