How Can We Measure the Rate of a Chemical Reaction in the Lab?
- When a chemical reaction occurs, reactants are transformed into products.
- The rate of reaction tells us how fast this change happens.
Mathematical Definition
In simple terms:
Rate of reaction = how quickly the amount or concentration of a reactant or product changes with time.
Mathematically, we can write: $$\text { Rate }=\frac{\Delta \text { quantity }}{\Delta t}$$
where:
- $\Delta \text { quantity }$ = change in amount, mass, volume, or concentration
- $\Delta t$ = change in time
Note
Common Units of Reaction Rate
Depending on what you measure, the units can be:
- mol dm⁻³ s⁻¹ → change in concentration per second
- g s⁻¹ → change in mass per second
- cm³ s⁻¹ → change in gas volume per second
For school-level kinetics, mol dm⁻³ s⁻¹ (concentration per time) is the most common.
Practical Methods to Measure Rate in the Lab
There are several ways to follow how fast a reaction occurs. The best method depends on what observable change happens.
Change in Mass (gas produced)
- If a reaction produces a gas that escapes, the mass of the reaction mixture decreases over time.
- You can place the reaction flask on a balance and record the mass every few seconds.
Example
Reaction of a metal carbonate with acid, producing CO₂.
Change in Volume (gas syringe)
- If a gas is produced, you can collect it in a gas syringe and measure the volume at regular time intervals.
- The steeper the volume–time curve, the faster the reaction.
- Measure the volume of hydrogen gas produced every 10–20 seconds.
Example
Magnesium + Hydrochloric Acid
$$\mathrm{Mg}(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_2(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})$$
Measure the volume of hydrogen gas produced every 10–20 seconds.
Change in Concentration (titration)
- Small samples can be withdrawn at set times and titrated to find the concentration of a reactant or product.
- This works well for slower reactions.
Change in Colour (colorimetry)
- If the reaction mixture changes colour, a colorimeter can measure how the intensity of colour changes with time.
- The absorbance (how much light is absorbed) relates to concentration.
Change in Conductivity
- If ions are formed or used up, the electrical conductivity of the solution will change.
- A conductivity meter can be used to track reaction progress.
Note
In all cases, the rate of reaction is calculated from how quickly the measured quantity changes with time.
How Do Graphs Help Us Compare Reaction Rates?
Graphs are powerful tools for visualising how quickly a reaction proceeds.
We can plot:
- Reactant concentration (or amount) vs time, or
- Product concentration (or amount) vs time.
Reactant vs Time Graphs
- Y-axis: Concentration (or amount) of reactant
- X-axis: Time
- The graph usually:
- Starts high (lots of reactant at the beginning),
- Then decreases over time as the reactant is used up,
- And finally levels off when the reaction is complete.
- The steeper the curve, the faster the reactant is being used up → higher rate.
Note
The slope (gradient) of the curve at any point gives the instantaneous rate of reaction at that moment.
Product vs Time Graphs
- Y-axis: Amount or concentration of product
- X-axis: Time
- The graph usually:
- Starts at zero,
- Then increases as the product forms,
- And finally levels off when no more product is being formed (reaction finished).
- Again, a steeper curve means a faster reaction.
Placeholder(image): Placeholder (graph): A graph showing reactant concentration decreasing and product concentration increasing over time. Both curves flatten when the reaction is complete.
Average Vs Instantaneous Rate
Given any of the concentration vs time graphs, we can deduce two separate aspects of the reaction rate: average and instantaneous.
Average Rate
- The average rate of reaction over a time interval is determined by calculating the slope of a secant line connecting two points on the curve.
- This represents the overall rate during a specific time period.Schematic drawing of calculating average rate of reaction.
Instantaneous Rate
- The instantaneous rate is the rate at a specific moment in time.
- To find this, a tangent line is drawn to the curve at the desired time, and its slope is calculated.
- The slope of the tangent is given by: $$\text{Slope} = \frac{\Delta[\text{Concentration}]}{\Delta t}$$
Example
Instantaneous Rate from a Graph
- Suppose the concentration of hydrogen gas ($\text{H}_2$) produced in a reaction is recorded over time, and the data is plotted as a curve.
- At $t = 20 \, \text{s}$, the tangent to the curve passes through the points $10, 0.05$ and $30, 0.20$ on the graph.
- The slope of the tangent is: $$\text{Slope} = \frac{0.20 - 0.05}{30 - 10} = \frac{0.15}{20} = 0.0075 \, \text{mol dm}^{-3} \, \text{s}^{-1}$$
- Thus, the instantaneous rate at $t = 20 \, \text{s}$ is $0.0075 \, \text{mol dm}^{-3} \, \text{s}^{-1}$.
Comparing Reaction Rates Using Graphs
- Graphs allow us to compare different reactions or the same reaction under different conditions (e.g. different temperature).
- If two reactions form the same product, the one whose product curve reaches its maximum more quickly is faster.
- If we compare slopes:
- A steeper slope = faster rate
- A gentler slope = slower rate
Note
Early in the reaction, the initial slope (initial rate) is especially useful for comparing reaction speeds.
Why Are Reaction Rates Important in Real Life?
- The study of reaction rates is called chemical kinetics.
- It is crucial in medicine, food science, and industrial chemistry.
Medicine – Getting the Dose and Timing Right
- In the body, medicines undergo processes that resemble “reactions”:
- They are absorbed, distributed, metabolised, and excreted.
- The rate of these processes affects:
- How quickly a drug starts working.
- How long its effect lasts.
- How often a dose must be taken.
- This is part of pharmacokinetics.
- Understanding these “rates” helps doctors decide:
- How much of a medicine to give (dose).
- How often it should be taken (frequency).
Food Science – Keeping Food Safe and Fresh
- Many processes in food are also chemical reactions:
- Microbial growth (bacteria, mould) causes spoilage.
- Oxidation reactions can make fats go rancid.
- Enzyme-catalysed reactions are used in bread making, cheese production, fermentation, and brewing.
- By controlling reaction rates, food scientists can:
- Slow down harmful reactions → extend shelf life (e.g. refrigeration slows microbial growth).
- Speed up useful reactions → make processes more efficient (e.g. using enzymes in fermentation).
Industrial Chemistry – Efficiency and Safety
- In industry, controlling reaction rates is essential for:
- Economy – producing as much useful product as possible in the shortest time, using reasonable energy.
- Safety – preventing dangerous runaway reactions.
- Reaction rate control also appears in:
- Polymer production (plastics).
- Manufacture of fuels, fertilisers, dyes and pharmaceuticals.
- Environmental chemistry (e.g. rate of pollutant breakdown).
Example
The Haber Process (ammonia production)
$$\mathrm{N}_2(\mathrm{~g})+3 \mathrm{H}_2(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_3(\mathrm{~g})$$
- Uses catalysts, high pressure, and moderate temperature to achieve a good rate and good yield.
- Ammonia is vital for fertilisers, which support global food production.
Self review
- How can you define rate of reaction using a simple formula?
- Name three different experimental methods you could use to measure reaction rate in the lab.
- On a graph of product vs time, what does a steeper slope tell you about the reaction?
- Why is it important to understand reaction rates in medicine and in food preservation?
- In the Haber process, why must reaction rate and yield both be considered when choosing conditions?
