AP Chemistry enthalpy study guide

What is enthalpy?

A measure of total energy of a thermodynamic system, often symbolized as $\displaystyle H$.

Define exothermic reaction.

A reaction that releases heat, resulting in a temperature increase in the surroundings.

Define endothermic reaction.

A reaction that absorbs heat from the surroundings, causing a temperature decrease.

What is the formula for change in enthalpy?

$\displaystyle \Delta H = H_{products} - H_{reactants}$.

True or false: Enthalpy is an extensive property.

True, because it depends on the amount of substance present.

Difference between $\displaystyle \Delta H_{f}^\circ$ and $\displaystyle \Delta H_{c}^\circ$.

$\displaystyle \Delta H_{f}^\circ$ is the heat of formation; $\displaystyle \Delta H_{c}^\circ$ is the heat of combustion.

Units of enthalpy.

Commonly expressed in kilojoules per mole ($\displaystyle kJ/mol$).

What is Hess's Law?

The total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps.

Fill in the blank: Standard enthalpy of formation for elements in their standard state is __________.

0 kJ/mol.

Key feature of bond enthalpy.

The energy required to break one mole of a specific bond in a gaseous substance.

Heat capacity vs. enthalpy.

Heat capacity measures energy required to change temperature; enthalpy measures total energy.

Calculate $\displaystyle \Delta H$ for the reaction: $\displaystyle A + B \rightarrow C + D$.

$\displaystyle \Delta H = \sum H_{products} - \sum H_{reactants}$.

True or false: $\displaystyle \Delta H$ is always positive for exothermic reactions.

False, because $\displaystyle \Delta H$ is negative for exothermic reactions.

What is the significance of negative $\displaystyle \Delta H$?

Indicates a reaction is exothermic, releasing heat.

Identify the sign of $\displaystyle \Delta H$ in an endothermic reaction.

Positive, since heat is absorbed.

What does the term 'enthalpy change' refer to?

The difference in enthalpy between products and reactants.

How does pressure affect enthalpy?

Changes in pressure can affect enthalpy, especially in gases.

Define calorimetry.

The science of measuring the heat of chemical reactions or physical changes.

What is an isothermal process?

A process that occurs at constant temperature, leading to specific enthalpy changes.

Bond enthalpy vs. enthalpy of formation.

Bond enthalpy is for breaking bonds; enthalpy of formation is for forming compounds.

What is the relationship between enthalpy and temperature?

Enthalpy changes with temperature; higher temperature can increase enthalpy.