1 Common Mistakes in Chemistry. 2 1.Read the question (5 - 10 s) 2.Think (10 s) 3.Pair-up and discuss your answer (10 – 15 s) 4.One group will be invited.

1

Common Mistakes in Chemistry

2

1. Read the question (5 - 10 s)

2. Think (10 s)

3. Pair-up and discuss your answer (10 – 15 s)

4. One group will be invited to share the answer with whole class (10 – 15 s)

3

What’s wrong with the following statement?

Oxygen relights a growing splint.

4

Correct statement

Oxygen relights a glowing splint.

5


Nitrogen dioxide is an acidic oxide. It dissolves in water to form nitric acid.

6

Correct statement

Nitrogen dioxide is an acidic oxide. It dissolves in water to form nitric acid and nitrous acid.

2NO2 + H2O HNO3 + HNO2

7


Ethanol turns green acidified potassium dichromate solution to orange.

8

Correct statement

Ethanol turns orange acidified potassium dichromate solution to green.

9


Ammonia gas turns red litmus paper blue.

10

Correct statement

Ammonia gas turns moist red litmus paper blue.

11


Water vapour turns cobalt chloride to pink.

12

Correct statement

Water vapour turns dry/ anhydrous/ blue cobalt chloride to pink.

13


Nitrogen is an inert gas.

14

Correct statement

Nitrogen is an inactive gas.

N2(g) + 3H2(g) 2NH3(g)

Nitrogen acts as oxidising agent.

N2(g) + O2(g) 2NO(g)

Nitrogen acts as reducing agent.

15


Chlorine gas turns moist blue litmus paper red.

16

Correct statement

Chlorine gas turns moist blue litmus paper red and then white.

17


Sulphur dioxide turns potassium dichromate from orange to green.

18

Correct statement

Sulphur dioxide turns acidified potassium dichromate from orange to green.

19


Ethene turns acidified potassium dichromate from orange to green.

20

Correct statement

Ethene turns acidified potassium permanganate from purple to colourless.

CH2 CH2

H2O +[O]CH2 CH2

OH OH

21


Carbon dioxide turns limewater milky.

The white precipitate is not soluble in excess carbon dioxide.

22

Correct statement

Carbon dioxide turns limewater milky.

Ca(OH)2(aq) + CO2(aq) CaCO3(s) + H2O(l)

Excess carbon dioxide turns the milky solution colourless.

CaCO3(s) + CO2(aq) + H2O(l) Ca(HCO3)2(aq)

23


Hydrogen burns with a ‘pop’ sound.

24

Correct statement

Pure hydrogen burns with a blue flame.

A mixture of hydrogen and air/ oxygen burns with a ‘pop’ sound.

We can use a burning splint to test for hydrogen gas collected in a test tube because hydrogen mixes up with air when we remove the stopper of the test tube.

25


Seperating funnel is used to seperate two immiscible liquids.

26

Correct statement

Separating funnel is used to separate two immiscible liquids.

27


We use mortar to grind the calcium carbonate into powder.

28


We use mortar and pestle to grind the calcium carbonate into powder.

29


We use spoon to transfer chemicals from reagent bottle to the test-tube.

30


We use spatula to transfer chemicals from reagent bottle to the test-tube.

31


Tripod stand, wire gauze, Bunsen burner, crucible & heat-proof mat are used to convert magnesium to magnesium oxide.

32


Tripod stand, pipe-clay triangle, Bunsen burner, crucible & heat-proof mat are used to convert magnesium to magnesium oxide.

33


pH of 1M hydrochloric acid is higher than that of 1M ethanoic acid.

34

Correct statement

pH of 1M hydrochloric acid is lower than that of 1M ethanoic acid.

35


The oxidation number of silver in Ag2O is 1.

36

Correct statement

The oxidation number of silver in Ag2O is +1.

37


Acid is a hydrogen-containing compound which dissolves in water to give H+(aq).

38

Correct statement

Acid is a hydrogen-containing compound which dissolves in water to give H+(aq) as the only cation.

39

The following answer is imprecise, amend it.

Base is a substance which reacts with acid to give salt and water only.

