Factors which affect the rates of chemical reactions

Welcome to reaction rates! In chemistry, it isn't just about what gets made, but how fast it gets made. Some reactions, like explosions, are over in a fraction of a second. Others, like iron rusting, take years.

What you'll learn

The five main factors that can change the speed of a chemical reaction.
How to predict the effect of changing one of these factors.
How to measure reaction rates in the lab using the "disappearing cross" and gas syringe methods (Required Practical 5).

The Five Factors

A chemical reaction only happens when reactant particles bump into each other with enough energy. Anything that makes these energetic collisions happen more often will speed up the reaction.

There are exactly five factors you need to know that affect the rate of a chemical reaction:

Concentration of solutions: How crowded the dissolved particles are in a liquid. Higher concentration means a faster rate.
Pressure of reacting gases: How squashed together gas particles are. Higher pressure means a faster rate.
Surface area of solid reactants: How much of the solid is exposed to the other reactants. A larger surface area (achieved by cutting a solid into smaller pieces or powder) means a faster rate.
Temperature: How hot the reaction mixture is. A higher temperature means a faster rate.
Presence of a catalyst: A special chemical helper. Adding a catalyst means a faster rate.

Definition

Catalyst

A catalyst is a substance that speeds up a chemical reaction without being changed or used up during the reaction. Because it isn't used up, you don't write it as a reactant in the chemical equation.

Key Idea

The general rule

If you increase the temperature, concentration, pressure, or surface area, the rate of reaction increases. If you decrease them, the rate decreases. Adding a catalyst always increases the rate.

Common Mistake

Surface area vs. particle size

Be very careful with your wording for solids! If you take a large lump of marble and crush it into a fine powder, you have decreased the particle size, but you have increased the surface area. Exams often try to catch you out here. Always link a larger surface area to a faster reaction.

Required Practical 5: Investigating Concentration

To prove that these factors actually work, we need to do some experiments. The AQA specification requires you to know how to investigate the effect of concentration on the rate of reaction.

We can do this using two completely different methods.

Method 1: The "Disappearing Cross" (Colour / Turbidity Change)

Some reactions produce a solid product (a precipitate) from two clear liquids. As the solid forms, the mixture goes cloudy. In chemistry, we call this cloudiness turbidity.

A classic example is reacting sodium thiosulfate solution with hydrochloric acid. It produces solid sulfur, which turns the liquid yellow-white and opaque.

Na2S2O3(aq)+2HCl(aq)→2NaCl(aq)+SO2(g)+H2O(l)+S(s)\text{Na}_2\text{S}_2\text{O}_3\text{(aq)} + 2\text{HCl(aq)} \to 2\text{NaCl(aq)} + \text{SO}_2\text{(g)} + \text{H}_2\text{O(l)} + \text{S(s)}

alt text

How to do it:

Use a measuring cylinder to put 10 cm³ of sodium thiosulfate solution into a conical flask.
Place the flask onto a printed black cross.
Add 10 cm³ of hydrochloric acid to the flask, give it a quick swirl, and immediately start a stopwatch.
Look down through the top of the flask. The mixture will gradually turn cloudy.
Stop the stopwatch the exact moment you can no longer see the black cross.
Repeat the experiment using lower concentrations of sodium thiosulfate.

The result: You will find that the higher the concentration of sodium thiosulfate, the shorter the time it takes for the cross to disappear (meaning a faster rate of reaction).

Common Mistake

The subjective observer

The disappearing cross method is highly subjective. Different people have different eyesight, so one student might think the cross has disappeared while their lab partner can still faintly see it. To make results reliable, the same person should judge the cross for every repeat.

Method 2: Measuring Gas Volume

If a reaction produces a gas, we can measure the volume of gas given off over a set time.

A great reaction for this is magnesium ribbon reacting with hydrochloric acid to produce hydrogen gas.

Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg(s)} + 2\text{HCl(aq)} \to \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}

alt text

How to do it:

Put 50 cm³ of hydrochloric acid into a conical flask.
Set up a gas syringe (or an inverted measuring cylinder filled with water) and connect it to a bung with a delivery tube.
Add a strip of magnesium ribbon to the flask, quickly insert the bung, and start the stopwatch.
Read the volume of gas collected in the syringe every 10 seconds until the reaction stops (when the volume stops changing).
Repeat the whole experiment using different concentrations of hydrochloric acid.

