# Rates of reaction

Rocket fuel needs to react quickly in order to achieve lift off.

#### What is RATE?

How fast a chemical reaction happens is a very important consideration.

If paint dries too quickly it will become impossible to apply.

If the fuel in a rocket burns too slowly - the rocket will not get off the ground

Task 1. Consider the word RATE.

• ​When we look at applying for a job we will normally want to know about the rate of pay.
• When we save or borrow money we will want to know about the interest rate which applies.
• When looking at using chemical reactions we will want to know about the rate of reaction.

Try to write a definition of each of the three rates mentioned above​. What quantity is involved in all three?

Experiment 1

Experiment 2 higher concentration

Compare the rate of reaction in each experiment. Use your own stopwatch to time how long it takes for the solution to obscure the cross.

What do these results tell you about the relationship between rate and concentration?

#### Rates in the lab

The video shows a classic reaction being performed.

Task 2. Watch the first video closely.

Two colourless solutions are mixed in a beaker. After a while, the solutions react together to produce a cloudy suspension.  The cross at the bottom of the beaker can no longer be seen.

The two solutions used are sodium thiosulfate and dilute hydrochloric acid

• Try to work out and write down what elements are present in the two solutions. Look up the names if necessary.
• the cloudy suspension is actually fine particles of one of these elements. Which element do you think it is?
• What single measurement could you make with this reaction which would give you an idea of the rate of the reaction?

The reaction studied here is between calcium carbonate solid and dilute hydrochloric acid. Carbon dioxide gas is given off. The gas can be collected in a a measuring cylinder to give a measure of the reaction rate. Alternatively, since the reaction mixture loses gas to the atmosphere, its mass will decrease over time. Thus mass change can be used as a way of measuring the rate of the reaction.

Watch the video closely. Pause it in order to answer the questions below. Answer in full sentences which make it clear what the question is.

1. Write down the two ways of measuring the rate of a reaction .
2. What feature of the graph of mass of reactants against time gives a measure of the rate of reaction?
3. What happens to the rate of most most reactions as the reaction progresses?
4. How does the graph of volume against time compare with the graph of mass against time. What are the differences and what are the similarities

1. The first method is to measure the speed the mass of the reactants decreases. The second method is the measure the speed that the volume of the products increases.

2. The rate of reaction at a point is equal to the gradient at that point.

3. Over time the rate of reactions in the majority of reactions decreases. This happens because the reactant is used up, resulting in fewer particles colliding

4. In graphs of mass over time and volume over time the line is flipped with mass over time graphs having a negative gradient and volume over time graphs having a positive gradient.