Unit 1: Chapter 5 - Salt and Electrolysis

Question 1

Acids and Alkalis

Answer 1

WHAT ARE ACIDS AND BASES?

A solution that is dissolved in water is an AQUEOUS SOLUTION (aq). The solution may be acidic, alkaline or neutral.

• Soluble hydroxides are called alkalis. (e.g. sodium hydroxide) All alkalis form hydroxide (OH-) ions which makes the solution alkaline.
• Bases, which include alkalis are substances that neutralize acids. Metal oxides and hydroxides are bases. (e.g. copper oxide and copper hydroxide)
• Acids contain H+ ions. The more ions there are, the more concentrated the acid is.
• Pure water is neutral

MEASURING pH

The pH scale goes from 0 (most acidic) to 14 (most alkaline). We can use the universal indicator to find the pH of a solution. It turns into a range of colours depending on the pH.

Question 2

Making Salts From Metals or Bases

Answer 2

Acid + Metal -> Salt + Hydrogen

Acid + Base -> Salt + Water
NEUTRALIZATION REACTION

Question 3

Making Salts From Solutions

Answer 3

Acid + Alkali -> Salt + Water

MAKING INSOLUBLE SALTS

We can make insoluble salts by combining two different solutions that contain different soluble salts. The soluble salts react together in a precipitation reaction. The insoluble salt is called a PRECIPITATE.

E.g.
Lead nitrate + Potassium iodide -> Lead iodide + potassium
solution solution solution nitrate
(insoluble) solution

Question 4

Electrolysis

Answer 4

During electrolysis:

• Positive ions move to negative electrode
• Negative ions move to positive electrode

When ions move to electrode, they are turned into elements.

Gases may be given off or metals deposited at electrodes. This depends on whether the compound is molten or dissolved in water.

When a bromide is molten, it gives of gas in electrolysis.
When bromide is dissolved in water, it remains as ‘aq’ and stays in solution.

Covalent bonds cannot be electrolysed.

Question 5

Changes at the Electrode

Answer 5

REDUCTION
- When ions gain electrons, we say it has been reduced

OXIDATION
- When ions lose electrolysis, we say it has been oxidized.

HALF EQUATION (LEAD BROMIDE)

• We represent what is happening in electrolysis, using HALF EQUATIONS.
• At the negative electrode:
  Pb(2+) + 2e- -> Pb (REDUCED)

At the positive electrode:
2Br(-) - 2e(-) -> Br2 (OXIDIZED)

THE EFFECT OF WATER
When electrolising aqueous solutions, there are 2 positive ions. The less reactive ion will always be formed.

e.g. If there is K and H, H2 will always be formed.

In the case of negative ions:
Halide ions > hydroxide > all other negatively charged ions

Question 6

The Extraction of Aluminum

Answer 6

Aluminum:
Aluminum oxide -> aluminum + oxygen
2AlO3 -> 4Al + 3O2

We must first purify the aluminum ore (bauxite) before electrolysis.

When purified, pure aluminum oxide is left or ALUMINA.

We then melt the Aluminum and mix CRYOLITE to reduce melting point to 850C.