Rates of Reaction (SQA National 5 Chemistry): Revision Note

Monitoring chemical reactions

• The rate of a chemical reaction is a measure of its speed

  • Some reactions are very slow, like rusting

  • Other reactions are extremely fast, like an explosion

• To find the rate of a reaction, we track how much a quantity changes over a set amount of time

• No matter which method you use, you are always measuring two things:

  1. A change in quantity, for example:

     • Mass (usually in grams, g)

     • Volume (usually in cm³)

  2. The time taken for that change (usually in seconds, s)

Methods for tracking a reaction

• Here are three common ways to follow a reaction's progress in the lab

1. Measuring mass loss (when a gas escapes)

• This is used when a reaction produces a gas that can leave the flask

• Example reaction:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

• The reaction is performed in a flask on a digital balance.

• As the gas escapes, the mass on the balance goes down.

• You record the mass at regular time intervals (e.g., every 30 seconds).

2. Collecting a gas

• This is used for reactions that produce a gas

• Example reaction:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

• The reaction is performed in a flask with a delivery tube attached

• The two most common ways to measure the volume of gas produced in the lab are:

  • Using a gas syringe

  • Collecting over water in an inverted measuring cylinder

• For full diagrams and details on which method to use for which gas, see the Collecting gases revision note

3. Watching a precipitate form

• This is used when two solutions react to form an insoluble solid, called a precipitate

• The reaction is performed in a flask with a black cross underneath it

• Example reaction:

sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur dioxide + water + sulfur

Na₂S₂O₃ (aq) + 2HCl (aq) → 2NaCl (aq) + SO₂ (g) + H₂O (l) + S (s)

• As the reaction proceeds, the black cross becomes less visible through the flask

• You time how long it takes for the mixture to get cloudy enough to hide the black cross

  • This is often called the "disappearing cross" experiment

Rate of reaction graphs

• When you collect data from a rates experiment, plotting it on a graph gives you a reaction-rate graph

• This graph visually tells you the story of the reaction's speed over time

Understanding the shape of the curve

• A typical reaction rate graph is a curve that starts steep and gradually flattens out, as shown:

• This graph shows three key stages of the reaction:

1. The start (steepest part)

• The curve is steepest at the beginning

• This shows the reaction is fastest when the reactants are first mixed.

2. The middle (curving part)

• The curve becomes less steep as time goes on

• This shows that the reaction is slowing down

• This happens because the reactants are being used up

3. The end (flat part)

• The curve becomes a horizontal line

• This is the end-point of the reaction, where the rate is zero

• At least one of the reactants has been completely used up

• So, the reaction has stopped

How to read information from the graph

You can get three pieces of information from a rates graph:

1. How much product is made?

• Find the point where the graph goes flat

• Draw a line across to the y-axis

• Read the final volume

  • For example, the graph above shows that a total of 25 cm³ of gas is made

2. How long does it take to make a certain amount of product?

• Find the required volume on the y-axis

• Draw a line across to the curve

• Then, draw a second line straight down to the x-axis

• Read the time

  • For example, the graph above shows that it takes 50 seconds to produce 22.5 cm³ of gas

3. How much product is made in a certain time?

• Find the time on the x-axis

• Draw a line up to the curve

• Then, draw a line across to the y-axis

• Read the volume

  • For example, the graph above shows that 15cm³ of gas is produced in 20 seconds

Comparing graphs for different conditions

• Exams often ask you to draw or identify a graph for a reaction where one condition has been changed

• You always need to think about two things:

  • The speed of the reaction

  • The final amount of product

1. Changing the reaction speed

• If you change a condition that only affects the rate, the reaction will:

  • Speed up or slow down

  • Make the same total amount of product

• Changing temperature or adding a catalyst can have this effect

• A faster reaction will have a steeper curve and finish sooner

• A slower reaction will have a less steep curve and finish later

2. Changing the amount of product

• If you change the amount of a starting reactant, you will change the total amount of product you can make

• Reducing the concentration of one reactant can have this effect

• Using less reactant will:

  • Make the reaction slower (a less steep curve)

  • Produce less product

For more information about why temperature, concentration and other factors affect the rate, see the Factors Affecting Rates of Reaction revision note