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Monitoring Rates

01Monitoring Rates

Measuring reaction rate

This section covers the meaning and units of reaction rate and methods used to calculate or compare reaction rates.

Calculating a rate

The rate of reaction describes how rapidly the concentration of a substance changes as a reaction proceeds.

rate of reaction=magnitude of concentration changetime taken\text{rate of reaction}=\frac{\text{magnitude of concentration change}}{\text{time taken}}

Product formation can be followed using its increase in concentration. If a reactant is monitored instead, use the size of its decrease in concentration.

When concentration is measured in mol dm−3\mathrm{mol\,dm^{-3}} and time in seconds, the rate has units of mol dm−3 s−1\mathrm{mol\,dm^{-3}\,s^{-1}}. If time is measured in minutes, the units are mol dm−3 min−1\mathrm{mol\,dm^{-3}\,min^{-1}}.

From mass formed to reaction rate

Worked example: A reaction forms 0.0440 g0.0440\,\mathrm{g} of ethyl ethanoate, CHX3COOCX2HX5\ce{CH3COOC2H5}, in 1.00 min1.00\,\mathrm{min}. The mixture has a total volume of 400 cm3400\,\mathrm{cm^3} and Mr(CHX3COOCX2HX5)=88.0M_r(\ce{CH3COOC2H5})=88.0. Calculate the rate in mol dm−3 s−1\mathrm{mol\,dm^{-3}\,s^{-1}}.

1. Find the amount of ethyl ethanoate.

n=mMr=0.044088.0=5.00×10−4 moln=\frac{m}{M_r}=\frac{0.0440}{88.0}=5.00\times10^{-4}\ \mathrm{mol}

2. Convert the volume and find the concentration formed.

400 cm3=0.400 dm3400\,\mathrm{cm^3}=0.400\,\mathrm{dm^3}

Δc=5.00×10−40.400=1.25×10−3 mol dm−3\Delta c=\frac{5.00\times10^{-4}}{0.400} =1.25\times10^{-3}\ \mathrm{mol\,dm^{-3}}

3. Convert the time to seconds and calculate the rate.

1.00 min=60 s1.00\,\mathrm{min}=60\,\mathrm{s}

rate=1.25×10−360=2.08×10−5 mol dm−3 s−1\text{rate} =\frac{1.25\times10^{-3}}{60} =2.08\times10^{-5}\ \mathrm{mol\,dm^{-3}\,s^{-1}}

Comparing rates using a fixed endpoint

Sodium thiosulfate reacts with hydrochloric acid to produce solid sulfur, which gradually makes the reaction mixture cloudy.

NaX2SX2OX3+2 HCl→2 NaCl+SOX2+S+HX2O\ce{Na2S2O3 + 2HCl -> 2NaCl + SO2 + S + H2O}

Place the reaction vessel over a marked cross and look through the mixture towards the mark. Record the time until the sulfur obscures the cross.

If the same visual endpoint is used each time, the timing ends after approximately the same amount of sulfur has formed. The experiments can therefore be compared using:

relative rate≈1t\text{relative rate}\approx\frac{1}{t}

This is an approximate measure of reaction rate because concentration does not appear in the expression. A shorter measured time gives a larger value of 1/t1/t.

cloudy mixturecrosslook down

Exam Tip: Before calculating a rate, convert concentration and time into the units requested. If using the disappearing-cross method, remember that 1/t1/t is only an approximate relative rate.

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