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 25oC)

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

Google Presentation

Enthalpy change of formation

The enthalpy change of formation is the enthalpy change when 1 mole of a substance is formed from its elements in their normal state under standard conditions (298K, 100kPa)

This definition could get you 2 or 3 marks in an exam.

So learn it.

As you can see from the table most enthalpies of formation are negative - most substances form exothermically.

But not all.

We must form 1 mole of product so the equation must be balanced to leave 1 mole in front of that substance even if it makes for awkward balancing.

So the enthalpy of formation of methanol is:

C(s) + 1/2 O2(g) + 2 H2(g) → CH3OH(l)

Note that state-symbols will be needed for all reactants and the product to show that they are in their standard form.

Enthalpy change of combustion

The enthalpy change of combustion is the enthalpy change when 1 mole of a substance is completely burned in an excess of Oxygen under standard conditions (298K, 100kPa)

This definition could also get you 2 or 3 marks in an exam.

So learn it too - it's not much different to the one above.

As you can see from the table enthalpies of combustion are negative - substances burn exothermically.

The reactant that burns is the important substance that must be proceeded by an (unwritten) 1 in the equation, no water how awkward the coefficients (numbers) it generates in front of the products or the Oxygen.

eg. C3H6(g) + 4.5 O2(g) → 3 CO2(g) + 3H2O(l) - only the unwritten 1 in from of Propene is important even if it generates a 4.5

Enthalpy change of neutralisation

The enthalpy change of combustion is the enthalpy change when 1 mole of water is formed in a reaction between an acid and a base under standard conditions (298K, 100kPa)

This definition could etc etc.

You might wonder why it's not one mole of acid or one mole of base but remember that not all acids and bases react 1:1

But all acid-base neutralisations make water- so the molar quantity is for the water's formation.

As you can see from the table enthalpies of neutralisation are negative - neutralisation reactions get warm.

1/3 H3PO4(aq) + 1/2 Ca(OH)2(aq) → 1/6 Ca3(PO4)2(aq) + H2O(l) - this time only the 1 in front of the H2O matters no matter what it does to the other coefficients

Enthalpy change of reaction

This definition won't be asked for.

It's a bit over-compliacated and arguably somewhat unsatisfactory

It is the enthalpy change when substances react in the number of moles written in the equation under standard conditions (298K, 100kPa)

The problem is that you can balance equations differently:

H2SO4 + 2 NaOH → Na2SO4 + 2 H2O is no more valid than 1/2 H2SO4 + NaOH → 1/2 Na2SO4 + H2O which is no better or worse than 2 H2SO4 + 4 NaOH → 2 Na2SO4 + 4 H2O.

And each of them has a different enthalpy of reaction - consequently, enthalpies of reaction are frequently calculated using the kind of data above which is strictly definied.

You may be given the enthalpy of a reaction and asked to use it to work out something else.

But you can't usually look up the enthalpy of a reaction.

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