Base comprises metal oxide and metal hydroxide.

40

Precise answer

Base is a substance which reacts with acid to give salt and water only.

Base comprises metal oxide, metal hydroxide and ammonia.

41


Salt is a compound derived from acid. The hydrogen ion in acid is replaced by metal ion.

42

Precise answer

Salt is a compound derived from acid. The hydrogen ion in acid is replaced by metal ion or ammonium ion.

43


In the electrolysis of dilute sodium chloride solution, hydrogen ion discharges instead of sodium ion because hydrogen ion is above sodium ion in the electrochemical series.

44

Correct statement

In the electrolysis of dilute sodium chloride solution, hydrogen ion discharges instead of sodium ion because hydrogen ion is below sodium ion in the electrochemical series.

45


In the electrolysis of dilute sodium chloride solution, hydroxide ion discharges instead of chloride ion because hydroxide ion is below chloride ion in the electrochemical series.

46

Correct statement

In the electrolysis of dilute sodium chloride solution, hydroxide ion discharges instead of chloride ion because hydroxide ion is above chloride ion in the electrochemical series.

47

What’s wrong with the following structure?

C C C

HH

H H H

HH

48

Correct structure

C C C

H

H H H

HH

49


The functional group of ethanol is –HO.

50

Correct statement

The functional group of ethanol is –OH.

51


C C C

HH

H H H

HH

OH

propan -1- ol

C C C

HH

H H H

H

H

OH

propan -1- ol

I II

52

Correct structure

C C C

HH

H H H

HH

OH

propan -1- ol

53


Helium has an octet structure.

54

Correct statement

Helium has a duplet structure.

55


Na is a stronger oxidising agent than Mg because Na loses electron more readily than Mg.

56

Correct statement

Na is a stronger reducing agent than Mg because Na loses electron more readily than Mg.

57


During the electrolysis of dilute sulphuric acid, hydrogen discharges at the cathode.

58

Correct statement

During the electrolysis of dilute sulphuric acid, hydrogen ion discharges at the cathode.

59


During the electrolysis of dilute zinc sulphate, hydroxide ion discharges preferentially at the anode because hydroxide ion is below sulphate ion in the electrochemical series.

60

Correct statement

During the electrolysis of dilute zinc sulphate, hydroxide ion discharges preferentially at the anode because hydroxide ion is above sulphate ion in the electrochemical series.

61


All sulphates are soluble in water except calcium sulphate.

62

Correct statement

All sulphates are soluble in water except calcium sulphate, barium sulphate, strontium sulphate and lead sulphate.

63


All carbonates are insoluble in water except sodium and potassium carbonate.

64

Correct statement

All carbonates are insoluble in water except sodium, potassium and ammonium carbonate.

65


Nylon is a biodegradable condensation polymer.

C

O

(CH2)4 C

O

Cl

+ N

H

H

(CH2)6 N

H

H

condensation polymerization

C(CH2)4C N(CH2)6N

H

O

H

O

n n

n

+ (2n- 1)

Nylon

hexane- 1,6- diamine

Cl

hexane- 1,6- dioyl dichloride

HCl

66

Correct statement

Nylon is a non-biodegradable condensation polymer.

C

O

(CH2)4 C

O

Cl

+ N

H

H

(CH2)6 N

H

H

condensation polymerization

C(CH2)4C N(CH2)6N

H

O

H

O

n n

n

+ (2n- 1)

Nylon

hexane- 1,6- diamine

Cl

hexane- 1,6- dioyl dichloride

HCl

67


Urea methanol is a thermosetting polymer.

68

Correct statement

Urea methanal is a thermosetting polymer.

O

CH H

CN N

O

H

H H

H CN N

O

H

H H

H

urea urea

methanal

CN N

O

H

H H

C CN N

OH H

H

H

H

69


Isotopes have similar chemical properties.

70

Correct statement

Isotopes have same chemical properties.

71


Electrolyte is a compound which in aqueous solution or when molten, conducts electricity.

72

Correct statement

Electrolyte is a compound which in aqueous solution or when molten, conducts electricity and is decomposed in the process.

