AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Standard enthalpy changes refer to standard conditions, Ɵ ie 100 kPa and a…
Full specification wording
Enthalpy change: Standard enthalpy changes refer to standard conditions, Ɵ ie 100 kPa and a stated temperature (eg ∆H ).
A standard enthalpy change is quoted under agreed conditions so values from different sources can be added and compared.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
Study notes
From the checked teaching notes for this specification point.
The idea
A standard enthalpy change is quoted under agreed conditions so values from different sources can be added and compared. AQA fixes the pressure at 100 kPa and requires the temperature to be stated, normally 298 K, and every substance must be in its standard state, the physical state it adopts under those conditions: water liquid, bromine liquid, iodine solid, oxygen gaseous, carbon as graphite. Solutions are taken at 1.00 mol dm⁻³. The symbol is written with a superscript plimsoll, as in ∆H⦵, and subscripts name the type: ∆fH⦵ for formation of one mole of a compound from its elements in their standard states, ∆cH⦵ for the complete combustion of one mole in excess oxygen. Because states are fixed, the enthalpy of formation of an element in its standard state is zero by definition.
What earns the marks
- Quoting a pressure of 100 kPa and a stated temperature, usually 298 K.
- Adding that all substances are in their standard states at those conditions.
- Defining a standard enthalpy of formation as the formation of one mole of a compound from its elements in their standard states.
- State symbols written correctly in the equation that accompanies a standard definition.
Common mistakes and how to put them right
Quoting 1 atm or 101 kPa instead of the 100 kPa the specification uses.
Calling 298 K a fixed part of the definition when the specification only requires the temperature to be stated.
Writing H₂O(g) in an equation for a standard enthalpy change, when the standard state of water at 298 K is liquid.
Writing an equation for enthalpy of formation that makes two moles of the compound, so the value no longer matches the definition.
Giving carbon as diamond rather than graphite in a formation equation.
Exam tips
- When a definition question follows, write the equation with state symbols underneath: it often carries a mark of its own.
- Fractions are allowed in formation equations so that exactly one mole of product is made, for example H₂(g) + ½O₂(g) → H₂O(l).
- Say formed from its elements in their standard states, not made from its elements, which examiners treat as incomplete.
Try the lesson
The start of the Enthalpy change: Standard enthalpy changes refer to standard conditions, Ɵ ie 100 kPa and a stated temperature (eg ∆H ). lesson, made from checked teaching notes for this specification point.
Have a go first
What pressure is used in the standard conditions for standard enthalpy changes?
Listen
A standard enthalpy change is quoted under agreed conditions so values from different sources can be added and compared. AQA fixes the pressure at 100 kPa and requires the temperature to be stated, normally 298 K, and every substance must be in its standard state, the physical state it adopts under those conditions: water liquid, bromine liquid, iodine solid, oxygen gaseous, carbon as graphite. Solutions are taken at 1.00 mol dm⁻³. The symbol is written with a superscript plimsoll, as in ∆H⦵, and subscripts name the type: ∆fH⦵ for formation of one mole of a compound from its elements in their standard states, ∆cH⦵ for the complete combustion of one mole in excess oxygen. Because states are fixed, the enthalpy of formation of an element in its standard state is zero by definition. A strong answer earns marks for these points. quoting a pressure of 100 kPa and a stated temperature, usually 298 K. adding that all substances are in their standard states at those conditions. defining a standard enthalpy of formation as the formation of one mole of a compound from its elements in their standard states. state symbols written correctly in the equation that accompanies a standard definition. Exam tip: When a definition question follows, write the equation with state symbols underneath: it often carries a mark of its own.
See it
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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