The result: You will find that the higher the concentration of the acid, the steeper the volume curve and the faster the gas is produced.

Tip

Fair testing

In both of these practicals, concentration is your independent variable (the thing you change). The time taken or gas volume is your dependent variable (the thing you measure). To keep the test fair, everything else must be a control variable — you must keep the temperature, the volume of liquids, and the size of the solid pieces exactly the same every time.

Working with the results

Once you have collected data from an experiment, you might be asked to calculate the mean rate of reaction. The rate is simply a measure of how much product is made (or reactant used up) divided by the time taken.

Example

Calculating mean reaction rate

A student reacts magnesium with hydrochloric acid using the gas syringe method. After $30$ seconds, $\text{ cm}^3$ of hydrogen gas has been collected. Calculate the mean rate of reaction and state the units.

Identify the formula:

\text{Mean rate} = \frac{\text{Amount of product formed}}{\text{Time taken}}

Substitute the values:
$rate=4530=1.5\begin{aligned} \text{Mean rate} &= \frac{45}{30} \\ &= 1.5 \end{aligned}$
Determine the units: We divided a volume in $cm3\text{cm}^3$ by a time in seconds ( $s\text{s}$ ). Therefore, the units are $cm3/s\text{cm}^3/\text{s}$ .

Final Answer: $\text{ cm}^3/\text{s}$

Exam technique

In the exam

Read the axes: If you are given a graph of reaction results, check whether the y-axis is "amount of product" or "amount of reactant". The curve will go up for products and down for reactants.
Learn the 5 factors: If a question asks "State two ways to speed up this reaction", you can pick from the five factors. But be smart! If the reaction is between two liquids, do not suggest "increase the pressure" (pressure only affects gases) or "increase surface area" (that only affects solids).
Check the cross: If asked for a source of error in the turbidity practical, remember that judging when the cross disappears is subjective and prone to human error.

Self review

Check yourself

What are the five factors that affect the rate of a chemical reaction?
If you crush a solid reactant into a powder, does the reaction speed up or slow down? Why?
In the disappearing cross experiment, what causes the solution to go cloudy?
Why is measuring gas in a syringe considered a more accurate way to measure rate than the disappearing cross method?

Factors which affect the rates of chemical reactions

What you'll learn

The five main factors that can change the speed of a chemical reaction.
How to predict the effect of changing one of these factors.
How to measure reaction rates in the lab using the "disappearing cross" and gas syringe methods (Required Practical 5).

The Five Factors

A chemical reaction only happens when reactant particles bump into each other with enough energy. Anything that makes these energetic collisions happen more often will speed up the reaction.

There are exactly five factors you need to know that affect the rate of a chemical reaction:

Concentration of solutions: How crowded the dissolved particles are in a liquid. Higher concentration means a faster rate.
Pressure of reacting gases: How squashed together gas particles are. Higher pressure means a faster rate.
Surface area of solid reactants: How much of the solid is exposed to the other reactants. A larger surface area (achieved by cutting a solid into smaller pieces or powder) means a faster rate.
Temperature: How hot the reaction mixture is. A higher temperature means a faster rate.
Presence of a catalyst: A special chemical helper. Adding a catalyst means a faster rate.

Definition

Catalyst

Key Idea

The general rule

Common Mistake

Surface area vs. particle size

Required Practical 5: Investigating Concentration

To prove that these factors actually work, we need to do some experiments. The AQA specification requires you to know how to investigate the effect of concentration on the rate of reaction.

We can do this using two completely different methods.

Method 1: The "Disappearing Cross" (Colour / Turbidity Change)

Some reactions produce a solid product (a precipitate) from two clear liquids. As the solid forms, the mixture goes cloudy. In chemistry, we call this cloudiness turbidity.

A classic example is reacting sodium thiosulfate solution with hydrochloric acid. It produces solid sulfur, which turns the liquid yellow-white and opaque.