73


Members of a homologous series have the same chemical properties but different physical properties.

74

Correct statement

Members of a homologous series have similar chemical properties but graduation in physical properties.

75


An advantage of using iron over PVC in making water pipe is that iron is harder than PVC.

76

Correct statement

An advantage of using iron over PVC in making water pipe is that iron has a higher tensile strength than PVC.

77

What’s wrong with the following formula?

•NaSO4

•Zn(Cl2)

•KCr2O7

•(NH3)2SO4

78

Correct formula

•Na2SO4

•ZnCl2

•K2Cr2O7

•(NH4)2SO4

79


repeating unit of poly(propene)

C C

H

H H

H

C

H

H

80

Correct structure

repeating unit of poly(propene)

C C

H

H H

CH3

81

What’s wrong with the following electronic structure?

nitrogen

N N

82

Correct electronic structure

Nitrogen – octet structure for each nitrogen atom

N N

83

What’s wrong with the following ionic equation?

Ca2+(aq) + Na2CO3(aq)

CaCO3(s) + 2Na+(aq)

84

Correct ionic equation

Ca2+(aq) + CO32-(aq) CaCO3(s)

Na2CO3(aq) is an electrolyte.

85

What’s wrong with the following ionic equation?

CaCO3(aq) + 2H+(aq)

Ca2+(aq) + H2O(l) + CO2(g)

86

Correct equation

CaCO3(s) + 2H+(aq)

Ca2+(aq) + H2O(l) + CO2(g)

Calcium carbonate is insoluble in water and hence no mobile carbonate ions are available for the reaction.

87

What’s wrong with the following equation?

CH3COOH(aq) CH3COO-(aq) + H+(aq)

88

Correct equation

CH3COOH(aq) CH3COO-(aq) + H+(aq)

Ethanoic acid is a weak acid. Ionization is incomplete.

89


NH3(aq) + H2O(l) NH4+(aq) + -OH(aq)

90

Correct equation

NH3(aq) + H2O(l) NH4+(aq) + -OH(aq)

Aqueous ammonia is a weak base. Ionization is incomplete.

91


Solid iodine is purple.

92

Correct statement

I2(s) is black I2 in KI is brown (iodine is slightly soluble in water, but highly soluble in KI solution.) I2 vapour is purple

93


NH3(g) + HCl(g) NH4Cl(g)

94

Correct equation

NH3(g) + HCl(g) NH4Cl(s)

Ammonium chloride is an ionic compound. It is a solid at room temperature and pressure.

95


Cu(s) + 2HCl(aq) CuCl2(aq) + H2(g)

96

Correct equation

Copper is below hydrogen in the electrochemical series, it does not react with dilute hydrochloric acid.

97


conc. H2SO4

C6H12O6(s) 6C(s) + 6H2O(l)

Table sugar (cube sugar) undergoes dehydration.

98

Correct statement

The main constituent of cube sugar/ table sugar is sucrose.

conc. H2SO4

• C12H22O11(s) 12C(s) + 11H2O(l)

99

What’s wrong with the following diagram?

A

Copper plate

Copper(II)

Solutionchloride

anodecathode

variable resistor

ammeterdry cell

100

Correct diagram

A

Copper plate

Copper(II)

Solutionchloride

anode cathode

variable resistor

ammeterdry cell

101


Ester can be formed by heating alkanoic acid with an alkanol.

102

Precise answer is needed

Ester can be formed by heating alkanoic acid with an alkanol under reflux in the presence of concentrated sulphuric acid. H2SO4

e.g. CH3COOH + C2H5OH CH3COOC2H5 + H2O

103


Copper(II) ion is blue in colour.

104


Aqueous copper(II) ion is blue in colour.

105


Aluminium foil is corrosion resistant because it has an oxide layer on its surface.

106


Oxide layer on the surface of aluminium foil is impervious/ impermeable to oxygen or water

107


Electrical conductivity of copper(II) sulphate solution is due to ‘free ions’

108


Electrical conductivity of copper(II) sulphate solution is due to ‘mobile ions’ not ‘free ions’

109


Mixing of toilet cleaner and bleaching solution is dangerous.

