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Consider the following reaction:

\[ 2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s) \quad \Delta H = -1204 \, \text{kJ} \]

How many kilojoules of heat are absorbed when 99.1 g of MgO(s) is decomposed into Mg(s) and O₂(g) at constant pressure?

Answer :

Final answer:

To calculate the heat absorbed when 99.1 g of MgO(s) is decomposed into Mg(s) and O₂(g), convert the mass of MgO(s) to moles and use the molar ratio from the balanced equation. Finally, use the enthalpy change per mole of O₂(g) to calculate the heat absorbed.

Explanation:

The enthalpy change of a reaction can be used to calculate the amount of heat absorbed or released. In this case, we have the enthalpy change (-1204 kJ) for the reaction 2Mg(s) + O₂(g) ⟶ 2MgO(s). The reaction you're asking about is the reverse reaction, so the enthalpy change would have the opposite sign. To find the heat absorbed when 99.1 g of MgO(s) is decomposed into Mg(s) and O₂(g), we need to convert the mass of MgO(s) to moles and then calculate the heat using the molar ratio.

First, calculate the number of moles of MgO(s) using its molar mass:

99.1 g MgO(s) / 40.31 g/mol MgO(s) = 2.45 mol MgO(s)

Next, use the molar ratio from the balanced equation:

2 mol MgO(s) ⟶ 2 mol Mg(s) + 1 mol O₂(g)

So, for every 2 mol of MgO(s) decomposed, 1 mol of O₂(g) is formed. Therefore, the number of moles of O₂(g) formed is also 2.45 mol. Finally, use the enthalpy change per mole of O₂(g) given in the question to calculate the heat absorbed:

-1204 kJ / 2 mol O₂(g) = -602 kJ/mol O₂(g)

Therefore, when 99.1 g of MgO(s) is decomposed, -602 kJ of heat is absorbed.

Learn more about enthalpy change here:

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