Chapter 4 Acids And Redox

Question 1

What are acids, and how can their strength differ? What are the common acids?

Answer 1

-all acids contain hydrogen and when dissolved in water and acid releases protons (hydrogen ions)
-ACIDS ARE A SPECIES THAT IS A (H+) PROTON DONOR
E.g.
HCl -> H+ +Cl-

• a strong acid like HCl fully dissociates in an aqueous solution and releases all its H+ ions
• a weak acid like CH3COOH only releases a small proportion of its available hydrogen meaning it partially dissociates, most organic acids are weak
• H2SO4, H3PO4, HCl, CH3COOH

Question 2

What is a base? And what are the common bases?

Answer 2

• a species that is a proton (H+) acceptor
• Common bases are metal oxides and hydroxides
• all amines and ammonia NH3 is also a base as it has a lone pair

Question 3

What is an alkali? What are the common alkalis?

Answer 3

• an alkali is a type of base that dissolves in water to form hydroxide (OH-) ions and gives a pH higher than 7
• common alkalis include: ammonia, sodium hydroxide, potassium hydroxide
• ammonia forms a weak alkaline solution because only a small proportion of the dissolved NH3 actually reacts with water

Question 4

What is neutralisation? give an example of hydrochloric acid and sodium hydroxide, and give the ionic equation. give an example of a neutralisation reaction with zinc carbonate and sulfuric acid?

Answer 4

• in neutralisation of an acid, H+ ions react with a base to form a salt and neutral water. The H+ ions are replaced by metal or ammonium ions from the base.

Hydrochloric acid and sodium hydroxide
HCl + NaOH -> NaCl + H2O
Ionic equation:
H+ + OH- -> H2O

Zinc carbonate and sulfuric acid
ZnCO3 + H2SO4 -> ZnSO4 + H2O + CO2

Question 5

What is an oxidation number?

Answer 5

Based in a set of rules that apply to atoms, and can be thought of as the number of electrons involved in bonding to a different element. Use of oxidation numbers helps when writing formulae and balancing electrons as a check that all electrons have been accounted for.

Question 6

What is the element rule?

What rules apply to ions?

Answer 6

• in a pure element form the oxidation number is always zero

Element and [O] number in an ion
O -> -2
H -> +
Exceptions:
H in metal hydrides -> -1 e.g. NaH
O in peroxides-> -1 e.g. H2O2
O bonded to F -> +2 e.g. F2O

Question 7

What is a redox reaction? What is oxidation and reduction?

Answer 7

• redox reaction are where both oxidation and reduction happen simultaneously
• oxidation is the increase in oxidation number, gain of oxygen, loss of hydrogen and loss of electrons
• reduction is the opposite of oxidation

Question 8

Give an example of the half equations of Mg and Cl stating which is oxidation or reduction.

Answer 8

Mg + Cl2 -> MgCl2

Cl2 + 2e- -> 2Cl- reduction
Mg -> Mg2+ + 2e- oxidation

If the electrons are on the RHS of an equation the species is being oxidised

Question 9

What are oxidising and reducing agents? What is a disproportionate reaction?

Answer 9

• a species being oxidised is a reducing agent because it forces another species to be reduced
• a species being reduced is an oxidised agent because it forces another species to be oxidised
• a disproportionate reaction is a type of redox reaction where the species is both oxidised and reduced at the same time

Question 10

What is a titration?

Answer 10

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

Question 11

How do you prepare a standard solution?

Answer 11

1. Weight the solid
2. Dissolve it in a beaker using less distilled water that will be needed to fill the volumetric flask
3. Transfer to volumetric flask rinsing the last traces of solution into the flask with distilled water
4. Carefully fill the flask to the graduation line by adding distilled water a drop at a time until the meniscus lines up exactly
5. Volumetric flask is inverted several times to mix the solution thoroughly

Question 12

What are the steps for an acid base titration?

Answer 12

1. Add a measured volume of solution to a conical flask using a pipette
2. Add other solution to the burette are récord the reading to the nearest 0.05cm3
3. Add a few drops of indicator to the solution in the conical flask with a pipette
4. Run the solution from the burette into the conical flask swirling the flask to mix them thoroughly. Eventually a colour change will take place at the endpoint
5. Record the final burette reading. The volume of solution added (titre) is calculated by subtracting the initial from the final reading
6. A quick trial titration is the first to be conducted to find a rough titre
7. The next titres are conducted accurately and drop wise towards the end. This is repeated until two sets of concordant data are found agreeing within 0.10cm3