110


Toxic chlorine gas is produced on mixing toilet cleaner and bleaching solution.

Cl- + ClO- + 2H+ Cl2 + H2O

111


Both chlorine bleach and chlorine water are alkaline.

112

Precise answer is neededChlorine bleach is alkaline and chlorine water is acidic. Chlorine bleach is a solution of sodium hypochlorie, which is a salt of strong base and weak acid.

Cl2 + H2O HCl + HOCl

Chlorine water is a solution of hydrochloric acid and hypochlorous acid.

113

The following answer is incorrect, amend it.

Mean volume of titrant used = (15.0 + 15.7 + 15.8 + 15.9)/4 = 15.4 cm3

1st 2nd 3rd 4th

15.0 15.7 15.8 15.9

114

Correct answer

1st 2nd 3rd 4th

15.0 15.7 15.8 15.9

• Mean volume of titrant used = (15.7 + 15.8 + 15.9)/3 = 15.8 cm3

the datum 15.0 should be discarded

115


Formula mass of methanol is 30 g

116

Correct answer

•Formula mass of methanol is 30

117


In sacrificial protection, a more reactive metal (e.g. Zn) is put near a less reactive metal (Fe)

118


In sacrificial protection, a more reactive metal (e.g. Zn) is connected to a less reactive metal (Fe)

119


PVC is formed by additional polymerisation of chloroethene

120

Correct answer

PVC is formed by addition polymerisation of chloroethene

121

Common Mistakes in Chemistry(Part 2)

122

1. Read the question (5 - 10 s)

2. Think (10 s)

3. Pair-up and discuss your answer (10 – 15 s)

4. One group will be invited to share the answer with whole class (10 – 15 s)

123


Metal reacts with dilute hydrochloric acid to give hydrogen.

124


Metals below hydrogen in the electrochemical series (Cu, Ag, Au) cannot displace hydrogen from dilute acids.

Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)

125


126


Copper does not react with acid.

127


Copper does not react with dilute acid. Cu reacts with concentrated sulphuric acid and concentrated nitric acid.

Cu(s) Cu2+(aq) + 2e-

SO42-(aq) + 2e- + 4H+(aq) SO2(g) + 2H2O(l)

128


Rusting of iron is a redox reaction. Iron is oxidised in the presence of water.

129


Rusting of iron is a redox reaction. Iron is oxidised in the presence of water and oxygen.

Fe(s) Fe2+(aq) + 2e-

O2(g) + 2H2O(l) + 4e- 4-OH(aq)

130


After scratching, galvanized iron rusts faster than pure iron.

131


After scratching, tin-plated iron rusts faster than pure iron because tin is below iron in the ECS.

Zinc is above iron in the ECS. By sacrificial protection, galvanized iron does not rust after scratching.

132


On the addition of excess iron(II) sulphate, the acidified potassium permanganate turns from purple to colourless.

133


On the addition of excess iron(II) sulphate, the acidified potassium permanganate turns from purple to yellow (aqueous iron(III) ion is yellow).

134


Calcium granules react with excess dilute sulphuric acid readily and completely. The resulting solution is colourless.

135


Calcium granules react with dilute sulphuric acid readily and the reaction stops after a short while. This is because CaSO4 is insoluble in water. It forms a layer on the surface of the calcium granules and stops further reaction.

136


Excess calcium is added to dilute hydrochloric acid. The resulting solution is calcium chloride.

137


Excess calcium is added to dilute hydrochloric acid. The resulting solution is a mixture of calcium chloride and calcium hydroxide.

138


The following reactions are reversible: esterification ionization of ethanoic acid SO2 combines with O2 to form SO3

neutralization

139


The following reactions are reversible: esterification ionization of ethanoic acid SO2 combines with O2 to form SO3

except

neutralization

140


Electricical conductivity of 1M ethanoic acid is lower than that of 1M ethanol because the concentration of mobile ions in 1M ethanoic acid is lower than that in 1M ethanol.

141


Electricical conductivity of 1M ethanoic acid is higher than that of 1M ethanol because the concentration of mobile ions in 1M ethanoic acid is higher than that in 1M ethanol.

