Work Book -Alkanes, Alkenes, Alcohols & Acids

Work Book/Organic Chemistry Page

Mahatma Gandhi School, Jakarta By: Manoj K. Agnihotri

1

Name of the student ………………………………….

Grade : …………………



2

Question 1 Write the molecular formula of the alkane molecules with 10 carbon atoms.

What is the physical state likely to be at r.t.p?

Answer 1. The molecular formula of the alkane molecule with 10 carbon atoms is C10H22

This formula is obeying the general formula of alkane, which is Cn H2n +2

The structural formula will be different.

……………………………………………………………………………………………………………..

Question 2. From the table of homologous series the molecular formula of nonane is C9H20.

(a) Write the molecular formula, relative molecular mass and structural

formula of nonane. (b) Draw a graph of the boiling point (vertical axis) against the Mr of the

alkanes.

Use your graph to find the approximate boiling point of the nonane.

Answer2. (a) From the table of homologous series

The molecular formula of nonane is C9H20

The relative molecular mass (Mr) =

The structural formula is CH3-(CH2)7-CH3

Or

……………………………………………………………………………………………………

(b) From the table, as B. P. increases Mr also ncreases.

Mr B.P (oC)

16 -160

30 -89

44 -42

58 -0.5

72 36

86 69

100 98

114 126

Tem

p. in

OC

Mr

200

- 200

-150

-100

-50

150

100

50



3Question 3. The structural formula of pentane is

CH3 – CH2 –CH2 – CH2 − CH3

Write structural formula of two hydrocarbons which are isomer of pentane.

Answer. The structural formula of the isomer of pentane are:

First structural formula

CH3 – CH2 –CH – CH3 or CH3 – CH –CH2 – CH3

l l

CH3 CH3

or or

CH3

CH3 l

l CH3 – CH –CH2 – CH3

CH3 – CH2 –CH – CH3

Name : Isopentane

* It does not matter whether the alkyl group (CH3) attached above or below.

Just one structural formula (All are similar)

Second structural formula

CH3

l

CH3 – C– CH3

l

CH3

Name ; Neo pentane (Only one possible way to write)

Question 4. Write the structural formulas of two isomers with molecular formula C2H4Br2.

Answer The structural formula of C2H4Br2 are:

………………………………………………… ………………………………………………

Question 5. Write molecular formula and structural formula of the next member of the C2H4, C3H6, C4H8 this series.

Answer. Three alkenes; C2H4, C3H6, C4H8 forms the homologous series.

The molecular formula of the next member is C5H10.

(This molecular formula is obeying the general formula for alkenes, CnH2n)

The structural formula is CH3-CH2-CH2-CH =CH2

Also write its structural formula in the flat form

……………………………………………………………………………………………………………….



4

Question 6. Decene is an alkene containing ten carbon atoms in one molecule. Write its

molecular formula. Answer. The molecular formula of decene is …………………………………. Question 7.

(a) Some reactions of butene are shown in the following diagram. Write

the structural formulas of the products in the three boxes.

H C

H

H

C

H

H

C

H

C

H

H

+ hydrogen REACTION A

+ bromineREACTION B

+ steam REACTION C

(b) What other substance is needed in reaction A?

(c) Name the product of reaction A.



5Answer (a)

H C

H

H

C

H

H

C

H

C

H

H

+ hydrogen REACTION A

+ bromineREACTION B

+ steam REACTION C

H C

H

H

C

H

H

C

Br

C

H Br

H

H

Addition of Br2

will take place

across the

double bond

H C

H

H

C

H

H

C

H

C

H H

H

H

Addition of H2

will take place

across the

double bond

H C

H

H

C

H

H

C

H

C

H OH

H

H

Addition of H2O

will take place

across the

double bond Answer (b) Other substance needed is a catalyst Ni

Answer (c) The product of reaction A is alcohol (Butanol)

Question 8. Write a balanced chemical equation in each case to show the cracking of C20H42, to produce

(a) an alkane with 6 carbon atoms and an alkene molecule;

(b) 8 ethene molecules and one other alkene molecule and hydrogen;

(c) an alkene with 12 carbon atoms, one other alkene molecule and

hydrogen. Answer cracking (a) C20H42 C6H14 + C14H28

Alkane Alkene

cracking (b) C20H42 8 C2H4 + C4H10

Ethene Butane

Cracking (c) C20H42 ………..+ …………… + H2



6

Question 9 Complete the following table.

