Page 58 - ACE YR IGCSE A TOP APPROACH TO CHEM

Disadvantage (any one from) [1] A weak acid changes colour to orange /
• Slow yellow. [1]
• Non-recyclable raw materials H and OH are detected by the pH indicator.
+
–
• A lot of land is needed to grow plants (f) (i) Ethyl propanoate [1]
• Need further step to obtain pure ethanol Rules for naming an ester:
The culture broth must be replaced by a new batch The name of the alcohol part is given first, then
even if there is still glucose left. followed by a separate word for the name of
Many plants are needed to produce a small the carboxylic acid part.
percentage of ethanol. So, a lot of land is used to First part ➞ comes from the alcohol
grow crops for the production of ethanol. Second part ➞ comes from the carboxylic acid
The ethanol produced is impure. Fractional
distillation is needed for purification. H H O H H Alcohol
Carboxylic
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Chemical method acid H C C C O C C H
Advantage (any one from) [1]
• Fast H H H H
• Continuous process (ii) Ethanol [1]
• Obtain pure ethanol Propanoic acid [1]
The chemical method is fast. Removing the ethanol (iii) Concentrated sulfuric acid [1]
consistently makes it a continuous process. The
unreacted ethene and steam will be recycled to (iv) H H O H
increase the percentage of yield. Pure ethanol will
be obtained. Therefore, no further steps such as H C C C O C H
fractional distillation are required. H H H
Disadvantage (any one from) [1] Ester linkage [1]
• Use of non-renewable resources A carbon is connected to three other
• Expensive atoms: a single bond to a carbon, a double
• The yield is not high at 300°C. bond to an oxygen, and a single bond to an
Ethene is obtained from petroleum which will run oxygen. [1]
out one day. A great deal of heat is needed because The single-bonded oxygen is connected to
the process which is carried out at the temperature another carbon. [1]
of 300°C will use a lot of energy. This increases the Tips to draw the structure of an ester:
cost. 300°C is a compromised temperature, but the Step 1: Draw the structure of carboxylic acid,
percentage of yield obtained is not high. omitting the OH molecule.
(d) Any one from: [1] Step 2: Connect the alcohol structure to the C
• As a fuel in C=O, with the H removed from the functional
• As a solvent group –OH.
Step 3: Always ensure there is –COO linkage.
5 (a) H O 6 (a) A substance containing very large molecules [1]

H C C O H formed by a lot of small molecules joined
together [1]
H
Correct number of C and H atoms [1] (b) F F F F F
Correct functional group [1]
Ethanoic acids contain two carbon atoms and its C C C C C
functional group is –COOH.
F F F F F
(b) C H 2n + 1 COOH [1] Or any two connected carbon atoms with four
n
(c) (i) Potassium manganate(VII) [1] fluorine atoms [1]
(ii) Change from purple [1]
to colourless [1] (c) F F
–
+
2+
MnO + 8H + 5e ➞ Mn + 4H O C C
4
2
purple colourless
F F
(d) (i) Sodium ethanoate [1] Correct number of C and F atoms [1]
NaOH + CH COOH ➞ CH COONa + H O
3 3 2 Contain 1 C=C double bond [1]
(ii) Neutralisation [1]
(d) Any one from: [1]
(e) A strong acid has a higher concentration of H • High melting point
+
than a weak acid. [1] • Strong / hard
A strong acid changes colour to red. [1] • A conductor of heat
Cambridge IGCSE TM
170 Ace Your Chemistry

Answers.indd 170 3/4/22 3:54 PM