C4- Redox

Question 1

Explain acids and the two types.

Answer 1

When dissolved in water an acid released hydrogen ions as protons (H+), into the solution.

Strong acids= completely dissociates in aqueous solution. (Releases all its H+ ions)

Weak acids= partially dissociates in aq solution.
(Releases small proportion of H+ ions).

Question 2

What is a base and alkali?

Answer 2

Base-neutralises an acid to form a salt.

Alkali- a base that dissolves in water releasing hydroxide ions (OH-) into the solution.

Question 3

What are the common bases?

Answer 3

Metal oxides:

• MgO
• CaO
• CuO

Metal carbonates:

• Na2CO3
• CaCO3
• CuCO3

Alkalis:

• NaOH
• KOH
• NH3

Question 4

What happens in a neutralisation reaction?

Answer 4

In neutralisation of an acid, H+ ions react with a base to form a salt and neutral water.

The H+ ions from the acid are replaced by metal or ammonium ions from the base in order to form the salt

Question 5

Neutralisation with metal oxide/hydroxides.

Answer 5

Acid + metal oxide/ hydroxide -> salt + water

Question 6

Neutralisation with alkalis.

Answer 6

Acid + alkali -> salt + water

Question 7

Neutralisation with carbonates.

Answer 7

Acid + metal carbonate -> salt + water + CO2

Question 8

Explain the dissociation of sulfuric acid.

Answer 8

Sulfuric acid is a strong acid, but this is only true for one of the two H atoms. When it is mixed with water, each H2SO4 molecule dissociates, releasing only one H atom as an H+ ion.

Strong acid:
H2SO4 -> H+ -> HSO4-

Weak acid:
HSO4- (reversible symbol) H+ + SO4^2-

Question 9

What is a titration and what is it used for?

Answer 9

A technique used to accurately measure the volume of one solution that reacts exactly with another solution.

Used for:

• finding the concentration of a solution.
• identification of unknown chemicals.
• finding purity of a substance.

Question 10

What is a standard solution?

Answer 10

A solution of known concentration.

A volumetric flask is used to make up a standard solution very accurately.

Question 11

How is a standard solution prepared?

Answer 11

1. Solid weighed accurately.
2. Solid dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark.
3. Solution is transferred to volumetric flask. Last traces of solution are rinsed into flask with distilled water.
4. The flask is filled to the graduation line a drop at a time until the bottom of the meniscus lines up. (Look from eye level for accuracy). If too much water is added, solution will be too dilute and must be prepared again.
5. Volumetric flask is slowly inverted several times to mix thoroughly to make the solution consistent.

Question 12

What is a acid- base titration?

Answer 12

A solution of an acid is titrated against a solution of a base using a pipette and burette.

Burette reading is always recorded to the nearest half division!

Question 13

Procedure of carrying out acid- base titration.

Answer 13

1. Add measured volume of one solution to a conical flask with a pipette.
2. Add the other solution to the burette. Record initial reading.
3. Add a few drops of indicator to solution in conical flask.
4. Run the solution from the burette into the solution in the conical flask, swirling the flask throughout to mix.
   The indicator will change colour at the end point.
5. Record final burette reading.
6. Do a quick titration to get approx titre.
7. Repeat titration accurately, adding solution drop wise when close to the end point.
   Continue until 2 concordant titres reacted (within 0.10cm3 of each other).

Question 14

What is meant by the end point in a titration?

Answer 14

The end point is the drop at which the indicator changes colour.
It is used to indicate the volume of one solution that exactly reacts with the volume of the other solution.

Question 15

How do you calculate titre and mean?

Answer 15

Titre = final reading - initial reading.

To calculate mean, only use accurate titres (within 0.10cm3 of each other).
Including all titres for the mean = inaccurate result

Question 16

What is oxidation number?

Answer 16

A measure of the number of electrons that an atom uses to bond with other atoms of a different element.
Oxidation state= oxidation number

Oxidation number for:

• elements = always zero.
• simple ions= charge of ion.
• compounds=total of oxidation numbers=total charge= 0
• polyatomic ions= total of oxidation numbers= charge on ion.

Oxidation number applies to each atom of an element.

Question 17

What is a redox reaction?

Answer 17

A reaction that involves both reduction and oxidation.
Reduction= gain of electrons.
Oxidation = loss of electrons.

Dilute acids can undergo redox reactions with some metals to produce salts and hydrogen gas.

Question 18

Explain redox in terms of oxidation number.

Answer 18

Reduction = decrease in oxidation number.
Oxidation = increase in oxidation number.