Enthalpy changes: key terms
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Standard conditions
A pressure of 100 kPa and a stated temperature, usually 298 K; solutions at 1 mol dm⁻³.
Examiners say Standard states are the physical states of substances under standard conditions.
Enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a stated equation.
Examiners say It always refers to an equation: quote the equation or the value has no meaning.
Enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements, with all reactants and products in their standard states under standard conditions.
Examiners say One mole of the compound; from its elements. The value for an element in its standard state is zero.
Enthalpy change of combustion
The enthalpy change when one mole of a substance reacts completely with oxygen, with all reactants and products in their standard states under standard conditions.
Examiners say "Completely" or "in excess oxygen" is needed. Experimental values are less exothermic than data-book values because of heat loss and incomplete combustion.
Enthalpy change of neutralisation
The enthalpy change when an acid and a base react to form one mole of water, under standard conditions.
Examiners say Per mole of water formed, not per mole of acid: watch H₂SO₄.
Activation energy
The minimum energy required for a reaction to take place.
Examiners say On an enthalpy profile it is the gap from the reactants to the top of the curve.
Average bond enthalpy
The energy required to break one mole of a given type of bond in gaseous molecules, averaged over many different compounds.
Examiners say Bond breaking is endothermic. Calculations from average values differ from the real value because the actual bond enthalpy depends on the molecule.
Hess’ law
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
Examiners say With formation data: products − reactants. With combustion data: reactants − products.
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