Energy Changes

ENTHALPY CHANGES

Some reactions feel hot; some feel cold. This means there are energy differences between the reactants and the products. We call this an enthalpy change.

If something feels hot, it is releasing energy. We call this exothermic.

If something feels cold, it is absorbing energy from our skin. We call this endothermic.

MAKING AND BREAKING BONDS

It takes energy to break chemical bonds. Therefore, this process is endothermic.

Conversely, when bonds form, energy is released. Bond formation is exothermic.

We can apply this to our understanding of melting (and boiling) points. We can also use this to understand what is happening at a particle level in an endothermic or exothermic chemical reaction.

BOND ENTHALPY

Every chemical bond requires a specific amount of energy to be broken. The amount of energy required to break one mole of a particular chemical bond is called its Bond Enthalpy.

THERMOCHEMICAL CALCULATIONS

We need to remember and apply some key calculations in our thermochemistry: