MYP Chemistry Chemical Reaction Kinetics Rates Notes

How Can We Measure the Rate of a Chemical Reaction in the Lab?

1. When a chemical reaction occurs, reactants are transformed into products.
2. 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:

1. $\Delta \text { quantity }$ = change in amount, mass, volume, or concentration
2. $\Delta t$ = change in time

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)

1. If a reaction produces a gas that escapes, the mass of the reaction mixture decreases over time.
2. You can place the reaction flask on a balance and record the mass every few seconds.

Change in Volume (gas syringe)

1. If a gas is produced, you can collect it in a gas syringe and measure the volume at regular time intervals.
2. The steeper the volume–time curve, the faster the reaction.
3. Measure the volume of hydrogen gas produced every 10–20 seconds.

Change in Concentration (titration)

1. Small samples can be withdrawn at set times and titrated to find the concentration of a reactant or product.
2. This works well for slower reactions.

Change in Colour (colorimetry)

1. If the reaction mixture changes colour, a colorimeter can measure how the intensity of colour changes with time.
2. The absorbance (how much light is absorbed) relates to concentration.

Change in Conductivity

1. If ions are formed or used up, the electrical conductivity of the solution will change.
2. A conductivity meter can be used to track reaction progress.

How Do Graphs Help Us Compare Reaction Rates?

Graphs are powerful tools for visualising how quickly a reaction proceeds.

We can plot:

1. Reactant concentration (or amount) vs time, or