Required Practical 19: Effect Of Concentration On Reaction Rate

Section 1 of 7The investigation

Reaction used

• Reaction used Sodium thiosulfate solution reacts with dilute hydrochloric acid to form a cloudy yellow precipitate of sulfur.
• Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)

Measuring the rate

• Measuring the rate Time how long it takes for the mixture to become cloudy enough to hide a cross drawn under the flask.
• A short time means a fast reaction, so rate is proportional to 1 ÷ time.

Variables

• Variables Independent: concentration of sodium thiosulfate. Dependent: time for the cross to disappear.
• Controlvolume and concentration of acid, temperature, same flask, same observer.

Section 2 of 7Method

1

Draw a cross on a piece of paper and put a conical flask on top of it.

2

Use a measuring cylinder to add 10 cm³ of sodium thiosulfate solution and 40 cm³ of water to the flask.

3

Add 10 cm³ of dilute hydrochloric acid and start the stopwatch immediately; swirl once.

4

Look down through the solution and stop the timer when the cross can no longer be seen.

5

Repeat using less water, so that thiosulfate concentration rises while the total volume stays 60 cm³.

6

Repeat each concentration and calculate a mean time.

Section 3 of 7Processing results

1Calculate rate

• Calculate rate Work out 1 ÷ time for each concentration, in s⁻¹.

2Plot a graph

• Plot a graph Plot rate (1 ÷ time) against concentration or volume of thiosulfate.

3Pattern

• Pattern A straight line through the origin means rate is directly proportional to concentration.

4Explain

• Explain Higher concentration means more particles per volume, more frequent collisions and a faster rate.

Section 4 of 7Safety and limitations

Issue	Comment
Sulfur dioxide gas	Harmful to asthmatics; work in a well-ventilated room or fume cupboard
Acid irritant	Wear eye protection and wipe spills
Judging the cross	Subjective, so use the same person or a light sensor
Disposal	Do not pour the mixture straight back; dispose of it as directed

Section 5 of 7Alternative: gas collection

Other methods

Gas syringe

• Gas syringe Measure the volume of gas given off by magnesium and acid at fixed times.

Mass loss

• Mass loss Place the flask on a balance with cotton wool in the neck, and record the falling mass.

Rate from a graph

• Rate from a graph The steepest gradient at the start gives the initial rate.

Section 6 of 7Key terms

Term	Meaning
Precipitate	A solid formed in a solution during a reaction.
Concentration	The amount of a substance in a given volume of solution.
Initial rate	The rate at the very start of a reaction.
Control variable	A variable kept the same so that it does not affect the results.
Mean	The sum of repeated readings divided by their number.

Section 7 of 7Exam technique

1

Changing the total volume as well as the concentration, which alters the depth of liquid over the cross.

2

Using time as the rate; the rate is proportional to 1 ÷ time.

3

Starting the timer after mixing instead of when the acid is added.

4

Not repeating the experiment, so one rogue result goes unnoticed.