New
New
Year 10
AQA
Foundation

Concentration and rate: practical using end point method

I can carry out a fair test to investigate how the rate of reaction depends on the concentration of a reacting solution.

New
New
Year 10
AQA
Foundation

Concentration and rate: practical using end point method

I can carry out a fair test to investigate how the rate of reaction depends on the concentration of a reacting solution.

Lesson details

Key learning points

  1. The reaction between sodium thiosulfate solution and hydrochloric acid produces a cloudy precipitate.
  2. At the end point of the disappearing cross experiment, each reaction has produced the same amount of product.
  3. At the end point of the disappearing cross experiment, the chemical reaction may not be complete.
  4. Increasing concentration of a reacting solution introduces more reactant particles, so they collide more frequently.

Common misconception

The reaction stops when the 'X' is no longer visible.

The disappearance of the cross only indicates that the sulfur precipitate has formed sufficiently to obscure the cross. The reaction itself continues beyond this point until all reactants are consumed.

Keywords

  • Rate of reaction - Rate of reaction is the speed with which a chemical reaction takes place.

  • End point method - An end point method is used to measure the progress of a reaction until it has reached completion or a desired point e.g. a colour change.

  • Precipitate - A precipitate is an insoluble solid formed when two solutions react together.

  • Fair test - In a fair test variables are controlled so that only one change affects the dependent variable.

Pupils could dilute the sodium thiosulfate themselves to produce three different concentrations. See the additional material for additional guidance.
Teacher tip

Equipment

Conical flask, card marked with an 'X', measuring cylinders, different concentrations of sodium thiosulfate, hydrochloric acid, timer and eye protection.

Content guidance

  • Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
Which of the following statements about concentration and rate of reaction is correct?
Correct answer: Concentration and rate of reaction are directly proportional.
Concentration and rate of reaction are inversely proportional.
Concentration has no effect on the rate of reaction.
Only high concentrations affect the rate of reaction.
Q2.
What effect does concentration have on the rate of reaction?
Correct answer: Doubling the concentration of a reacting solution doubles the rate of reaction.
Doubling the concentration of a reacting solution halves the rate of reaction.
Halving the concentration of a reacting solution doubles the rate of reaction.
Correct answer: Halving the concentration of a reacting solution halves the rate of reaction.
Q3.
Which of the following methods is used when we measure the progress of a reaction until it has reached a desired point?
gas collection
calorimetry
neutralisation
Correct answer: end point
Q4.
Which variable is the independent variable in an experiment where we change the concentration and measure the time taken for a colour change to occur?
Correct answer: the concentration of a reacting solution
the temperature of the reactants
the time taken for the colour change
the volume of reacting solutions
Q5.
Which variable is the dependent variable in an experiment where we change the concentration and measure the time taken for a colour change to occur?
the concentration of reacting solutions
the temperature of the reactants
Correct answer: the time taken for the colour change
the volume of reacting solutions
Q6.
What is the main reason for repeating a rate of reaction experiment multiple times?
to increase the rate of reaction
to decrease the rate of reaction
to change the dependent variable
to adjust control variables
Correct answer: to obtain more reliable results

6 Questions

Q1.
In the disappearing cross experiment, which of the following indicates the end point of the reaction?
the solution turns blue
the solution becomes colourless
Correct answer: an X placed under the reaction mixture is no longer visible
the solution starts to bubble
Q2.
Which of the following is observed when sodium thiosulfate reacts with hydrochloric acid?
a colour change from blue to red
Correct answer: the reaction mixture changes from clear to cloudy
light is emitted
no visible change
Q3.
When sodium thiosulfate and hydrochloric acid react together, a yellow precipitate of sulfur forms. In which state is this sulfur?
Correct answer: solid
liquid
gas
aqueous
Q4.
Increasing the concentration of reactants introduces more...
solvent particles.
product particles.
Correct answer: reactant particles.
inert particles.
Q5.
Which statement about the amount of product formed at the end point of the disappearing cross experiment, when using different concentrations of sodium thiosulfate in excess, is correct?
a higher concentration produces a larger amount of product
a lower concentration produces a larger amount of product
the different concentrations produce different products
Correct answer: the different concentrations produce the same amount of product
Q6.
At the end point of a reaction, the reaction may not be complete because ...
all the reactants have been converted to products.
Correct answer: some of the reactants have not yet reacted.
no reactants have reacted.
the products have decomposed.

Additional material

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