Question 10

Compound

Formula

Boiling point/oC

A CH4 −162

B C2H4 −104

C C2H6 −89

D C3H6 −48

E C3H8

F C4H10 0

(a) (i) The hydrocarbons in the table above belong to two homologous

series.

Name these two series

Alkane series Alkene series

Answer ……………………………………… ……………………………………

(ii) State which compound belongs to each series

Answer CH4, C2H6, C3H8, C4H10 (Alkane series)

C2H4, C3H6, (Alkene series)

Name of the homologous series

Name of the first member

Structural formula of the first member

Molecular Formula of the first member

General formula of the series

alkanes

methane

H l

H – C – H

l

H

Cn H2n +2

C C

H

HH

H

silanes

silane

Si

H

HH

H

SiH4

Sin H2n +2



7 (b) State one method that could be used to separate a mixture of

the six hydrocarbons

Answer ………………………………………………………………………………………….

(c) Which hydrocarbon is the main component of the natural gas?

Answer ……………………………………………………….

(Hint lower hydrocarbons from C1 to C4)

(d) Is compound F is a solid, liquid or gas at a room conditions?

Answer …………………………………………………………………………

(e) From the table predict the boiling point of compound E

Answer The boiling point of this compound should be close to C3H6

Since it also contains three carbon atoms, since the number

Of hydrogen atoms are higher in case of C3H8 comparatively to C3H6,

therefore,its boiling point must be higher than C3H6 and lower than

00C.

−30 or −20 or −10

(f) Compound D and E can be distinguished by a chemical test.

(i) Name a reagent used for the test.

Bromine water (unsaturation test)

Answer ……………………………………………………………

(ii) State what it seen in the test with compound D

Answer Reddish-brown colour of bromine water will de-colourise.

(iii) State what is seen in the test with compound E.

Answer The compound E is an ‘alkane’ (saturated aliphatic hydrocarbon)

Therefore, no de-colourisation of bromine water will take place

in case of compound E.

(g) Compound B and D belong to the same homologous series.

Calculate the relative molecular mass, Mr of the next member of

the series

Answer C4H10 Mr =58



8

Question 11. Study the reaction scheme below and then answer the following

questions.

Petroleum C10H22

Cracking

Compound XC2H5OH

Reagent Z

bromine

Compound Y

C2H6

Reaction II

Hydrogen

CO2 and

water

Reaction III

C2H5Cl

Reaction V

Hydrogen chloride

Reaction IV

Chlorine

C2H5NH2

reaction VI

ammonia

Process I

(a) Write the name and formula of the organic compound X

Answer ………………………………………………………………………………………………

(b)

(i) What is the name used for process I?

Answer …………………………………………………………………………………

(ii) What other substance is needed for reaction II?

Answer ………………………………………………………………………………………………

(iii) What type of reaction is reaction II?

Answer ………………………………………………………………………….

(iv) What type of reaction is reaction III?

Answer ………………………………………………………………………….



9

(v) What type of reaction is reaction IV?

Answer ………………………………………………………………………….

(c) Suggest the type of reaction that takes place in

(i) reaction V?

Answer ……………………………………………………………………………………………….

(ii) reaction VI?

Answer ……………………………………………………………………………………

(d) Write the formula of the compound Y?

Answer ……………………………………………………………………………………………..