142


Aqueous solution of ammonium chloride is alkaline.

143

Precise answer is neededAqueous solution of ammonium chloride is acidic. H2O H+ + -OHNH4Cl NH4

+ + Cl-

NH3 is a weak alkali, NH4+ combines with –OH.

NH4+ + -OH NH3 + H2O

HCl is a strong acid and Cl- does not combine with H+.-OH is removed, but not H+, so the solution is acidic.

144


Aqueous solution of sodium ethanoate is acidic.

145

Precise answer is neededAqueous solution of sodium ethanoate is alkaline. H2O H+ + -OHCH3COONa Na+ + CH3COO-

CH3COOH is a weak acidic, CH3COO- combines with H+.

CH3COO- + H+ CH3COOHNaOH is a strong alkali & Na+ does not combine with -OH.

H+ is removed, but not -OH, so the solution is alkaline.

146


A metal which is lower in the ECS can displace another metal higher in the ECS from its salt solution.

147

Correct answer is needed

A metal which is higher in the ECS can displace another metal lower in the ECS from its salt solution.

Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s)

148


Flame colour of sodium compounds is intense yellow while that of calcium is bluish-green.

149


Flame colour of sodium compounds is intense yellow while that of calcium is brick-red.

Flame colour of copper(II) compounds is bluish-green.

150


Metal hydroxide is precipitated out when sodium hydroxide is added to an aqueous solution of a salt.

151


Insoluble metal hydroxides (e.g. Cu(OH)2, Fe(OH)2, Fe(OH)3, Zn(OH)2) are precipitated out when sodium hydroxide is added to Cu2+(aq), Fe2+

(aq), Fe3+(aq), Zn2+(aq).

152


Carbon monoxide can be used to reduce metal oxide to metal.

153


Carbon monoxide can be used to reduce oxides of moderately active metals (e.g. iron, lead, tin, copper) to metal.

Electrolysis is used to extract active metals (e.g. Al, Mg, Na) from their molten oxides or chlorides.

154


Alkanols are miscible with water.

155


Simple alkanols (methanol, ethanol and propan-1-ol) are miscible with water. Miscibility decreases with the size of alkanol.

156


Soapy detergent acts as a wetting agent because it can increase the surface tension of water.

157


Soapy detergent acts as a wetting agent because it can decrease the surface tension of water.

158


Detergent particles have hydrophilic heads which are soluble in oil or grease.

159


Detergent particles have hydrophobic tails which are soluble in oil or grease.

160


Detergent forms scum in hard water.

161


Soapy detergent forms scum in hard water.

162


The melting point range of petrol is narrow because petrol is a mixture.

163


The melting point range of petrol is wide because petrol is a mixture.

164


The viscosity of lubricating oil is lower than that of petrol because the intermolecular forces between hydrocarbon molecules in lubricating oil are stronger than that in petrol.

165


The viscosity of lubricating oil is higher than that of petrol because the intermolecular forces between hydrocarbon molecules in lubricating oil are stronger than that in petrol.

166


Incinerator is commonly used to dispose of plastic wastes in Hong Kong.

167


Landfilling is commonly used to dispose of plastic wastes in Hong Kong.

168


Injection moulding is used to manufacture thermosetting polymer.

169


Injection moulding is used to manufacture thermoplastic polymer. OR

Compression moulding is used to manufacture thermosetting polymer.

170


Catalytic converter in car is used to reduce the emission of carbon monoxide and nitrogen oxides

171


Catalytic converter in car is used to reduce the emission of carbon monoxide, nitrogen oxides and unburnt hydrocarbons.

172


In the electrolysis of very dilute sodium chloride solution, hydrogen gas is formed at the cathode because hydrogen accepts electrons more readily than sodium.

173


Hydrogen gas is formed at the cathode because hydrogen ion accepts electrons more readily than sodium ion.

174


Sodium hydroxide, because it is alkaline, can emulsify oil.

175

Precise answer is neededOil undergoes hydrolysis (breaks down to form glycerol and sodium salt of alkanoic acid) in alkaline medium.

