How do changes in concentration affect the speed of a chemical reaction ?

IDEA

The reaction between sodium thiosulphate and dilute hydrochloric acid is investigated.

The concentration of the sodium thiosulphate is changed by adding different amounts of water.

The concentration of the acid remains unchanged.

EXPLANATION

In this reaction sodium thiosulphate reacts with hydrochloric acid forming sodium chloride (salt), sulphur (a yellow solid), water and a gas sulphur dioxide.

WORD EQUATION

sodium thiosulphate
+
hydrochloric acid
sodium chloride (salt)
+
sulphur
+
water
+
sulphur dioxide
CHEMICAL EQUATION
Na2S2O3
+
2HCl
2NaCl
+
S
+
H2O
+
SO2

APPARATUS

PROCEDURE

Using the 50 ml plastic measuring cylinder measure out in to the conical flask.

  • 10 mls of sodium thiosulphate.
  • 40 mls of water.

Using the 10 ml glass measuring cylinder measure out 5 mls of dilute hydrochloric acid and put it into the test tube.

 

 

 

 

 

 

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

  • Pour the hydrochloric acid into the flask from the test tubre and start the clock.
  • Time (in seconds) how long it takes for the cross to disappear.

Wash out the flask thoroughly .

 

Repeat using the next volumes of sodium thiosulphate and acid as given in the results table.

 

THE RESULTS: (For student results click here)

Fill in the table below

Volume of acid used
Volume of sodium thiosulphate (mls)
Volume of water (mls)
Vol of water + thiosulphate (mls)
Time (secs)
5 mls
10 mls
40 mls
50 mls
.
5 mls
20 mls
30 mls
50 mls
.
5 mls
30 mls
20 mls
50 mls
.
5 mls
40 mls
10 mls
50 mls
.
5 mls
50 mls
0 mls
50 mls
.

Using the graph paper below plot a graph of volume of sodium thiosulphate (vertical) against the time taken for thecross to disappear (horizontal).

QUESTIONS

1. Which mixture of the sodium thiosulphate and water was the most concentrated ?

2. How does the speed of reaction vary with concentration.

3. Explain why changes of concentration alter the speed of a reaction.Talk about the reacting particles.


FOR THE TECHNICIAN

  • 100 cm3 conical flasks
  • 50 cm3 plastic measuring cylinder
  • 10 cm3 glass measuring cylinder

HAZARDS

Sulphur dioxide

Toxic by inhalation.

Students who suffer from

asthma need to take care.

  • Stopclock
  • test tube.
  • Sheet of white paper with a cross on.
  • 2M hydrochloric acid
  • Sodium thiosulphate solution containing 40 g / litre
Hydrochloric acid

Corrosive - may cause burns.

Irritant