Thermochemistry and Enthalpy

Manual: General · Subject: Chemistry

Analyze heat flow, enthalpy changes, calorimetry, and Hess's law.

Energy in chemical change

Heat and work

Thermochemistry studies energy changes in chemical and physical processes. At constant pressure, the heat flow of a process equals the enthalpy change, $q_p = \Delta H$ .

Calorimetry

In simple calorimetry, heat absorbed or released is calculated by $q = mc\Delta T$ where $m$ is mass, $c$ is specific heat capacity, and $\Delta T$ is the temperature change.

Using Hess's law

1

Step 1: Write the target reaction.
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Step 2: Manipulate known equations to match the target.
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Step 3: Reverse equations if needed and change the sign of $\Delta H$ .
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Step 4: Add the equations and sum the enthalpy changes.

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State functions

Enthalpy is a state function, so $\Delta H$ depends only on initial and final states, not on the path taken.

Which expression gives the heat absorbed by a substance in a calorimetry problem?

q = mc\Delta T

q = nRT

q = \Delta G - \Delta H

q = \frac{m}{c\Delta T}

What is the sign of $\Delta H$ for an exothermic reaction?

Endothermic vs exothermic

Endothermic

Absorbs heat
$\Delta H > 0$
Products have higher enthalpy than reactants

Exothermic

Releases heat
$\Delta H < 0$
Products have lower enthalpy than reactants

If a reaction is reversed, what happens to its enthalpy change?

It stays the same It becomes zero Its sign changes It doubles

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