Enthalpy

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SAT Subject Test in Chemistry › Enthalpy

Questions 1 - 1

$2\text{K} + \text{Cl}_2 \rightarrow 2\text{KCl} + 640\text{ kJ}$

Suppose that 1.5 moles of potassium reacts completely with an excess of chlorine gas. What is the enthalpy change of the reaction?

$-480\text{ kJ}$

$+480\text{ kJ}$

$-640\text{ kJ}$

$+640\text{ kJ}$

$+320\text{ kJ}$

Explanation

First, note that heat is a product of the reaction. This means that the reaction releases heat and is exothermic, so the enthalpy change must be negative. Next, consider that 2 moles of sodium would react to produce 640 kJ. Thus, 1.5 moles of sodium would react to produce

$640\text{ kJ}\times\frac{1.5\text{ mol}}{2\text{ mol}}=480\text{ kJ}$ .

Hence, the enthalpy change is $-480\text{ kJ}$ .

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