O-LevelChemistryAcids, Bases & Salts5070/21 · Oct/Nov 2017 · Q610 Marks

B6 Ethanoic acid is a weak acid.

(a)What is meant by the term weak acid?[1]
(b)(i)Ethanoic acid reacts with ethanol to form ethyl ethanoate. The reaction is exothermic. CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O Describe and explain the effect, if any, on the position of equilibrium when the concentration of ethanol is increased,[2]
(b)(ii)the temperature is increased.[2]
(c)(i)The table shows some properties of four carboxylic acids. carboxylic acid | formula | melting point /°C | boiling point /°C | density in the liquid state in g/cm³ methanoic acid | HCOOH | 8.5 | 100.7 | 1.22 ethanoic acid | CH3COOH | 16.7 | 118.0 | 1.05 propanoic acid | C2H5COOH | -20.7 | 141.1 | butanoic acid | C3H7COOH | -4.4 | 165.6 | 0.96 How does the boiling point change as the number of carbon atoms in the formula of the carboxylic acids increases?[1]
(c)(ii)Estimate the density, in g/cm³, of liquid propanoic acid. density[1]
(c)(iii)What is the physical state of ethanoic acid at 15.0°C? Explain your answer.[1]
(c)(iv)Why is it difficult to predict the melting point of pentanoic acid, C4H9COOH, using only the information from the table?[1]
(c)(v)Draw the structure of propanoic acid showing all of the atoms and all of the bonds.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] B6(a) acid which doesn't dissociate completely to give H⁺ / doesn't ionise completely to give H⁺
[1] B6(b)(i) equilibrium shifts to the right
[1] B6(b)(i) to reduce the amount of ethanol added
[1] B6(b)(ii) equilibrium moves to the left
[1] B6(b)(ii) the (forward) reaction is exothermic / backward reaction is endothermic / moves in the direction of the endothermic reaction
[1] B6(c)(i) increases (with increasing number of carbon atoms)
[1] B6(c)(ii) any value between 0.97 and 1.04
[1] B6(c)(iii) solid because 15 °C is below its melting point / solid because the melting point is above 15 °C
[1] B6(c)(iv) there is no clear trend / the values go up and down (and up)
[1] B6(c)(v) correct structure of propanoic acid showing all atoms and all bonds

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