NaOH3+

RCOO-Na+

R'COO-Na+

R'COO-Na++

CH2OH

CHOH

CH2OH

CH2O C

O

CHO C

O

CH2O C

O

R

R'

R"

176


Quartz has giant covalent bond.

177


Quartz has giant covalent network/ structure.

178


In a study of the effect of concentration change on reaction rate, the reaction rate between hydrochloric acid and solid calcium carbonate is used. Identical apparatus are used and the following factors are kept constant except the concentration of HCl(aq): mass of calcium carbonate, temperature, volume of HCl(aq)

179


In a study of the effect of concentration change on reaction rate, the reaction rate between hydrochloric acid and solid calcium carbonate is used. Identical apparatus are used and the following factors are kept constant except the concentration of HCl(aq): mass of calcium carbonate, temperature, particle size of CaCO3

volume of HCl(aq)

180


In a study of the effect of temperature change on reaction rate, the reaction rate between thiosulphate and acid is used. Time taken for the disappearance of a cross below the conical flask is compared. Identical apparatus are used and the following factors are kept constant except the reaction temperature: volume & concentration of thiosulphate, volume & concentration of acid

181


In a study of the effect of temperature change on reaction rate, the reaction rate between thiosulphate and acid is used. Time taken for the disappearance of a cross below the conical flask is compared. Identical apparatus are used and the following factors are kept constant except the reaction temperature: volume & concentration of thiosulphate, volume & concentration of acid, same student is involved in observing the disappearance of cross

182


Reactivity increases down group 7.

Reactivity decreases down group 1.

183

Correct answer is needed

Reactivity decreases down group 7.

Reactivity increases down group 1.

184


The attraction between atoms in gold is ionic bond.

185


The attraction between atoms in gold is metallic bond.

186


Relative atomic mass is the weighted average of isotopic masses.

187


Relative atomic mass is the weighted average of isotopic masses on the carbon-12 scale.

188


Ethyne (C2H2) burns with a more sooty flame than ethane (C2H6) because ethyne has a smaller molecular size.

189


Ethyne (C2H2) burns with a more sooty flame than ethane (C2H6) because ethyne has a higher carbon content than ethane.

190


Leakage of town gas is dangerous because carbon monoxide, a constituent of town gas, is toxic.

191


Leakage of town gas is dangerous because town gas forms an explosive mixture with air and carbon monoxide, a constituent of town gas, is toxic.

192


Sulphur dioxide is toxic.

193


Sulphur dioxide attacks the respiratory system. Individuals who suffer from chronic respiratory diseases such as bronchitis or asthma are much more susceptible to attack

194


In the titration of sodium hydroxide with hydrochloric acid. At the end point, the methyl orange indicator turns orange.

195


Methyl orange changes from yellow to orange.

Initial and final colour should be specified.

196

Name a gas which could be collected by method (a):

197

Examples

Gaseous substances should be heavier than air and do not react with air, e.g. HCl, Cl2, SO2.

Average molar mass of air is 28.8g mol-1.

198

Name a gas which could be collected by method (b):

199

Examples

Gaseous substances should be lighter than air and do not react with air, e.g. H2, NH3.

200

Name a gas which could be collected by method (c):

201

Examples

Gaseous substances should be insoluble in water e.g. H2, N2 .

202

How can the Bunsen value prevent sucking back?

203

Bunsen valve is made of rubber, which is flexible. It allows gaseous product to move outward but prevent water to flow in the opposite direction.

204

Why is an inverted funnel used here?

205

• It provides a larger contact area for the dissolution of ammonia, sucking back is prevented.

206

Should you need further assistance, please send your e-mail to Mr. S.K. Leung ([email protected].)

mailto:[email protected]

207

Homework

Based on the mistakes you have made in your assignment, prepare ten similar questions and answers.

1 Common Mistakes in Chemistry. 2 1.Read the question (5 - 10 s) 2.Think (10 s) 3.Pair-up and discuss your answer (10 – 15 s) 4.One group will be invited.

Documents

following statement

s whats wrong

correct statementammonia

correct statementchlorine

correct statementethanol

correct statementethene

correct statementoxygen

correct statementpure