Suggested Answers to In-Text Activities and Exercises

New 21st Century Chemistry

Suggested answers to in-text activities and unit-end exercises

Topic 10 Unit 37

In-text activities

Checkpoint (page 48)

1 a) Zn(s) + 2HCl(aq) à ZnCl2(aq) + H2(g)

d) Zinc reacts with the hydrogen ions in the acids.

Sulphuric acid is a dibasic acid while hydrochloric acid is a monobasic acid.

Therefore 2 mol dm–3 sulphuric acid has a higher concentration of hydrogen ions than 2 mol dm–3 hydrochloric acid. The time required for the reaction between zinc and 2 mol dm–3 sulphuric acid to complete was shorter than that for Experiment 1.

2 a) As the reaction proceeds, the concentration of iodine in the reaction mixture decreases.

Hence the brown colour intensity of the reaction mixture also decreases.

Checkpoint (page 53)

Checkpoint (page 60)

1 • Increase the concentration of the reactant in aqueous solution.

This increases the number of reactant particles per unit volume. The particles are more crowded. The chance of collision increases and the number of effective collisions increases. Hence the rate of the reaction increases.

• Use powdered solid reactant instead of lumps of solid reactant.

This increases the contact surface area between the solid reactant and the reactant in aqueous solution. There is a greater chance for collision and the number of effective collisions increases. Hence the rate of the reaction increases.

• Increases the temperature.

The reactant particles have more energy and collide more often. Besides, a larger portion of the particles have energy equal to or greater than the activation energy. So, there are more effective collisions and the rate of the reaction increases.

2 a) The red line indicates the concentration of X(g).

The black line indicates the concentration of Y(g).

The orange line indicates the concentration of Z(g).

b) The time would be longer.

In a larger container, the concentrations of the reactants become smaller. The chance of collision decreases and hence the rate of the reaction decreases.

Library Search & Presentation (page 64)

Heterogeneous catalyst and homogeneous catalyst

There are two large groups of catalyst:

• heterogeneous catalyst (where the catalyst and the reactants are in different phases); and

• homogeneous catalyst (where the catalyst and the reactants are in the same phase).

By far the most important catalysts are the heterogeneous catalysts. The market share of homogeneous catalystsis estimated to be only 10 – 15 %.

Examples of uses of heterogeneous catalysts

Methanol synthesis

The synthesis of methanol from CO and H2 has been known since the early 1920s.

CO(g) + 2H2(g) CH3OH(g) ΔH < 0

The high pressure process is carried out with ZnO / Cr2O3 catalyst at 250 – 350 atmospheres and 350 – 400 °C. The development of more active, copper-based catalysts allowed the process to be carried out in the pressure range 50 – 100 atmospheres and at lower temperatures. This improved the economics of the process. The low pressure process was introduced in the mid-1960s.

Methanol synthesis is carried out in a recycle process similar to ammonia synthesis. The strong temperature dependence of the methanol equilibrium and the increasing extent of side reactions at higher temperature require rapid heat removal or cooling by introduction of fresh gas. Methanol plants are usually operated at a partial conversion of 15 –18 % of the CO starting material with recycle of the unchanged synthesis gas. Large quantities of gas must be circulated.

Alkene polymerization

The polymerization of alkenes has been carried out industrially for decades and can be performed by various mechanisms. The high pressure radical addition polymerization of ethene leads to low density polythene (LDPE).

In the mid-1950s Ziegler achieved the low pressure polymerization of ethene and propene (up to 10 atmospheres, 50 – 150 °C) by using organometallic catalysts based on TiCl4 / Al(C2H5)3. The Ziegler catalysts give less branched, linear high-molecular polythene (high density polythene, HDPE). Using this catalyst system, Natta succeeded in manufacturing crystalline, isotactic polypropene, and around the same time, the company Phillips in the USA developed silica-supported chromium catalysts. Oxides of Cr and Ti on various support materials have high activities for the polymerization of ethene to HDPE.

Catalytic carbonylation

The formation of C–C bonds is of key importance in organic synthesis. An important catalytic process for generating C–C bonds is provided by carbonylation (carbonylation refers to reactions that introduce carbonyl group into an organic compound).

For example, in the Celanese process, ibuprofen is produced by carbonylation of a substituted alcohol according to the following equation:

Hoffmann La Roche has developed a process for the anti-Parkinsonian drug, lazabemide, by amidocarbonylation of 2,5-dichloropyridine. This one-step route has replaced an original synthesis that involved eight steps.

Zeolite catalysts in the petrochemical industry

Zeolites are crystalline aluminosilicates composed of SiO44– and AlO45– tetrahedra arranged in various geometric patterns. The tetrahedra are lined together at the corners by shared oxygen atoms to form ordered lattices, which are often best visualized as three-dimensional combinations of chains, layers, and polyhedra.

Because of their unique porous properties, zeolites are used in a variety of applications. Processes based on zeolite catalysts were first developed in the 1960s.

Cracking

Before cracking, liquid fractions from the fractional distillation of petroleum are re-vaporized before cracking. Zeolite catalysts are used in the cracking process.

Isomerisation

Hydrocarbons used in petrol are given an octane rating which relates to how effectively they perform in the engine. A hydrocarbon with a high octane rating burns more smoothly than one with a low octane rating.

Molecules with straight chains have a tendency to undergo pre-ignition. When the petrol / air mixture is compressed they tend to explode, and then explode a second time when the spark is passed through them. This double explosion produces knocking in the engine.

Octane ratings are based on a scale on which heptane is given a rating of 0, and 2,2,4-trimethylpentane a rating of 100.

In order to raise the octane rating of the molecules found in petrol, the oil industry rearranges straight chain molecules into their isomers with branched chains.

One process uses a platinum catalyst on a zeolite base at a temperature of about 250 °C and a pressure of 13 – 30 atmospheres. It is used particularly to change straight chains containing 5 or 6 carbon atoms into their branched isomers.

References:

Jens Hagen (2006) Industrial catalysis. WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.

http://www.cheresources.com/zeolitezz.shtml

http://www.chemguide.co.uk/physical/catalysis/petrochem.html

http://www.chemheritage.org/discover/chemistry-in-history/themes/petrochemistry-and-synthetic-polymers/petrochemistry/weisz.aspx

STSE Connections (page 66)

1 Airbags had been designed based on car occupants of average size, so the force and direction of impact of the bag itself could hurt children or small adults.

Car manuals say young children should not be put in front seats where they might be injured or killed by an inflating airbag.

2 The second-generation airbags deploy with less force. Reuters reports that second-generation airbags are less risky for children while still providing appropriate levels of safety for larger adults.

Manufacturers are actively developing ‘smart’ airbags that are able to tailor deployment based on crash severity, occupant size and position, or seat belt use. These bags should eliminate the risks produced by current airbag designs.

Another direction of the development of airbag is to protect the pedestrians. One team of British experts has developed a new external airbag for pedestrians. The system deploys a bonnet-mounted airbag at the base of the windscreen, which research shows is where a pedestrian’s head is most likely to hit. The system uses radar and infrared technology to ‘pre-detect’ a collision and inflates quickly enough to cushion the impact.

References:

Erin Mays (2006). Second generation air bags safer for kids, still safe for adults.

Retrieved July 8, 2009, from

http://www.autoblog.com/2006/07/12/university-of-washington-study-shows-airbags-with-reduced-risks/

IHS Automotive, Airbags — The Next Generation

Retrieved July 8, 2009, from

http://auto.ihs.com/news/newsletters/atuo-v4i2-03.htm

Ben Mack (2009). External airbag designed to protect pedestrians.

Retrieved July 8, 2009, from

http://www.wired.co.uk/news/archive/2009-05/05/external-airbag-designed-to-protect-pedestrians.aspx

Unit-end exercises (pages 69 – 82)

Answers for the HKCEE (Paper 1) and HKALE questions are not provided.

1 A possible concept map:

Rate increases / remains the same / decreases
Using powdered calcium carbonate / Rate increases
Using an acid of higher concentration / Rate increases
Adding water to the reaction mixture / Rate decreases
Adding more acid of the same concentration / Rate remains the same
Raising the temperature of the acid / Rate increases

3 a) Cutting whole potatoes into chips increases the surface area of potatoes, thus increases the cooking rate.

b) Low temperatures slow down reactions that lead to the spoiling of food. Thus storing food products in refrigerators helps keep food from spoiling.

4 a) Magnesium hydroxide (Other reasonable answers are acceptable.)

b) Mg(OH)2(s) + 2HCl(aq) à MgCl2(aq) + 2H2O(l)

c) i) Experiments 3 and 5

ii) Experiments 4 and 5

iii) Experiments 1 and 2

5 When the temperature is increased, the reactant particles gain more energy and move faster. That means particles have more energy and collide more often.

