Module 3, Chapter 2: Physical Chemistry (ENTHALPY)

Enthalpy Change
is the heat energy transferred in a reaction at constant pressure
Units of enthalpy change
kJ mol-1
What does ∆Hø show/
that the measurements were made under standard conditions and the elements were in their standard states (i.e. their physical conditions)
Standard pressure
100kPa (1atm)
Standard Temperature
- releases energy

- ∆H is negative

- temperature increases

- oxidation
Examples of Exothermic Reactions
- combustion of fuel

- oxidation of carbohydrates (respiration)
Endothermic Reactions
- absorb energy

- ∆H is positive

- temperature falls
Examples of Endothermic Reactions
- Thermal Decomposition
e.g. CaCO3 → CaO + CO2

- photosynthesis
What do Enthalpy Profile Diagrams show?
show energy change in reactions
Activation Energy
the minimum amount of energy needed to begin breaking reactant bonds and start a chemical energy
What is less / more stable in Endothermic Reactions?
- reactants = more stable

- products = less stable
What is less / more stable in Exothermic Reactions?
- reactants = less stable

- products = more stable
Relationship between Enthalpy and Stability
less enthalpy a substance has, the more stable it is
What are enthalpy changes affected by?
- Temperature

- Pressure
Standard Enthalpy Change of a Reaction
- ∆rHø

- the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
Standard Enthalpy Change of Formation
- ∆fHø

- the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

- 2C (s) + 3H2 (g) + ½ O2 (g) → C2H5OH (l)
Standard Enthalpy Change of Combustion
- ∆cHø

- the enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
Standard Enthalpy Change of Neutralisation
- ∆neutHø

- the enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water
What is bond breaking?
- endothermic

- energy is NEEDED to BREAK bonds
What is bond making?
- exothermic

- energy is RELEASED when bonds are FORMED
When is the ∆H positive?
If MORE energy is needed to break bonds than is release when bonds are made, the ∆H is positive
When is the ∆H negative?
If LESS energy is needed to break bonds than is release when bonds are made
Bond Dissociation Enthalpy
The enthalpy change required to break a covalent bond with all species in the gaseous state
What do we need to measure the enthalpy change for a reaction?
- the number of moles of the stuff that's reacting

- the change in temperature
How to Find out Enthalpy Change of Combustion of a Flammable Liquid?
1. Burn it

2. As fuel burns, it heats the water

3. You can work out the heat absorbed by water, if you know
the mass of water

the temperature changes of the water

the specific heat capacity of the water
Specific Heat Capacity of Water
4.18J g -1 k -1
Hess' Law
the total enthalpy change is independent of the route taken
What is the ∆fHø for all elements?
Overall Enthalpy Change of Reaction
the difference between energy absorbed and released
Enthalpy Change of a Reaction =
total energy absorbed - total energy released