Enthalpy change and Hess's law.

What is Hess's law?

The Enthalpy Change for a chemical reaction is the same no matter which route is chosen to get from reactants to products.

How can you reduce the uncertainty in the mass measurement?

Use a balance with a greater resolution
Use a larger mass

Why can the enthalpy change of a thermal decomposition reaction not be measured directly?

As thermal energy must be supplied for the reaction to occur, the temperature change measured is not only due to decomposition.

How do you calculate enthalpy change experimentally?

q=m x c x (temperature change).
Where m is the mass of the solution that changes temperature (1g=1cm3), c is the specific heat capacity (usually of water) and the temperature change, measured using a thermometer and q is the heat energy taken in or released (in joules).
Divide this number, in kJ, by the number of moles of the limiting reactant.
Add a sign to show whether the enthalpy change is exothermic or endothermic.

Why may an experimental value for enthalpy change be different to the theoretical value?

Heat loss to apparatus/surroundings.
Incomplete combustion.
Non-standard conditions.
Evaporation of alcohol/water.

How do you prevent heat loss to surroundings/apparatus?

Insulate by placing the reactants in a polystyrene cup with a lid, into the beaker.
Avoid large temperature differences between surroundings and calorimeter.
Use a bomb calorimeter.

Other than preventing heat loss, how can the accuracy of this experiment be improved?

Read the thermometer at eye level to avoid parallax errors.
Stir the solution so the thermometer is evenly distributed.
Use a digital thermometer and data logger for more accurate and faster readings.
Use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty.

What is accuracy?

The more accurate the data, the closer it is to the actual value.