When we look at chemical reactions, it’s important to understand two types: endothermic and exothermic reactions. These types help us see how reactions can happen naturally and connect to energy changes.

Exothermic Reactions

Exothermic reactions are those that give off heat to their surroundings.

In these reactions, the products (what you get at the end) have less energy than the reactants (what you start with) because energy is released when the products form stronger bonds.

A common example of an exothermic reaction is when gasoline burns:

$\text{C}_8\text{H}_{18} + 12.5 \text{O}_2 \rightarrow 8 \text{CO}_2 + 9 \text{H}_2\text{O} + \text{Heat}$

When we use Gibbs Free Energy (G), we usually see that exothermic reactions have a negative change in energy, known as enthalpy ($\Delta H < 0$).

We can use the Gibbs Free Energy equation to understand if a reaction will happen on its own:

$G = H - TS$

In this equation:

• $G$ is Gibbs Free Energy,
• $H$ is the energy,
• $T$ is the temperature,
• $S$ is the disorder of the system (called entropy).

Since exothermic reactions lower the energy ($H$), they can also lower $G$, which means they are likely to happen naturally ($\Delta G < 0$).

Endothermic Reactions

On the other hand, endothermic reactions absorb heat from their surroundings.

This means they end up with products that have more energy than the reactants. A common example is when ammonium nitrate dissolves in water:

$\text{NH}_4\text{NO}_3 (s) + \text{Heat} \rightarrow \text{NH}_4^+ (aq) + \text{NO}_3^- (aq)$

In this example, energy is taken in, increasing the internal energy. For endothermic reactions, we find that $\Delta H > 0$.

To check if these reactions can happen naturally, we can use the Gibbs Free Energy equation again. Even though they absorb heat, endothermic reactions can still occur if the increase in disorder ($\Delta S > 0$) is big enough.

Spontaneity and Entropy

When we try to figure out if either type of reaction will happen on its own, we need to look at the change in disorder ($\Delta S$), which tells us how messy or orderly a system is.

We can write the Gibbs Free Energy equation like this:

$\Delta G = \Delta H - T \Delta S$

• For exothermic reactions, since $\Delta H$ is negative, the part $-T\Delta S$ (which can be positive if the temperature is high) makes it easier for $G$ to be negative, which means the reaction can happen naturally.

• For endothermic reactions, even though $\Delta H$ is positive, if $\Delta S$ is also positive and big enough, it can make $\Delta G$ negative. This means the reaction can still happen at higher temperatures.

Conclusion

In summary, the main differences between endothermic and exothermic reactions relate to their energy changes and whether they happen on their own. Exothermic reactions usually happen naturally because they release heat. However, endothermic reactions can also happen under the right conditions, especially when there's a significant change in disorder. Understanding these ideas helps us learn about the energy changes in chemical reactions.