(e) Name reagent Z

Answer …………………………………………………………………………………………

(f) (i) Name the homologous series to which C10H22 belongs

Answer …………………………………………………………………………………………

(ii) Write the formula of the next member of this series

Answer …………………………………………………………………………………………

Question 12 In an experiment, 0.04 mole of hydrocarbon Z was completely burnt in

oxygen, producing 2.88 dm3 of CO2 measured at room conditions.

(a) Calculate

(i) the number of moles of gas in 2.88 dm3 of CO2.

Answer From the formula

Volume of the gas = Number of moles × Molar volume

………………………………………………………………………………………………………..

……………………………………………………………………………………………………….

(ii) the number of moles of CO2 produced by the combustion of 1

mole of Z

Answer From the above result,

0.04 mole of hydrocarbon Z produces 0.12 moles of CO2

Therefore,

1 mol of hydrocarbon Z will produce 0.12

-------- = 3.0 moles of CO2

0.04



10

(iii) the number of carbon atoms in one molecule of Z

Answer It is important to note that all carbon atoms present in any

hydrocarbon will

change into carbon dioxide.

Therefore same numbers of carbon atoms are present in the

CO2 on complete combustion of any hydrocarbon.

For example, C2H4 on complete combustion produces 2 carbon

atoms in the form of CO2

C2H4 + 3O2 → 2CO2 + 2H2O

2 moles of CO2 contains 2 carbon atoms in CO2 as well as 2

carbon atoms in C2H4

……………………………………………………………………………………………………………………………..

………………………………………………………………………………………………………………………………

Question 13 (a) A saturated hydrocarbon contains 83.33 % by mass of carbon.

(i) Explain what is meant by the term saturated.

Answer It means maximum amount of the hydrocarbon in a given amount of the

substance.

(ii) Deduce the molecular formula of the hydrocarbon and give one

possible structural formula for it.

Answer By assuming that total mass of the hydrocarbon is 100 g

Element

C H Mass of each element (g)

Molar mass (i. e. Ar in grams)

Number of moles

Mass = ----------------- Molar mass

Simplest atomic ratio

(It is obtained by dividing

each number of moles by the

smallest number)

Simplest whole number

atomic ratio

5 12

The molecular formula of the hydrocarbon is

The structural formula of hydrocarbon is ………………………………………………………



11

(b) Saturated hydrocarbons undergo cracking in industry to produce smaller

molecules, including alkenes. These alkenes are used to make useful

chemicals by addition reactions

(i) What is meant by the term cracking.

Write a balanced equation for a cracking reaction and name the

alkene products

Answer ……………………………………………………………………………………………………

……………………………………………………………………………………………….

(ii) Explain what is meant by the term addition reaction.

………………………………………………………………………………………

(iii) Give the chemical name, formula and one important use of an organic

compound produced by an addition reaction of the alkene named in (i)

above.

Answer Chemical name: …………………………………………………………………

Chemical Formula: ………………………………………………….

Use: ……………………………………………………

Question 14 A Hydrocarbon, C6H6 is the first member of the homologous series with the

general formula CnH2n −x, where n is the number of carbon atoms in the

molecule.

(a) (i) Deduce the value of x in the general formula.

Answer : ………………………………………………………………………………………………

(i) Write the formula of the second member of this homologous

series.

Answer : ………………………………………………………………………………………………

(b) The hydrocarbon, C6H6 is poisonous and non biodegradable.

(i) What is meant by non -biodegradable?

Answer : …………………………………………………………………………………………….

(ii) Suggest the equation for a chemical reaction in which you

would attempt to destroy unwanted C6H6.

Answer : Combustion reaction

2C6H6 + 15O2 → 12CO2 + 6H2O

…………………………………………………………………………………………….



12

(c) The hydrocarbon, C6H6 undergoes a substitution reaction with chlorine

under the right conditions. One of the products of the reaction is

hydrogen chloride

Construct a balanced chemical equation for the reaction and use it to

explain the meaning of the substitution reaction.

