Question

12. A calorimeter has a measured heat capacity of 6.27 kJ/°C. The combustion of 1.84 g of

magnesium caused the temperature to rise from 21.30°C to 28.56°C. (9 points)
a. Is this reaction endothermic or exothermic? (1 point)
Exothermic
EXPORT
b. Calculate the enthalpy change of the reaction. (4 points)
c. What is the enthalpy change for this combustion in kilojoules per mole of magnes
atoms? (Make sure the sign of your answer is consistent with part a.) (4 points)

Answer 1

The combustion of magnesium in a calorimeter is exothermic. The enthalpy change of the reaction is 45.5182 kJ. Per mole of magnesium, the enthalpy change is -601.6 kJ/mol.

Step-by-step explanation:

The reaction in question is exothermic, as combustion reactions typically release energy, causing a temperature rise in the surroundings. To calculate the enthalpy change for the combustion of magnesium, we use the formula:

q = C × ΔT

Where:

• q is the heat absorbed or released (the enthalpy change)
• C is the heat capacity of the calorimeter
• ΔT is the change in temperature

Given that C = 6.27 kJ/°C and ΔT

= 28.56°C - 21.30°C

= 7.26°C

q = 6.27 kJ/°C × 7.26°C

= 45.5182 kJ

To calculate the enthalpy change per mole of magnesium, we use magnesium's molar mass (24.305 g/mol) to find how many moles 1.84 g represents:

Moles of Mg = 1.84 g / 24.305 g/mol

= 0.0757 mol

Now, the enthalpy change per mole is:

ΔH = q / Moles of Mg

= 45.5182 kJ / 0.0757 mol

= 601.6 kJ/mol

As the reaction is exothermic, we express this as -601.6 kJ/mol to indicate that energy is released.