# Determining Speed of a Chemical Reaction

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There are three common experimental methods to determine the speed of a chemical reaction:

• Measure the time for a reaction to be completed
• Measure the quantity of products formed in a period of time
• Measure the quantity of reactants remaining in a period of time

The choice of experimental methods for the determination of the speed will depend on the nature of the chemical reaction that is taking place.

### Measure the time for a reaction to be completed

We shall take a look at the chemical reaction between magnesium ribbon and two different solutions of dilute sulphuric acid.

The chemical equation for the chemical reaction is given by:

$$\text{Mg} \left( \text{s} \right) + \text{H}_{2}\text{SO}_{4} \left( \text{aq} \right) \rightarrow \text{MgSO}_{4} \left( \text{aq} \right)$$

The following procedures are carried out:

1. Obtain two pieces of magnesium ribbon that is 2 cm in length each.
2. Prepare two different solutions of sulphuric acid, Acid A and Acid B.
3. Drop one piece of magnesium ribbon into Acid B and the other in Acid B.
4. Record the time taken for each piece of magnesium ribbon to dissolve completely in the two acids.

The following is observed:

• A lot more bubbles of gas are observed for the chemical reaction in Acid A.
• It takes 25 seconds for the chemical reaction to be completed in Acid A.
• It takes 35 seconds for the chemical reaction to be completed in Acid B.

Since the same amount of magnesium is used for both acids, the speed of reaction is inversely proportional to the time taken for the reaction to be completed.

Hence, we can use the following formula to calculate the speed of reaction:

$$\text{Speed of reaction} = \frac{1}{\text{Time}}$$

Reaction in Acid A:

\begin{aligned} \text{Speed of reaction} &= \frac{1}{25} \\ &= 0.040 \, \text{s}^{-1} \end{aligned}

Reaction in Acid B:

\begin{aligned} \text{Speed of reaction} &= \frac{1}{35} \\ &= 0.029 \, \text{s}^{-1} \end{aligned}

From the results, we can say that:

The shorter the time taken for the reaction to be completed, the faster the speed of reaction.

### Measure the quantity of products formed in a period of time

We shall take a look at the reaction between marble (calcium carbonate) and dilute hydrochloric acid.

The chemical equation for the chemical reaction is given by:

$$\text{CaCO}_{3} \left( \text{s} \right) + 2 \text{HCl} \left( \text{aq} \right) \rightarrow \text{CaCl}_{2} \left( \text{aq} \right) + \text{H}_{2}\text{O} \left( \text{l} \right) + \text{CO}_{2} \left( \text{g} \right)$$

The following procedures are carried out:

1. Set up the experiment as shown in the figure above.
2. Collect the gas produced in a measuring cylinder and record the volume of gas in the syringe every minute from the start of the reaction.
3. Plot a graph of volume of gas ($\text{CO}_{2}$) against time.

The gradient of the graph will show the speed of the reaction. The larger the gradient at any particular time, the faster the reaction at that time.

Analysis of graph

• The gradient of the graph is the biggest at the start of the reaction, hence the speed of reaction is the greatest at the beginning of the reaction.
• The gradient decreases with time, so speed slows down with time.
• The gradient becomes zero at around 7 minutes, which means that the speed of reaction is zero. This implies that the chemical reaction has stopped and no more carbon dioxide is produced.

Explain why the slope of the graph is steep at the beginning and then levels off over time.

At the start of the reaction, there are more particles available for effective collisions, which will translate to high speed of reaction.

As the reactants particles collide, they are converted into product particles, resulting in fewer and fewer reactant particles available for collisions to happen. Hence, the volume of gas produced decreases and eventually, no gas is produced as all of the reactants (or one of them) have been used up.

### Measure the quantity of reactants remaining in a period of time

We shall take a look at the reaction between marble (calcium carbonate) and dilute hydrochloric acid.

The chemical equation for the chemical reaction is given by:

$$\text{CaCO}_{3} \left( \text{s} \right) + 2 \text{HCl} \left( \text{aq} \right) \rightarrow \text{CaCl}_{2} \left( \text{aq} \right) + \text{H}_{2}\text{O} \left( \text{l} \right) + \text{CO}_{2} \left( \text{g} \right)$$

The following procedures are carried out:

• Set up the experiment as shown in the figure above. (The cotton wool stops the loss of drops of liquid, but allows the escape of gas.)
• The initial mass is recorded.
• The mass of the contents of the flask is recorded at regular times.
• A graph of mass of the contents of the flask against time is plotted.

Analysis of graph

• The gradient of the graph is biggest at the start of the reaction. Hence, the speed of reaction is the greatest at the start.
• The gradient decreases with time, hence this implies that the speed of reaction decreases with time.
• The gradient of the graph is 0 in section Z of the graph. This means that the speed of reaction is zero and the reaction has stopped. 