Enthalpy Changes: Standard Conditions
Chemistry: AQA A Level, topic 1.4.3
Standard Conditions
For many measurements, we can record them under standard conditions. This is true for enthalpy changes - we call it the standard enthalpy change.
Symbols
- Enthalpy is given the symbol H.
- Enthalpy changes are given the symbol ΔH.
- A negative enthalpy change (pictured) is one which gives out heat.
- A positive enthalpy change is one which takes in heat.
Standard conditions
- You'll hear the term 'standard conditions' a lot in chemistry. These conditions refer to:
- A pressure of 1 bar or 100 kPa.
- A temperature of 298K.
- A substance's most stable state at 298K and 1 bar pressure.
- E.g. For water, it is liquid.
- E.g. For carbon, it is graphite.
Standard enthalpy changes
- A standard enthalpy change is an enthalpy change carried out under standard conditions.
- There are two you need to know;
- Standard enthalpy of combustion (ΔcHθ).
- Standard enthalpy of formation (ΔfHθ).
- The superscript θ signifies that it is a standard enthalpy change.
ΔcHθ
- The standard enthalpy change of combustion is defined as:
- The enthalpy change when one mole of a substance in its standard state burns completely in oxygen under standard conditions of 298K and 1 bar pressure.
ΔfHθ
- The standard enthalpy change of formation is defined as:
- The enthalpy change when one mole of a substance is formed in its standard state from the pure elements in their standard states under standard conditions of 298K and 1 bar pressure.
Test yourself on Enthalpy Changes: Standard Conditions
What phase would water be in under standard conditions?
Which of the following represents enthalpy change?