Answer :

Cl2FeCl3

Cl

HCl+ +

C6H6

Chlorobenzene

Benzene

Question 15 (a) The hydrocarbon, C6H14 forms isomers.

Explain the meaning of the term isomers, using two isomers of the

hydrocarbon as examples.

Suggest one difference in the physical properties of these isomers.

Answer …………………………………………………………………………………………………………

……………………………………………………………………………………………………….

……………………………………………………………………………………………………….

(b) Methane is the first member of the alkane homologous series.

Another homologous series of hydrocarbons has the general formula

Cn H2n−2 (where n is the greater than 1).

(i) Write molecular formula of the first member of this other series

of the hydrocarbons.

Answer …………………………………………………………………………………………………………

(i) Sketch a diagram to show the possible electronic structure of a

molecule of this hydrocarbon (outer shells only).

Answer …………………………………………………………………………………………………………

(ii) The hydrocarbons in (i) can be produced by the reaction of

water with calcium carbide, CaC2. Deduce the equation for the

reaction

CaC2 + 2H2O → C2H2 + Ca(OH)2

Answer …………………………………………………………………………………………………………



13 Question 16 (a) The general formula of the alkane series of is CnH2n+2. One member of this

series contains 82.76% by mass of carbon.

(i) Calculate the value of n for the hydrocarbon which contains 82.76 % by

mass of carbon. Answer: In order to calculate the value of n for the hydrocarbon, the molecular formula

of the hydrocarbon is to be calculated first. Then from the molecular formula

the value of n can be obtained.

By assuming that total mass of the hydrocarbon is 100 g

Element

C H Mass of each element (g)

82.76 17.24

Molar mass (i. e. Ar in grams)

12 1

Number of moles

Mass = ----------------- Molar mass

82.76

---------- = 6.89 12

17.24

-------- = 17.24

1.0

Simplest atomic ratio

(It is obtained by dividing

each number of moles by the

smallest number)

6.89 ------- = 1.0

6.89

17.24

-------- = 2.50

6.89

Simplest whole number

atomic ratio

1

2 or 4

2.5

5 or 10

The molecular formula of the hydrocarbon is C2H5 is not acceptable because it is

Given in the question that it belongs to the general formula CnH2n+2. Therefore,

Acceptable molecular formula is C4H10

This molecular formula belongs to this alkane series.

Therefore, the value of n is 4……

(ii) Give one full structural formula for this hydrocarbon.

Answer: …………………………………………………………………………………………………….

(b) One of the industrial uses of hydrocarbons involves ‘cracking’ them into

mixtures of smaller molecules which involves various alkenes.

These alkenes are then used to make synthetic macromolecules.

(i) Suggest the name of one of the alkenes which may be present in the

mixture formed by cracking in the hydrocarbon in (a).

Answer: From the molecular formula of the hydrocarbon as calculated above the

name of the alkenes is ascertained.

Cracking C4H10 -------------�� C2H4 + C2H6

Ethene



14

(ii) Explain what is meant by synthetic macromolecules?

Answer: Those macromolecules/ polymers prepared by synthetic method called

synthetic macromolecules.

These are synthetic polymers like polyethene, PVC, Nylon etc.

(ii) Name a type of macromolecules which occurs naturally. Give one use

named example of this type of macromolecules.

Answer: ………………………………………………………………………………………. ……………………

Question 1 Propanol reacts with a hot mixture of sulphuric acid and potassium dichromate

(VII). Write the molecular formula and structural formula of the organic

compound that you would except to be formed.

What type of organic compound is this product?

H2SO4 + K2Cr2O7

(Hot)

Answer CH3-CH2-CH2-OH CH2 = CH2 + H2O

Ethene

Molecular formula of this (ethane ) is …………………..

