Paper 1 - Electrolysing Solutions

Question 1

Why does water naturally form ions ? Reversible reaction?

Answer 1

water is a covalent compound. Some of its molecules naturally form ions:
H20 (l) -><- h[+] + OH[-]
The presence of these ions means that a
solution of an ionic compound contains:
cations and anions from the dissolved ionic compound, and H and OH ions from the water.

Question 2

Competing ions , what does the discharge if H and OH form ?

Answer 2

During electrolysis of an aqueous solution,
all the ions in the electrolyte compete to be discharged and form products.

Hydrogen gas is produced if H* ions
are dischanged:
2H [+] (aq) + 2e -> H2(g)
Oxygen gas is produced if OH ions
are dischanged:
4OH [-] (aq) -> 2H2O + O2(g) + 4e[-]

Question 3

What happens in terms of hydrogen production at the cathode ?

Answer 3

At the negative electrode:
hydrogen is produced, unless
the compound contains ions from a
metal less reactive than hydrogen.

in that case:

the metal is produced instead.
Copper and silver are below hydrogen in
the reactivity series

Question 4

What happens at the anode in terms of oxygen production?

Answer 4

At the positive electrode:

oxygen is produced (from OH ions)
unless

the compound contains halide ions
(Cl[-], Br[-] or I[-]).

In that case:

chlorides produce chlorine, Cl2
bromides produce bromine, Br2

iodides produce iodine, I2

Question 5

Note on reactivity and discharge

Answer 5

Ease of discharge increases as you go down the table - get less reactive

Question 6

explain how the potassium ions and iodide ions in solid potassium iodide are converted into potassium and iodine by electrolysis
You may include suitable half-equations in your answer

(6 marks)

Answer 6

you need to remember what the staying point in the question. In this case it is solid potassium not moiten potasolum iodide. Remember to explain how to melt the compound, and Why
this is needed

You should include knowledge and
uncderstanding about what happens at the cathode. Include a half-equation and reduction as the galin of electrons

You should include knowledge and
understancding about what happens at the
anode. include a half-equation and oxidation
as the loss of electrons. Also mention that a covalent bond forms between two iodine atoms when they share a pair of clectrons.

Question 7

Why must the ionic compound be heated / why current passed through

Answer 7

The potassium lodide must be heated. This is so that its ionic bonds break and it becomes molten. Electrolysis cannot happen in the solid state because the lons are not free to move. It happens in the liquid state because the ions are
free to move