Thermochemical Equations and Calorimetry

What Happens During a Chemical Reaction?

In a typical chemical reaction, there are usually 2 steps:

Bonds are broken

• Bonds in a molecule are broken.

• To do this, lots of energy is needed, so it is endothermic.

Bonds are reformed

• Products are made, which means energy is released when bonds are formed.

• This becomes exothermic.

The amount of energy released/absorbed is dependent on both the size of molecules and type of bond.

Double bonds require more energy to break than a single bond.

∆H is used to determine the enthalpy change in a chemical reaction

When;
• ∆H is positive (+), then it is endothermic
• ∆H is negative (-), then it is exothermic

Molar Enthalpy of Combustion

Combustion is an important reaction in thermochemistry

• They release a large amount of energy (in the form of heat)

• As a result, they are given their unit of measurement; ∆Hcomb

The enthalpy values are given standard notation

• It’s important to note that they are all at standard temperature (25 degree Celsius) and pressure (100 kPa)

Reactant Amounts and Enthalpy of Reaction

• The enthalpy change associated with a reaction is dependent on the amounts of the reactants involved
  

• If the reactants double, then so does the enthalpy of a reaction

Reactant Amounts and Enthalpy of Reaction (Formula)

The enthalpy change of a reaction is equal to the amount in moles (n) of a specified product or reactant multiplied by the standard molar enthalpy change