Structural formula of ethene is CH2 = CH2

The structural formula of ethane in flat form is ……………………………………..

This is an alkene organic compound.

Question 2 Write the molecular formula and structural formula of the acid with 5 carbon

atom in one molecule.

Answer Molecular formula is ……………………………………………………………………………

Structural formula is ……………………………………………………………………………

Question 3 Construct an equation for the reaction of ethanoic acid with copper (II) oxide .

Answer CH3COOH + CuO Cu (CH3COO)2

Copper acetate / Copper ethanoate

SALT



15 Question 3 Write a chemical names and formulas of the two organic compounds from

which the ester below is made.

H C

O

O C

H

H

C

H

H

C

H

H

H

Answer From the structural formula given above, the following conclusions can be

made. Ester is always made by the combination of acid and alcohol together.

H C

O

O C

H

H

C

H

H

C

H

H

H

This part came

from the acidThis part came from

the alcohol

Conclusion: The acid molecule contains only one carbon atom.

HCOOH (methanoic acid)

The alcohol molecule has three carbon atoms.

CH3-CH2-CH2-OH (Propanol)

Question 4 Write the structural formula of the ester produced by reacting ethanol with

propanoic acid.

Answer CH3-CH2-CH2-COOH + OH- CH2-CH3 …………………………………………………………………………………….



16 Question 5 The structural formulas of eight organic compounds, A to H are shown

Below. Use the letters for your answers.

H O C

H

H

C

H

H

C

H

H

H

A

H C

H

H

C

H

H

C

H

H

HC O

O

B

C

H

C C

H

H

H

H

H

C

H C

H

H

C

O

H

C

H

H

H

H

D

H C

H

H

C

O

O H

E

H C

H

H

C

H

C

H

H

H

C

H

HH

F

H C

O

O H

G

H C

H

H

C

Cl

Cl

Cl

H

(a) Which compounds are (i) alcohols and (ii) acids

Answer (i) Alcohols are …………………………………..

(ii) Acids are ………………………………………

(b) Which two compounds are members of the same homologous series?

Answer ……………………………………………………………………………………………………………

(c) Which two compounds are isomers?

Answer Isomers have same molecular formula but different structural formula.

The compound …………………….and ……………………. has same molecular formula.

Therefore, they are isomers.



17

(d) Which compound quickly decolourizes a solution of bromine in water?

Answer This is called unsaturation test. This test given by the olefins/ unsaturated

hydrocarbons i.e. alkenes or alkynes.

……………………………………………………………………………………………………………………..

(e) Which compound has a sweet smell?

Answer ……………………………………………………………………………………….

(f) Which compound is produced by the oxidation of the ethanol?

Answer ……………………………………………………………………………………….

(g) Choose two compounds which would react together to form an ester?

Answer ……………………………………………………………………………………….

(h) Which compound can be made from an alkane by a substitution reaction?

Suggest a use for this compound

Answer ……………………………………………………………………………………….

Use; Trichloroethane is an important organic solvent.

Question 6 Following figures shows the sources of the ethanol manufactured in three countries.

Fermentation

Petroleum

(a) Ethanol is made from petroleum by cracking, followed by reaction with stem.

(i) What organic compounds made in the cracking?

Answer Lower alkanes, lower alkenes or be hydrogen

(iii) Write a balanced chemical equation for the reaction with steam.

Answer …………………………………………………………………………………………………….

Brazil (100%) Singapore (100%)

1

2

USA

30%

70%



18

(b) (i) Name the organic compound from which ethanol is produced in Brazil.

Answer ……………………………………………………………………………………………………….

(ii) A major use of ethanol is produced in Brazil is in motorcars.

Write a balanced chemical when ethanol is used.

Answer ……………………………………………………………………………………………………….

(iii) State one other important use of ethanol.

Answer ……………………………………………………………………………………………………….

