Chemical Energetics (1) Endo and Exothermic Reactions

Chemical Energetics (1): Exothermic and Endothermic Reactions

I love reactions that give out heat energy or take in heat energy.

Prometheus gives humanity fire by Jose A Fadul.

It was the ancient Greeks who believed that their god Prometheus gave the world fire!

This abstract watercolor painting depicts Prometheus as the giver of fire for the use of humankind.

In Greek mythology, Zeus once assigned Prometheus the task of forming humans from water and earth, which he did, but in the process, became fonder of humans than Zeus had anticipated.

Zeus did not share Prometheus' feelings and wanted to prevent humans from having power, especially over fire.

Prometheus cared more for humans than for the wrath of the increasingly powerful and autocratic king of the gods, so he stole fire from Zeus' lightning, concealed it in a hollow stalk of fennel, and brought it to humanity.

And wasn’t it Zeus who had Prometheus chained up because of what he had set loose on the world?

But think of the benefits of fire: the warmth and light and wonder!!!

Well back to the less sentimental let’s first define exo and endo thermic reactions.

An exothermic reaction is one in which energy is transferred into the surroundings.

An endothermic reaction is one where energy is taken in from the surroundings

But what’s all that stuff mean: what are the surroundings?

As you can see the surroundings are anything but the reaction itself.

So in this example of a reaction in a beaker or polystyrene cup the water, the cup, the thermometer are all part of the surroundings as is the rest of the Universe!!

Sometimes you see the kind of picture that can leave you with the misleading impression that the surroundings are everything outside the container (flask, beaker etc.) and the system is what’s inside the container. 

Not so!!:

The system in this kind of set up is the reaction chemicals in the container.

The aqueous solution in which the reaction takes place is part of the surroundings absorbing or releasing heat. 

This point will become very important when we describe how to calculate the change of heat or enthalpy for a particular reaction in aqueous solution. 

We can recognise an endo or exo thermic reaction by the temperature change taking place in the surroundings.

In an exothermic reaction the temperature of the surroundings increases.

In an endothermic reaction the temperature of the surroundings decreases. 

Before we go any further we need to understand what the term Enthalpy Change means.

The symbol for Enthalpy Change is ΔH pronounced “delta H”.

Greek letter capital delta is the symbol for “change of” and H stands for “heat” or “enthalpy”.

Enthalpy Change refers to the heat change in a chemical reaction taking place under constant pressure. 

The enthalpy change may be negative

But these changes in temperature must not be confused with the change in heat content in these two reactions.

Let’s look at how the change in enthalpy is represented using a simple enthalpy level diagram.

Here they are:

You will see that in an exothermic reaction the temperature of the surroundings increases but the change in enthalpy is negative: - ΔH

And in an endothermic reaction though the temperature of the surroundings falls the change in enthalpy is positive: + ΔH

We can see how this works out using cold packs here .

Let’s finish this first blog on chemical energetics with some definitions of different enthalpy changes. 

The standard enthalpy of reaction (denoted ΔH^o_reaction) is the enthalpy change for the reaction in the equation that occurs under standard conditions i.e. at a pressure of 1 atmosphere, at a temperature of 298K, with the substances in their physical states normal to these standard conditions.  Solutions must have a concentration of 1mol dm^-3

The standard enthalpy change of formation (denoted  ΔH^o_f ) of a compound is the enthalpy change that takes place when one mole of that compound is formed from its elements in their standard states under standard conditions

The standard enthalpy change of combustion (denoted  ΔH^o_c ) is the enthalpy change that occurs when one mole of compound is completely burned in oxygen.

The standard enthalpy change of neutralisation (denoted  ΔH^o_neut ) is the enthalpy change that occurs when solutions of acid and alkali react together under standard conditions to produce one mole of water.

Standard enthalpy of atomization (denoted  ΔH^o_at ) is the enthalpy change when 1 mole of gaseous atoms is formed from its element in its defined physical state under standard conditions (298K, 1 atm).