Besides, a larger portion of the particles have energy equal to or greater than the activation energy.

So, there are more effective collisions and the rate of the reaction increases.

6 C Option C — The rate of decomposition in Experiment I was higher because a higher concentration of ammonium nitrite solution was used.
The volume of nitrogen collected in Experiment I was higher because a greater amount of ammonium nitrite was used.

7 A Option A — The rate of reaction in Experiment 2 was lower when using a lower temperature.

Option B — The rate of reaction in Experiment 2 would be higher when a higher temperature and an acid of higher concentration were used.

Option D — The rate of reaction in Experiment 2 would be higher when an acid of higher concentration was used.

8 D

9 D Option D — A catalyst can increase the rate of the reaction.

A catalyst does not change the amount of product formed in the reaction.

10 A

11 a) CaCO3(s) + 2HCl(aq)à CaCl2(aq) + H2O(l) + CO2(g)

c) • Volume of acid

• Concentration of acid

• Temperature

(Both curves finish within 5 minutes and with the same volume of gas; curve S finishes before curve L; curve S steeper than curve M, which steeper than curve L)

e) Measure the loss in mass of the reaction mixture over a certain time interval.

12 a)

b) Curve B

At the start of the reaction, the tangent to curve B is steeper.

Therefore the initial rate of the experiment represented by curve B is higher.

c) Hydrochloric acid was in excess because all the barium dissolved in both case (i.e. all the barium reacted).

d) Increase the temperature.

13 a) Hydrogen gas escaped through the cotton wool.

b) To stop any acid from splashing out.

d) The rate of reaction decreased with time.

At the start, there were plenty of reactant particles per unit volume.

As the reactant particles were consumed gradually, there were fewer particles per unit volume. So, the reaction slowed down.

The reaction stopped when one of the reactants was used up.

e) i) The reaction rate decreased.

For the same mass of magnesium, the surface area of magnesium ribbon was less than that of magnesium powder.

ii) The reaction rate decreased.

Ethanoic acid is a weak acid while hydrochloric acid is a strong acid.

Ethanoic acid only partially dissociates in water while hydrochloric acid almost completely dissociates.

Thus 1 mol dm–3 ethanoic acid has a lower concentration of hydrogen ions than 1 moldm–3 hydrochloric acid.

14 a) The volume of chlorine was still increasing after 2.5 minutes.

b) The total time for the reaction is about 4.5 minutes.

c) i) The rate was the same (as the concentration of the acid remained unchanged).

ii) The rate was higher (as the temperature was increased).

15 a)

Volume of 0.1 mol dm–3
Na2S2O3(aq) (cm3) / Volume of water (cm3) / Volume of 0.1mol dm–3
HCl(aq) (cm3) / Reaction time (s)
100 / 0 / 5 / 20
80 / 20 / 5 / 25
60 / 40 / 5 / 33
40 / 60 / 5 / 50
20 / 80 / 5 / 100

b) • Mark a cross on a piece of paper.

• Put a beaker containing some sodium thiosulphate solution on top of the paper.

• Add dilute hydrochloric acid to the beaker.

• Start the stop watch at the same time. The cross gets fainter as the precipitate forms.

• Stop the stop watch when the cross can no longer be seen from above.

c) The relative rate of reaction is inversely proportional to the time taken for enough sulphur to form to make the reaction mixture opaque.

d) The rate of reaction increases with the concentration of sodium thiosulphate solution.

Increasing the concentration of sodium thiosulphate solution means increasing the number of particles per unit volume. The particles are more crowded.

The chance of collision increases and the number of effective collisions increases.

16 —

17 • Carrying out the experiment three times at each temperature to obtain an average reaction time.

• Repeating the experiment at three or four different temperatures to see if any trend could be detected.

• More accurate temperature control — by heating the acid to the same starting temperature as the contents of the beaker.

Other possible refinements may include:

• stirring the solutions on mixing;

• insulating the reaction mixture;

• using a colorimeter connected to a data-logger to obtain more accurate reaction times.

18 —

19 a) i) The rate of reaction increases with temperature.

ii) When the temperature is increased, the reactant particles gain more energy and move faster. That means particles have more energy and collide more often.