(c) Suggest one reason why the diagram for Singapore is different

from that of Brazil

Because source of ethanol is different

Answer ……………………………………………………………………………………………………….

(d) How do you think the diagram for the United States will look 100 years from

now?

Explain your answer.

Answer

After 100 years most of the petroleum reserves will get exhausted

……………………………………………………………………………………………………….

so,

…………………………………………………………………………………………………………….

………………………………………………………………………………………………………………….

Question 7

Butanoic acid, C3H7CO2H, can be made by oxidizing butanol, C4H9OH. In an

experiment, 3.3 g of butanoic acid was produced from the oxidation of

3.7 g of butanol.

(a) (i) Name a suitable oxidizing agent for this reaction.

Answer ……………………………………………………………………………………………………….

(ii) State one observation you would make in this experiment

Answer The purple colour of KMnO4 (an oxidizing agent) will decolourise

or

The colour of K2Cr2O7 (an oxidizing agent) will change from orange to

green.

(b) (i) Calculate the relative molecular mass, Mr of butanol

Answer ……………………………………………………………………………………………………….

(ii) Calculate the number of moles of butanol used in the experiment.

Answer ……………………………………………………………………………………………………….

(iii) Calculate the relative molecular mass, Mr of butanoic acid.

Answer ……………………………………………………………………………………………………….



19

(iv) Calculate the maximum mass of butanoic acid that could be obtained

from the butanol in the experiment.

Answer

C4H9OH → C3H7CO2H

Mr = 74 g Mr = 88 g

74.0 g of butanol produces butanoic acid = 88.0 g

∴ 1.0 g of butanol will produce butanoic acid =

∴ 3.7 g of butanol will produce butanoic acid =

…………………………………………………………………………………………….

(iv) Calculate the percentage yield of butanoic acid in the experiment.

Answer ……………………………………………………………………………………………………….

Question 8 Ethyl ethanoate is made by esterification.

H C C

O

H

H

O H

+ H C C

H

H

O H

H

H

H C C

O

H

H

C C

H

H

H

H

H

O +

H

O

H

Ethanoic acid Ethanol

Ethyl ethanoate

Water

The table gives the relative molecular masses for the reactants and products in the reaction.

(a) In an experiment, 3.0 g of ethanoic acid and 4.6 g of ethanol were heated

together with a catalyst.

(i) How many moles of ethanoic acid were used?

Answer 3.0

Number of moles of ethanoic acid = −−−−−−−−−−−−−−−− Mr of ethanoic acid

3.0

= −−−−− 60

Substance Relative molecular mass (Mr)

Ethanoic acid

Ethanol

Ethyl ethanoate

water

60

46

88

18



20

(ii) How many moles of ethanol were used?

Answer ……………………………………………………………………………………………………….

(iii) Which reactant (ethanol or ethanoic acid) was in excess?

Answer ……………………………………………………………………………………………………….

(iv) What mass of ethyl ethanoate would be made if there was 100 %

conversion?

Answer CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O

Ratio of number of moles,

1

Number of moles of ethyl ethanoate = −−−−−− Number of moles of ethyl ethanoic acid 1

For 100 % conversion →

Number of moles of ethyl ethanoate = 0.05 mol

Mass of the ethyl ethanoate = number of moles × Mr of

Ethyl ethanoate

= 0.05 × 88 = 4.4 g

(v) In the experiment, only 2.20 g of ethyl ethanoate were obtained. What

was the percentage yield of ethyl ethanoate?

Answer ……………………………………………………………………………………………………….

(b) Propanoic acid has the following structural formula.

CH

H

H

C

H

H

H

C

O

O

Draw the structural formula of the ester formed when propanoic acid reacts

with ethanol.

Answer ……………………………………………………………………………………………………….

Question 9 Olive oil contains oleic acid which is unsaturated.

(a) (i) By naming the reagent and giving the observation, describe a

simple test to confirm that olive oil contains an acid.

Answer Reagent …………………………………………………………………………..

