3.2.1 (d) Enthalpy change definitions

Syllabus

(d) explanation and use of the terms:

(i) standard conditions and standard states (physical states under standard conditions)

(ii) enthalpy change of reaction (enthalpy change associated with a stated equation, ΔrH)

(iii) enthalpy change of formation (formation of 1 mol of a compound from its elements, ΔfH)

(iv) enthalpy change of combustion (complete combustion of 1 mol of a substance, ΔcH)

(v) enthalpy change of neutralisation (formation of 1 mol of water from neutralisation, ΔneutH)

{Definitions required for enthalpy changes of formation, combustion and neutralisation only.}

{Standard conditions can be considered as 100 kPa and a stated temperature, 298 K.}

What does this mean?

Standard Conditions

In order for any measured quantity to be useful you would have to know the conditions in which it was measured.

You can measure the temperature of river-water in the Thames and The Amazon but the comparison is largely pointless because the temperature of the air is very different in London and Brasilia, this will affect the river water.

So all enthalpies are measured with the reactants in their standard (normal) states under defined standard conditions that examiners delight in asking you.

And this is denoted by the standard symbol shown in the diagram.

Standard Temperature is 298 K (or 25^oC)

Standard Pressure for OCR 100 kPa (kiloPascals) - this isn't precisely 1 atmosphere but previous OCR A levels have always accepted this as an answer. I'd stick with 100kPa, just in case they've tightened up. You'll see 101.3 kPa in some textbooks and they certainly wouldn't mark that wrong.

What are standard conditions? PowerPoint