ENTHALPY CHANGES | Chemistry Cambridge As Level

Hello, everyone, my name’s Putri Dwi Safitri. You can call me Putri. I graduated from IPB university, majoring in Chemistry. Then, I continued my master degree at the University of Edinburgh in Medicinal and Biological Chemistry. I’ve just finished my master degree in August this year. Okay. That’s all about myself. Now, let’s move one. Today, we’re gonna discuss about Enthalpy Changes.

Firstly, before we jump to this topic, let’s find out the difference between exothermic and endothermic reaction. Those terms related to the heat energy transfer during chemical reaction.

What is exothermic and endothermic reaction? enthalpy changes

Exothermic reaction is the reaction that requires heat energy is transferred to the surrounding. As the result, the temperature of the surrounding increases.

For example, when you react magnesium with sulfuric acid in a test tube, you will feel the test tube becomes warm. It indicates, the temperature of the surrounding (or in this case, the test tube) increases.

That’s it for the exothermic reaction. It’s easy to understand, right?

Enthalpy Changes

Exothermic reaction is the reaction that requires heat energy absorption from the surrounding. As the result, the temperature of the surrounding decreases.

For example, when sodium hydrogencarbonate reacts with an aqueous solution of citric acid in a test tube, you’ll feel the test tube becomes cold which means, there is a transfer of heat energy from test tube and the air to the chemical reaction.

Now, you’re able to distinguish between exothermic and endothermic reaction.

Let’s go back to Enthalpy Changes. Read previous article!

What is enthalpy changes?

Enthalpy is the total energy associated with the material that react. The symbol of enthalpy is H. It must be capital. We cannot measure enthalpy, but we can measure enthalpy changes.

Enthalpy changes is the exchange of the energy between the chemical reactions and its surroundings at constant pressure. The symbol of enthalpy changes is ΔH. The unit for enthalpy change is kilojoule per mole (kJ mol^-1). The enthalpy change can be calculated by this formula,

ΔH = H products – H reactants

By using this formula, if the result is negative, means the reaction is exothermic. On the contrary, if the result is positive, the reaction is endothermic.

We can draw a reaction pathway diagram to show enthalpy changes.

Figure 1. The combustion of methane (exothermic)

The reaction pathway usually also shows the activation energy, Ea, which means, the minimum energy needed for a reaction to happen. For exothermic reaction, the energy is released to the surrounding, so the enthalpy of reactants must be greater than the enthalpy of the products. We can write down the reaction,

Remember, the negative sign for ΔH shows the reaction is exothermic.

Figure 2. Decomposition of calcium carbonate (endothermic)

For endothermic reaction, the energy is absorbed from the surrounding by the substance, so the enthalpy of the products must be grate than the enthalpy of the reactants.

Read previous article.

We can write the reaction,

Always, remember, the positive sign for ΔH means the reaction is endothermic.

After we discussed about exothermic and endothermic reactions, enthalpy change, and how to construct reaction pathway diagram, let’s discuss about,

A variety of Enthalpy Changes

But firstly, we need to know about standard enthalpy changes which means the reaction is carried out under standard condition which are, a pressure of 101 kPa or 1 atm and a temperature of 298 K.

The symbol used for standard enthalpy change is ΔH°. We can read it delta H standard or some people called delta H nought.

Okay. That’s for standard enthalpy changes.

Now, let’s identify the varieties of enthalpy changes.

According to the type of chemical reaction taking place, we can describe the enthalpy change. For example,

Enthalpy change of formation
Enthalpy change of combustion
Enthalpy change of neutralisation

And in more general case, we can use

Enthalpy change of reaction

Let’s discuss one by one

Standard enthalpy change of reaction

Standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in stoichiometric equation react to give product under standard conditions. he symbol for this is ΔH_r°. For example,

This equation shows us when 1 mol of water is formed from hydrogen and oxygen, 286 kJ of energy is released. However, if we add more hydrogen and oxygen, the energy that is released from this reaction is twice, because 2 mol of water is formed.

Read previous article

Standard enthalpy change of formation

Standard enthalpy change of formation is the enthalpy change when one mol of compound is formed from its element under standard conditions. The symbol for this is ΔH_f°. For example,

Standard enthalpy change of combustion

Standard enthalpy change of combustion is the enthalpy change when one mol of a substance is burnt in excess of oxygen under standard conditions. The symbol for this is ΔH_c°. For example,

Read previous article

Standard enthalpy change of neutralisation

Standard enthalpy of neutralisation is the enthalpy change when one mol of water is formed from the reaction of an acid and an alkali under standard conditions. The symbol for this is ΔH_neut°. For example,

Now that we knew the varieties of enthalpy changes, let’s move on to

How to measure enthalpy changes

There are at least two ways to measure enthalpy changes, those are

Using Hess’s law
Bond energies

Hess’s Law

Drawing Hess’s cycle can help us illustrate Hess’s Law.

This figure shows 3 routes. First, direct route, the reactants A and B combine directly to form C. Indirect route 1, where F and G are formed as intermediates to form product C. Or indirect route 2, where two steps of intermediates formed to form the final product, C.

Hess’s law tells us that enthalpy change of reaction both for direct and indirect route are the same, no matter how many steps are in the indirect route.

You can use this model below to draw an enthalpy cycle for calculating an enthalpy change of reaction from enthalpy change of formation. Dash lines shows us the indirect routes.

When we use Hess’s law, we can see that

ΔH₂ (direct route) = ΔH₁ + ΔH_r (indirect route)

So,

ΔH_r = ΔH₂ – ΔH₁

While the picture below can be used to draw an enthalpy cycle for calculating enthalpy change of formation from enthalpy change of combustion.

When we use Hess’s law, we can see that

ΔH₁ (direct route) = ΔH_F + ΔH₂ (indirect route)

So,

ΔH_F = ΔH₁ – ΔH₂

Bond energies

Enthalpy change caused by breaking and forming bond. Breaking bond needs energy while forming bond releases the energy.

In chemical reaction,

If the energy needed to break bonds less than the energy released to form new bonds, the reaction will release energy and is exothermic.
If the energy needed to break bonds more than the energy released to form new bonds, the reaction will absorb energy and is endothermic.

To calculate the enthalpy changes using average bond energies, we can use this formula,

ΔH_r = ΔH_{bonds broken} – ΔH_{bonds formed}

Well, it’s the end for our discussion about Enthalpy Changes today. We’ve learned about exothermic and endothermic reaction, type of enthalpy changes, and how to calculate enthalpy change. Now, it’s time to discuss some exercises about this topic.

Read previous article

PROBLEM SOLVING

Draw full reaction pathway diagrams including activation energy for: (a) The combustion of sulfur to form sulfur dioxide (exothermic); (b) The endothermic reaction of H₂O (g) + C (s) à H₂(g) + CO (g)

Answer:

(a)

(b)

2. (a) Draw the enthalpy cycle for the reaction: 2Al (s) + Fe₂O₃ (s) à 2Fe (s) + Al₂O₃ (s); (b)Draw an enthalpy cycle of the formation of ethanol, C₂H₅OH using enthalpy change of combustion.

Answer:

(a)