Conductivity and Reaction Rates

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Any strong acid will release approximately - 57 kJ/mol of energy on neutralisation with strong base.

This energy release is due to the formation of water molecules from the H+ ions from the acid reacting with the OH- ions from the base.

H+(aq) + OH-(aq) H2O(l)

When a weak acid is neutralised some of the energy is needed to dissociate the molecules to make the hydrogen ions available for neutralisation. The result is a value for neutralisation enthalpy lower than - 57 kJ/mol (numerically lower, not more negative!)

Note that as the weak acid is in equilibrium as the hydrogen ions are 'mopped up' by the base the equilibrium shifts to the right hand side to make more until eventually all of the acid is able to react. This does, however, absorb energy as bonds are being broken to dissociate the weak acid.

CH3COOH H+(aq) + CH3COO-(aq) approx 99% approx 1%

Conductivity

Any solution's ability to conduct electricity is conditioned by the concentration of ions it contains. A strong acid has more ions than a weak one, and so it's solution will be a better electrical conductor than a weak acid. The same goes for strong/weak bases.

The conductivity may be measured using a power pack and two graphite electrodes connected to an ammeter. The apparatus is assembled and current values measured for a given voltage setting. The strong acids pass more current than the weak acids for the same voltage.

Strong acids : HCl, HNO3, H2SO4. - good conductors - large value for current passing

Weak acids : CH3COOH, H2CO3. - poor conductors - low value for current passing

Strong bases : group 1 hydroxides (ie NaOH etc), or lower group 2 hydroxides Ba(OH)2. - good conductors

Weak bases : NH3, CH3CH2NH2. - poor conductors

Reaction rates

Similarly, the rate of reaction will reveal the strength of an acid. The rate of a chemical reaction is usually proportional to the concentration of the reactants.

As it is the hydrogen ions that

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