Observation ………………………………………………………………………..



21

(ii) Explain what is meant by the term unsaturated.

Answer ……………………………………………………………………………………………………….

(iii) By naming the reagent and giving the observation, describe a simple

test to confirm that oleic acid is unsaturated.

Answer Reagent …………………………………………………………………………..

Observation ………………………………………………………………………..

(b) The molecular formula of oleic acid is C17H31CO2H.

How many double bonds between carbon atoms are present in one molecule

of oleic acid?

Explain your answer.

Answer ……………………………………………………………………………………………………….

CH3−(CH2)12(C2H2)2 • COOH

Question 10

Carefully study the reaction scheme below and use it answer the questions

that follow.

P

Fermentation

Ethene C2H4

R 1,2 - dibromoehtane Q

Polymerization

Dehydration

Hydrogen and a catalyst

glucose



22

(a) (i) Give the name of the substances and the conditions needed to

ferment glucose.

Answer Substances needed …………………………………………………………..

Conditions needed …………………………………………………………..

(ii) Give the name of the gas that is also produced during the fermentation

of glucose

Describe a chemical test for this gas.

Answer Name of the gas …………………………………………………………..

Test for the gas …………………………………………………………..

Observation ………………………………………………………………….

(b) (i) Give the name and molecular formula for the substance P.

Answer Name ……………………………………………………………………………….

Molecular formula …………………………………………………………..

(ii) Give the names of substances Q and R.

Answer Substance Q ……………………………………………………………………………….

Substance R ……………………………………………………………………………..

(c) Write the equation for the addition reaction between ethane and bromine

Answer . ……………………………………………………………………………………………………..

Question 11

Ethanol, C2H5OH, is used in Brazil as a liquid fuel instead of gasoline (petrol). The

ethanol is manufactured by a fermentation process from sugar cane.

(a) (i) Describe how cane sugar can be converted by the process of

fermentation.

Answer . ……………………………………………………………………………………………………..

. ……………………………………………………………………………………………………..



23 (ii) What is the name of the gas produced during this fermentation?

How would you test for this gas.

Answer Name of the gas……………………………………………………………………………..

Test for the gas ………………………………………………………………………..

(b) When ethanol is completely burnt in air, it forms CO2 gas and water. One

mole of ethanol will release 1370 kj of heat energy during this reaction

(i) Construct the balanced equation for the burning of ethanol.

Answer C2H5OH + 3O2 → 2CO2 + 3H2O; ∆Η = - 1370 kj

(ii) The mass of 1 litre of ethanol is 780 g. How much heat energy will

be released when 1 litre (1 dm3) ethanol is completely burnt?

Answer one mole of ethanol on produces heat 1370 kj (From above)

C2H5OH + 3O2 → 2CO2 + 3H2O; ∆Η = - 1370 kj

From the chemical equation, the mass of one mole of C2H5OH

is = 46 (Mr)

46 g of ethanol produces heat = - 1370 kj

1.0 g of ethanol will produce heat = - 1370 kj/ 46

- 1370 × 780 ∴ 780 g (1 litre/1 dm3 ) will produce heat = −−−−−−−−−−−−−− kj 46

= - 23230 kj

(c) When completely burnt, 1 litre of gasoline releases 38000 kJ of heat energy.

Comparing ethanol and gasoline as a liquid fuels, suggest

(i) One advantage of using ethanol rather than gasoline.

Answer using ethanol as a fuel will meet the increasing demand for the

petrol, moreover

(ii) One advantage of using gasoline rather than ethanol.

Answer. Same mass of the gasoline on combustion produces more heat than

same mass of ethanol.

Therefore, gasoline is a better fuel than ethanol.



24 Question 12

Ethanol

+

Compound Y

+ acid A

Organic Compound X

Cold Water

Heat (a) Above figure shows an experiment to make organic compound X by reacting

ethanol with organic compound Y in the presence of acid A. Compound X has the formula CH3CH2CO2CH2CH3

(i) Name the compounds X, Y and A.

