Exothermic and Endothermic Reactions

Flashcards covering the definitions, characteristics, and rules of exothermic and endothermic reactions as well as thermochemical equations.

Exothermic Reaction

A reaction accompanied by releasing heat energy to the surrounding where $\text{H}_R > \text{H}_P$ and the enthalpy change ($\triangle \text{H}$) is negative.

Endothermic Reaction

A reaction accompanied by absorbing heat energy from the surrounding where $\text{H}_P > \text{H}_R$ and the enthalpy change ($\triangle \text{H}$) is positive.

Enthalpy Change ($\triangle \text{H}$)

The measure of how much the heat content changes, calculated by the formula $\triangle \text{H} = \text{H}_P - \text{H}_R$.

Stability (Exothermic)

In an exothermic reaction, the products are more stable and the reactants are less stable.

Stability (Endothermic)

In an endothermic reaction, the products are less stable and the reactants are more stable.

Heat Transfer (Exothermic)

The process where heat transfers from the system into the surrounding.

Heat Transfer (Endothermic)

The process where heat transfers from the surrounding into the system.

Condensation

The exothermic process of a substance changing from a gas to a liquid ($\triangle \text{H} = -ve$).

Freezing

The exothermic process of a substance changing from a liquid to a solid ($\triangle \text{H} = -ve$).

Combustion

An exothermic process involving the burning of a substance with $\text{O}_2$ ($\triangle \text{H} = -ve$).

Neutralization

An exothermic reaction between an acid and a base that produces salt and water ($\triangle \text{H} = -ve$).

Vaporization (Evaporation)

An endothermic process where a substance changes from a liquid to a gas ($\triangle \text{H} = +ve$).

Fusion (Melting)

An endothermic process where a substance changes from a solid to a liquid ($\triangle \text{H} = +ve$).

Thermochemical Equation

A balanced chemical equation that includes the heat of reaction ($\triangle \text{H}$) and the physical states of the substances involved.

Physical States (Thermochemical)

The specific notations required in a thermochemical equation: solid $(s)$, liquid $(l)$, gas $(g)$, and aqueous $(aq)$.

Rule for Reversing Reactions

If a chemical reaction is reversed, the sign of the enthalpy change ($\triangle \text{H}$) is reversed too.

Rule for Scaling Equations

If a thermochemical equation is multiplied or divided by any factor, the enthalpy change ($\triangle \text{H}$) must be multiplied or divided by that same factor.

Negative Sign of $\triangle H$

Does not mean negative energy, but instead indicates that energy is lost to the surroundings.