Nov 2021 Paper 1

Question 1

Explain why H2O isn’t included in the Kw expression

Answer 1

Concentration of H2O remains constant

Question 2

The enthalpy of hydration of Ca2+(g) is –1650 kJ mol–1.
Suggest why this value is less exothermic than that of Mg2+(g)

Answer 2

Ca2+ has more shells and the distance between the nucleus and outer e- is larger.
Weaker bond to delta negative O in water.

Question 3

Suggest why it is dangerous to react a similar piece of sodium with 10 cm3 of water in a boiling tube

Answer 3

Temp increases and glass could shatter

Question 4

Give an equation for the reaction of phosphorus(V) oxide with water. Suggest a pH for the solution formed.

Answer 4

P4O10 + 6 H2O → 4 H3PO4
-1 to +1

Question 5

Explain, in terms of crystal structure and bonding, why silicon(IV) oxide has a higher melting point than phosphorus(V) oxide.

Answer 5

M1-SiO2 is macromolecular / giant covalent / giant molecule
M2-Strong covalent bonds (between atoms) or covalent bonds need a lot of energy to be broken/overcome
M3-P4O10 is molecular or simple covalent molecule
M4-Weak van der Waals forces between molecules or van der Waals forces between molecules break easily

Question 6

An element in Period 3 forms an oxide that is insoluble in water.
This oxide reacts with sulfuric acid and with aqueous potassium hydroxide.
Give the formula for this oxide.
Give an equation for the reaction of this oxide with sulfuric acid

Answer 6

Al2O3
Al2O3 + 3 H2SO4 → Al2(SO4)3 + 3 H2O

Question 7

Give the formula of a hydroxide of an element in Period 3 used in medicine.

Answer 7

Mg(OH)2

Question 8

Explain why the value of Kw increases as the temperature increases.

Answer 8

-Equilibrium is endothermic (in forward direction)
-Equilibrium shifts to the RHS to minimise/oppose temperature increase

Question 9

Find the pH of pure water at 50 C
Why is pure water neutral at 50 C

Answer 9

pH=6.63
Kw=[H+]^2

[H+]=[OH-]

Question 10

Suggest why the pH probe is washed with distilled water between each of the calibration measurements

Answer 10

To avoid contamination of the solutions

Question 11

Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

Answer 11

To avoid missing the end point

Question 12

Explain why the standard entropy value for carbon dioxide is greater than that for carbon.

Answer 12

CO2 (g) is more disordered