Monitoring the progress of a reaction
A reaction in which at least one component is a gas might result in an overall change in pressure in a system of constant volume, so its progress may be followed by recording the variation of pressure with time.
Example 22.1 Monitoring the variation in pressure
Predict how the total pressure varies during the gas-phase decomposition 2 N2O5(g) →4 NO2(g)+O2(g) in a constant-volume container.
Method The total pressure (at constant volume and temperature and assuming perfect gas behaviour) is proportional to the number of gas-phase molecules. Therefore, because each mole of N2O5 gives rise to 5–2 mol of gas molecules, we can expect the pressure to rise to 5/2 times its initial value. To confirm this conclusion, express the progress of the reaction in terms of the fraction, α, of N2O5 molecules that have reacted. Answer Let the initial pressure be p0 and the initial amount of N2O5 molecules present be n. When a fraction α of the N2O5 molecules has decomposed, the amounts of the components in the reaction mixture are:
N2O5 NO2 O2 Total
Amount: n (1 −α) 2 αn 
αn n(1+ 
α)
Whenα=0 the pressure is p0, so at any stage the total pressure is
p =(1+ α)p0
When the reaction is complete, the pressure will have risen to
times its initial value.
Spectrophotometry, the measurement of absorption of radiation in a particular spectral region, is widely applicable, and is especially useful when one substance in the reaction mixture has a strong characteristic absorption in a conveniently accessible region of the electromagnetic spectrum. For example, the progress of the reaction
H2(g) + Br2(g) →2 HBr(g)
can be followed by measuring the absorption of visible light by bromine. A reaction that changes the number or type of ions present in a solution may be followed by monitoring the electrical conductivity of the solution. The replacement of neutral molecules by ionic products can result in dramatic changes in the conductivity, as in the reaction
(CH3)3CCl(aq) + H2O(l) →(CH3)3COH (aq)+H+(aq)+Cl−(aq)
If hydrogen ions are produced or consumed, the reaction may be followed by monitoring the pH of the solution. Other methods of determining composition include emission spectroscopy, mass spectrometry, gas chromatography, nuclear magnetic resonance, and electron para magnetic resonance (for reactions involving radicals or paramagnetic d-metal ions).
