Hess's Law

The last concept on this topic is Hess's Law. It takes some practice to master this skill, so be patient!!

2016 NCEA

2018 NCEA

Enthalpy of Formation

We can calculate the enthalpy change that occurs when one mole of a substance is made from its elements under standard conditions. This can then be used to calculate the theoretical enthalpy change of a reaction involving that substance.

You need to be able to:

• define Standard Enthalpy of Formation
• write a balanced equation for the Standard Enthalpy of Formation for a given molecule/compound.

Once you master that, you can use Standard Enthalpy of Formation values to calculate Standard Enthalpy values for any given reaction:

Enthalpy of Combustion

A very useful standard enthalpy (measured under "standard conditions") is Standard Enthlpy of Combustion. We will use it to apply something called Hess's Law later in the topic. You need to be able to:

• define Standard Enthalpy of Combustion
• write a balanced equation for the Standard Enthalpy of Combustion for a substance

Calorimetry

We can use temperature change to calculate the enthalpy change of a reaction:

These videos (and photos) show the steps you should follow to solve a problem like this one, and find the enthalpy change:

When we calculate enthalpy change experimentally, there are many sources of error that we need to be aware of. Heat energy is always gained/lost to the surrounding environment. If enough heat is gained, some of the energy may be absorbed as latent heat (no temperature change would be seen, but evaporation, for example, could occur).

Very special calorimeters (such as bomb calorimeters) are need to get very accurate temperature change data to calculate enthalpy change. They keep the system closed so there is negligible heat loss/gain to the surroundings.

SOURCE: https://gulpmatrix.com/application-uses-bomb-calorimeter/

Gibbs Free Energy

At any given temperature, enthalpy and entropy are important in determining if a reaction is spontaneous (actually happens). at equilibrium (the forward and reverse reactions occur at the same rate), or non-spontaneous (does not happen...actually the reverse reaction occurs instead at that temperature).

This is because of a quantity called Gibbs Free Energy (ΔG). ΔG must be a negative value for a reaction to be spontaneous.

Temperature must be reported in Kelvin for this to work. Therefore, T cannot be a negative value, as 0 Kelvin is absolute zero (-273°C). That makes making predictions a little easier...

We need to be able to categorise reactions as being one of these:

1. Enthalpy driven, or
2. Entropy driven

We use the equation for Gibbs Free Energy to make this prediction:

Entropy

Entropy is a key part of the "2nd Law of Thermodynamics". The universe tends towards greater disorder. Entropy is a relative measure of this disorder.

KHAN ACADEMY VIDEOS

Optional viewing if you struggled with my explanations:

Change of State

When we consistently (and persistently) heat a substance, we observe two things:

• an increase in temperature
• a change in state (melting or evaporating)

When we investigate this in more detail, we notice something peculiar: at the melting/boiling point there is no real change in temperature (until after all of the substance has melted/evaporated).

This is because at melting/boiling point, the heat energy is not being transformed into kinetic energy (observed as a temperature increase). Instead, it is being used to break the bonds between the particles. The amount of energy required to do this is called Latent Heat. It is an enthalpy change.

LATENT HEAT OF FUSION

LATENT HEAT OF VAPORISATION