Na2S2O3(aq)+2HCl(aq)→2NaCl(aq)+SO2(g)+H2O(l)+S(s)\text{Na}_2\text{S}_2\text{O}_3\text{(aq)} + 2\text{HCl(aq)} \to 2\text{NaCl(aq)} + \text{SO}_2\text{(g)} + \text{H}_2\text{O(l)} + \text{S(s)}

alt text

How to do it:

Use a measuring cylinder to put 10 cm³ of sodium thiosulfate solution into a conical flask.
Place the flask onto a printed black cross.
Add 10 cm³ of hydrochloric acid to the flask, give it a quick swirl, and immediately start a stopwatch.
Look down through the top of the flask. The mixture will gradually turn cloudy.
Stop the stopwatch the exact moment you can no longer see the black cross.
Repeat the experiment using lower concentrations of sodium thiosulfate.

The result: You will find that the higher the concentration of sodium thiosulfate, the shorter the time it takes for the cross to disappear (meaning a faster rate of reaction).

Common Mistake

The subjective observer

Method 2: Measuring Gas Volume

If a reaction produces a gas, we can measure the volume of gas given off over a set time.

A great reaction for this is magnesium ribbon reacting with hydrochloric acid to produce hydrogen gas.

Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg(s)} + 2\text{HCl(aq)} \to \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}

alt text

How to do it:

Put 50 cm³ of hydrochloric acid into a conical flask.
Set up a gas syringe (or an inverted measuring cylinder filled with water) and connect it to a bung with a delivery tube.
Add a strip of magnesium ribbon to the flask, quickly insert the bung, and start the stopwatch.
Read the volume of gas collected in the syringe every 10 seconds until the reaction stops (when the volume stops changing).
Repeat the whole experiment using different concentrations of hydrochloric acid.

The result: You will find that the higher the concentration of the acid, the steeper the volume curve and the faster the gas is produced.

Tip

Fair testing

Working with the results

Example

Calculating mean reaction rate

Identify the formula:

\text{Mean rate} = \frac{\text{Amount of product formed}}{\text{Time taken}}

Substitute the values:
$rate=4530=1.5\begin{aligned} \text{Mean rate} &= \frac{45}{30} \\ &= 1.5 \end{aligned}$
Determine the units: We divided a volume in $cm3\text{cm}^3$ by a time in seconds ( $s\text{s}$ ). Therefore, the units are $cm3/s\text{cm}^3/\text{s}$ .

Final Answer: $\text{ cm}^3/\text{s}$

Exam technique

In the exam

Read the axes: If you are given a graph of reaction results, check whether the y-axis is "amount of product" or "amount of reactant". The curve will go up for products and down for reactants.
Learn the 5 factors: If a question asks "State two ways to speed up this reaction", you can pick from the five factors. But be smart! If the reaction is between two liquids, do not suggest "increase the pressure" (pressure only affects gases) or "increase surface area" (that only affects solids).
Check the cross: If asked for a source of error in the turbidity practical, remember that judging when the cross disappears is subjective and prone to human error.

Self review

Check yourself

What are the five factors that affect the rate of a chemical reaction?
If you crush a solid reactant into a powder, does the reaction speed up or slow down? Why?
In the disappearing cross experiment, what causes the solution to go cloudy?
Why is measuring gas in a syringe considered a more accurate way to measure rate than the disappearing cross method?

Factors which affect the rates of chemical reactions

What you'll learn

The Five Factors

Required Practical 5: Investigating Concentration

Method 1: The "Disappearing Cross" (Colour / Turbidity Change)

Method 2: Measuring Gas Volume

Working with the results

You've reached the end

Factors which affect the rates of chemical reactions

What you'll learn

The Five Factors

Required Practical 5: Investigating Concentration

Method 1: The "Disappearing Cross" (Colour / Turbidity Change)

Method 2: Measuring Gas Volume

Working with the results

You've reached the end

Factors which affect the rates of chemical reactions

What you'll learn

The Five Factors

Required Practical 5: Investigating Concentration

Method 1: The "Disappearing Cross" (Colour / Turbidity Change)

Method 2: Measuring Gas Volume

Working with the results

You've reached the end

Factors which affect the rates of chemical reactions Revision Guide

Factors which affect the rates of chemical reactions

What you'll learn

The Five Factors

Required Practical 5: Investigating Concentration

Method 1: The "Disappearing Cross" (Colour / Turbidity Change)

Method 2: Measuring Gas Volume

Working with the results

You've reached the end

Factors which affect the rates of chemical reactions Revision Guide