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The reaction rate is measured by determining how quickly a reactant is consumed or a product is formed during a chemical reaction. In other words, the change in the amount of a given reactant or product is measured experimentally over time. Different methods can be used to measure the amount of a substance. The optimal method is often chosen based on the state of the substance.
The following table presents different ways of calculating the reaction rate based on the state of the reactant or product and the variables measured to assess the change in its amount:
|
State of the reactant or product |
Measured variable |
Examples of measuring equipment |
Reaction rate formula |(r)| |
Examples of reaction rate units |
|---|---|---|---|---|
|
Solid |
Mass |(m)| |
Balance |
|r = \dfrac{\vert\Delta m\vert}{\Delta t}| |
Grams per second |(\text{g/s})| |
|
Liquid |
Mass |(m)| |
Balance |
|r = \dfrac{\vert\Delta m\vert}{\Delta t}| |
Grams per second |(\text{g/s})| |
|
Volume |(V)| |
Graduated cylinder |
|r = \dfrac{\vert\Delta V\vert}{\Delta t}| |
Millilitres per second |(\text{mL/s})| |
|
|
Gas |
Mass |(m)| |
Balance |
|r = \dfrac{\vert\Delta m\vert}{\Delta t}| |
Grams per second |(\text{g/s})| |
|
Volume |(V)| |
Gas burette |
|r = \dfrac{\vert\Delta V\vert}{\Delta t}| |
Millilitres per second |(\text{mL/s})| |
|
|
Pressure |(P)| |
Manometer |
|r = \dfrac{\vert\Delta P\vert}{\Delta t}| |
Kilopascals per second |(\text{kPa/s})| |
|
|
Molar concentration |(\text{[Substance]})| |
Spectrophotometer |
|r = \dfrac{\vert\Delta [\text{Substance}]\vert}{\Delta t}| |
Moles per litre per second |(\text{mol/L}\cdot\text{s})| |
|
|
Solution |
Molar concentration |
Spectrophotometer or pH-meter |
|r = \dfrac{\vert\Delta [\text{Substance}]\vert}{\Delta t}| |
Moles per litre per second |(\text{mol/L}\cdot\text{s})| |
Note: While all units presented in this table use seconds |(\text{s})| as the unit of time change |(\Delta t)|, reaction rate can be measured over minutes |(\text{min}),| hours |(\text{h}),| days |(\text{d})| or even years |(\text{yr}).|
The choice of the measured variable also depends on:
Consider the following chemical reaction, where solid magnesium |(\text{Mg})| and aqueous sulphuric acid |(\text{H}_2\text{SO}_4)| react to form aqueous magnesium sulphate |(\text{MgSO}_4)| and hydrogen gas |(\text{H}_2)\!:|
|\text{Mg}_{\text{(s)}} + \text{H}_2\text{SO}_{4(aq)} → \text{Mg}\text{SO}_{4(aq)} + \text{H}_{2(g)}.|
The following table analyzes some of the methods of measuring the reaction rate in this experiment:
It is often useful to determine the reaction rate in |\text{mol/s}| or |\text{mol/L}\cdot\text{s}.| Since concentration can only be measured for gaseous and aqueous solutions, other measurements can be converted to the desired values using the molar mass formula, Avogadro’s law and molar volume or the ideal gas law.
Consider the following chemical reaction, where |3.0\ \text{g}| of magnesium |\text{Mg}| reacts with |6.0\ \text{mol/L}| sulphuric acid |(\text{H}_2\text{SO}_4)| in excess: |\text{Mg}_{\text{(s)}} + \text{H}_2\text{SO}_{4(aq)} → \text{Mg}\text{SO}_{4(aq)} + \text{H}_{2(g)}.|
After |2.5\ \text{min},| the reaction is complete and all of the |\text{Mg}| is consumed. Determine the average reaction rate with respect to |\text{Mg}| in |\text{g/s}| and |\text{mol/s}.|
During a chemical reaction conducted at STP, |50.0\ \text{mL}| of gas forms over the course of |80.0\ \text{s}.| Determine the average reaction rate with respect to this gas in |\text{mol/s}.|
During a chemical reaction conducted at |15.00^\circ\text{C}| and |102\ \text{kPa},| |105.0\ \text{mL}| of |\text{H}_2,| gas forms over the course of |6\ \text{min}.| Determine the average reaction rate with respect to this gas in |\text{mol/L}\cdot\text{s}.|