Saturday, 1 August 2015

Rate of reaction - Test 1

This test is on rate of reactionYou may like to revise this chapter first before attempting the question.

The total score for this test is 10 marks. You should complete the test in 10 minutes.

Open Ended Questions [10 marks]
Experiment 1 was carried out to investigate the rate of reaction between 0.2 g of magnesium ribbon and 25 cm3 of 0.1 mol/dm3 of hydrochloric acid, at 25 oC. x cm3 of hydrogen (measured at room temperature and pressure) was produced.

a) Write a balanced equation for the reaction between magnesium and hydrochloric acid. Include state symbols in your answer.
[2 marks]
b) Determine the value of x.
[3 marks]
c) Sketch on the same graph the volume of oxygen produced with time, when 0.2 g of magnesium ribbon is added to 12.5 cm3 of 0.2 mol/dm3 of hydrochloric acid, at 25 oC.
Label this graph experiment 2.
[1 mark]
d) Another experiment was carried out using 0.2 g of magnesium ribbon and  25 cm3 of 0.1 mol/dm3 of ethanoic acid, 25 oC.

i) Sketch on the same graph the volume of oxygen produced with time for this experiment.
Label this graph experiment 3.
[1 mark]

ii) Explain the difference in the rate of reaction between experiment 1 and 3.
[ 1 mark]

e) A fourth experiment was conducted using 0.2 g of magnesium ribbon and 25 cm3 of 0.1 mol/dm3 of sulfuric acid, at 25 oC.

i) Sketch on the same graph the volume of oxygen produced with time for this experiment. Label this graph experiment 4.
[ 1 mark]
ii) Explain the difference in the rate of reaction between experiment 1 and 4.
[ 1 mark]

a) Mg (s)+ 2HCl (aq) ---> MgCl2(aq) + H2 (g)
b) no. of moles of Mg = 0.2 / 24 = 0.00833 mol
no. of moles of HCl = 0.2 x 25/1000 = 0.005 mol
1 mol of Mg reacts with 2 mol of HCl,
Hence 0.00833 mol of Mg reacts with 0.00833 x 2 = 0.0167 mol of HCl. However, only 0.005 mol of HCl is used.
As such, HCl is in excess, and Mg is the limiting reagent.

no. of moles of H2 formed = 0.005 / 2 = 0.0025 mol
volume of H2 produced = 0.0025 x 24 000 = 60 cm3
x = 60

c) di) ei)

ii) Ethanoic acid is a weak acid, while hydrochloric acid is a strong acid. The concentration of hydrogen ions in 0.1 mol/dm3 of ethanoic acid is lower than that in 0.1 mol/dm3 hydrochloric acid, resulting in a lower rate of reaction for ethanoic acid than hydrochloric acid.

e) ii) Sulfuric acid is a dibasic acid while hydrochloric acid is a monobasic acid.
There is 0.2 mol/dm3 of hydrogen ions in 0.1 mol/dm3 of sulfuric acid, and only 0.1 mol/dm3 of hydrogen ions in 0.1 mol/dm3 of hydrochloric acid.
Due to the higher concentration of hydrogen ions in sulfuric acid as compared to hydrochloric acid, the rate of reaction of the former is greater than that of the latter.