Answer From the formula the compound X, CH3CH2CO2CH2CH3

It is concluded that it is an ester.

An ester is formed by the reaction of an acid and an alcohol.

Therefore, total number of carbon atoms an acid and total number of

carbon atoms in alcohol can be found out from the molecular formula

of an ester.

CH3CH2CO OCH2CH3

This part came

This part came from alcohol

From acid

Compound X → Ester

Compound Y → Organic acid (CH3CH2COOH)

Compound A (Acid) → H2SO4

H2SO4

(ii) CH3CH2COOH + HOCH2CH3 CH3CH2COOCH2CH3

+

H2O

(iii) Cold water will reduce the rate of evaporation of the molecular

substance (ester).

(b) The same apparatus was used to make organic compound Z,by reacting

ethanol with compound B in the presence of acid A. Compound Z has the composition by mass: 53.3 % oxygen,40.0 % carbon and

6.7% hydrogen.

(i) Find the empirical formula of compound Z

By assuming that total mass of the hydrocarbon is 100 g

Answer



25

The empirical formula of the compound Z is …….CH2O

(ii) The relative molecular mass of compound Z is 60. Find the molecular

formula of the compound Z and write its structural

formula.

Answer

The relative molecular mass

= n

Relative mass of the empirical formula

60 = 2 =n

30

molecular formula of the compound Z is …..2 × CH2O = C2H4O2

The structural formula of the compound is

……………………………………………………………………………………………

Question 13 Some information is given below on four organic compounds. A,

B, C and D. In each case, suggest the identity of the compound by writing a full

structural formula. Explain your reasoning.

(a) Compound A: a liquid with relative molecular mass 60; the liquid is neutral

and soluble in water; under right conditions, it reacts with ethanoic acid and

to give a sweet smelling product.

Answer A sweet smelling product is formed by the reaction of ethanoic acid

with compound A. Therefore, the compound A could be an acid.

Because, an ester (a sweet smelling compound) is only formed by the

reaction of an acid with alcohol, in presence of acid (right conditions)

This organic acid (A) has molecular mass 60 (given)

So. The molecular formula of A will be CH3COOH

It is water soluble, because it is capable of forming H-bonding with

water molecule.

Element

O C H

Mass of each element (g)

53.3

40.0

6.7

Molar Mass (i.e. Ar in grams)

16g

12g

1.0 g

Number of moles

Mass

= ------------------

Molar mass

53.3

----- = 3.33

16

40.0

----- = 3.33

12

6.7

--- = 6.7

1.0

Simplest atomic ratio (it is

obtained by dividing each

number of moles by the

smallest number)

3.33

----- = 1.0

3.33

3.33

----- = 1.0

3.33

6.7

---- = 2.0

3.33

Simplest whole number atomic

ratio

1

1

2



26(b) Compound B: a liquid with relative molecular mass of 32; the liquid can

be oxidized to produce a compound with molecular formula HCOOH.

Answer The given compound, HCOOH is an acid. If oxidation of any compound

can produce an acid, it means that compound is an alcohol.

It is important to note that the number of carbon atoms in alcohol and

acid remains same. Therefore, alcohol is CH3OH, which has the molar

mass 32

(c) Compound C: a liquid with relative molecular mass of 74; a dilute solution

of the compound has a pH less than 7 and reacts with magnesium ribbon to

produce a gas which burns in air.

Answer If, the liquid has pH less than 7, it means the liquid is an acid, acid also

reacts with magnesium ribbon & produces H2 gas. H2 gas burns in the

air.

The possible compound is:-

HCOOH Mr = 32

CH3COOH Mr = 60

√ CH3CH2COOH Mr = 72

***************

Documents

Work Book -Alkanes, Alkenes, Alcohols & Acids