3B Enthalpy Changes Summary

What is meant by enthalpy

A measure of the heat energy in a chemical system

What does it mean if an enthalpy change of a reaction is exothermic

Energy is transferred from the system to the surroundings

What does it mean if an enthalpy change is described as endothermic

Energy is transferred from the surroundings to the system

What’s the sign for the enthalpy change of an exothermic and endothermic reaction

• exo = -

• endo = +

Draw an enthalpy diagram for an exothermic and exothermic reaction WITH labels

What’s the temperature change like for an exothermic and endothermic reaction

• exothermic = temperature increases

• endothermic = temperature decreases

What is meant by the term activation energy

The minimum energy required to start a reaction by breaking the bonds in the reactant molecules

Draw a labelled enthalpy profile diagram for an exothermic reaction, with enthalpy change and activation energy

Draw a labelled enthalpy profile diagram for an endothermic reaction, with enthalpy change and activation energy

Whats the equation for energy change (including units)

q = mc🔺T

• q = energy, J

• m = mass of solution, g

• c = specific heat capacity of solution, Jg-1K-1

• 🔺T = temperature change, K (same a Celsius)

Why is water most frequently used in measuring enthalpy changes

• readily available

• High specific heat capacity

• Non-hazardous

Whats the equation for enthalpy change (including units)

🔺H = q/mol

• 🔺H = enthalpy change, kJ mol-1

• q = energy change, kJ

• mol = moles

Outline a method for an experiment to determine the enthalpy change of a solution of ammonium bromide ( including equipment + measurements)

1. Measure a known volume of water into a measuring cylinder

2. Pour the water into a polystyrene cup and add a thermometer

3. Measure the initial temperature

4. Weigh a weighing bottle containing solid ammonium bromide

5. Pour the solid into the water and stir with the thermometer

6. Measure the highest / lowest temperature reached

7. Reweigh the empty weighing bottle

State the standard conditions used for measuring enthalpy changes

• temp = 298K / 25C

• pressure = 100kPa

• conc = 1 Moldm-3

Whats the definition for the standard enthalpy change of combustion

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states

Whats the definition for standard enthalpy change of formation

The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Whats the definition for the standard enthalpy change of neutralisation

The enthalpy change that accompanies the reaction of an acid and a base to form 1 mole of water under standard conditions, with all reactants and products in their standard states

State Hess’s law

If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Draw a general Hess cycle for a combustion reaction

Draw a general Hess cycle for a formation reaction

State what is meant by average bond enthalpy

The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given type of bond in the molecules of a gaseous species

Suggest possible procedural errors that may have caused an experiments results to be higher than the standard values

• heat loss to surroundings

• Incomplete combustion

• Evaporation of water / solution

• Non-standard conditions

• Specific heat capacity of the apparatus is ignored