Experiment 20
Investigation of some factors on the reaction rate
(6/3, 9/3)
Introduction
On the basis of enthalpy change of a reaction, you may predict whether or not a particular reaction will take
place. However, if the rate of a reaction is studied, you may know whether a reaction is likely to proceed or not. The
rate of a reaction is usually expressed as a change of concentration of a reactant or a product in a given interval of
time.
Apparatus : 50 cm3 graduated tube, sand paper, 100 cm3 conical flask, watch glass, stop-watch
Chemicals : magnesium ribbon, marble chips, marble powder, 2 M HCl(aq), cotton-wool, hexane, 1 % bromine
solution in1,1,1-trichloroethane, glacial ethanoic acid, ethanol, conc. H2SO4(l), 1 M KI(aq),
0.1 M K2S2O8(aq), starch solution, 0.1 M FeSO4(aq), 0.1 M FeCl3(aq)
A.
Concentration effect (濃度效應)
(I) Reaction between magnesium and different concentrations of hydrochloric acid
Clean a 4-cm long magnesium ribbon with a piece of sand paper. Weigh it accurately. Place 30 cm3 of 2 M
HCl(aq) into the graduated tube and add deionized water until the tube is full-filled. Now put the magnesium ribbon
into the graduated tube. Invert the tube onto a 250 cm3 beaker which is half-filled with water. Start timing when
reaction occurs. Record the time for collecting 30 cm3 of gas. Make sure that the magnesium ribbon should always
contact with the acid.
Repeat the above procedure except using 10 cm3 of 2 M HCl(aq).
Mg(s) + 30 cm3 of 2 M HCl(aq) Mg(s) + 10 cm3 of 2 M HCl(aq)
Mass of magnesium / g
Time / min
Relative rate / min1
Questions :
1.
Write a balanced ionic equation for the reaction, indicating the state symbols of all species.
2.
Write observations for the reaction.
3.
How does the reaction rate change with concentration ?
4.
Draw a labelled diagram to show another method to study the rate of this reaction.
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(II) Reaction between hydrochloric acid with different concentrations of Na 2S2O3(aq)
Draw a cross on a white paper and place a 50 cm3 beaker onto the paper. Pour 4 cm3 of 0.1 M Na2S2O3(aq)
into the beaker, then pour 4 cm3 of 2 M HCl(aq) into it. Record the time for the disappearance of the cross
through the mixture.
Repeat the above procedure but using 4 cm3 of 0.05 M Na2S2O3(aq).
0.1 M Na2S2O3(aq) + HCl(aq)
0.05 M Na2S2O3(aq) + HCl(aq)
Time / min
Relative rate / min1
1.
Write a balanced ionic equation for the reaction, indicating the state symbols of all species.
2.
Write observations for the reaction.
3.
How does the reaction rate change with concentration ?
B.
Particle size (粒子的大小)
(I)
Reaction between marble chips and hydrochloric acid
Weigh 10 g of marble chips in a 100 cm3 conical flask, plug the top with the cotton-wool. Place 50 cm3 of 1 M
HCl(aq) into a measuring cylinder. Now put the conical flask and the measuring cylinder together on a balance and
record the total mass. Pour all the hydrochloric acid into the conical flask then start timing. (Remember to plug the top
with the cotton-wool again.) Record the mass of the whole reaction mixture and apparatus again after 30 minutes.
(II) Reaction between powdered CaCO3 and hydrochloric acid
Repeat the procedure in (I) except using 10 g of powdered CaCO 3.
Marble chips + HCl(aq)
Powdered CaCO3(s) + HCl(aq)
Initial mass / g
Final mass / g
Relative rate / g min1
1.
Write a balanced ionic equation for the reaction, indicating the state symbols of all species.
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2.
Write observations for the reaction.
3.
How does the reaction rate change with particle size of reactants ?
C.
Effect of light on reaction rate
Mix 20 drops of bromine solution and 20 drops of hexane in a test tube. Divide the mixture into two portions in
test tubes. Put a moist blue litmus paper near the mouth of the test tube. Stopper each portion and shake well. Wrap
one portion with an aluminium foil. The other portion is placed on the top of an overhead projector such that it is
exposed to strong light.
1.
Write a balance equation for the reaction, including the state symbols for all species.
2.
Name the type of the reaction (寫出這反應類別的名稱).
3.
Write observations for the reaction.
4.
How does the reaction rate change when the reaction mixture is irradiated ?
D.
Effect of temperature on reaction rate
Mix 5 drops of ethanoic acid, 5 drops of ethanol and 2 drops of concentrated sulphuric(VI) acid in a test tube.
Divide the mixture into two portions. One portion is immersed in a hot water bath and the other portion is left at room
temperature. After 5 minutes pour each portion respectively into a beaker containing some tap water. Compare the
smell of the content in each beaker.
1.
Write a balanced equation for the reaction involved.
2.
Name the type of reaction.
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3.
Write the observations of the reaction.
4.
How does the reaction rate change with temperature ?
E.
Effect of catalyst on reaction rate
Mix 10 drops of 1 M KI(aq), 10 drops of 0.1 M K 2S2O8(aq) and 2 drops of starch solution in a test tube. Divide
the mixture into three portions. One portion left as a blank. Two drops of 0.1 M FeSO 4(aq) is added into the second
portion and 2 drops of 0.1 M FeCl3(aq) is added to the third portion. Compare the colour of each portion.
1.
Write an ionic equation for the reaction involved.
2.
Write the observations of the reaction.
3.
Explain the above observations.
References :
1.
Chemistry, a modern view, 2nd ed. Workbook 2, p. 98, Wilson
2.
Revised Nuffield Advanced Science, Chemistry, Students‘ Book II, p. 129 Longman
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