Acids and Redox

Question 1

What is the definition of a strong acid?

Answer 1

Releases all its hydrogen atoms into solution as H+ ions and completely dissociates in aqueous solution
E.g. HCL

Question 2

What is the definition of a weak acid?

Answer 2

Only releases a small proportion of its available hydrogen atoms and only partially dissociates in aqueous solution
E.g. Ethanoic acid - CH3COOH

Question 3

What is a base and what are some examples?

Answer 3

A substance that neutralises an acid to form a salt.

Metal oxides, metal hydroxides, metal carbonates and ammonia (NH3).

Question 4

What is an alkali?

Answer 4

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

Question 5

Write equations to show the dissociation of hydrogen atoms in H3PO4 (phosphoric acid).

Answer 5

H3PO4 –> H+ + H2PO4-
H2PO4- H+ + HPO4^2-
HPO4^2- H+ + PO4^3-

Question 6

How to prepare a standard solution?

Answer 6

Known concentrations. Volumetric flask used.

1) Solid weighed
2) Dissolved in beaker using less vol. needed to fill the flask
3) Last traces rinsed into flask
4) Add distilled water to the graduation line - meniscus
5) Invert volumetric flask to mix thoroughly.

Question 7

What are the uncertainty for the 10cm3 pipette, 25cm3 pipette and 50cm3 burette?

Answer 7

• ± 0.04cm3
• ± 0.06cm3
• ± 0.10cm3

Question 8

How do you do an acid-base titration?

Answer 8

1) Add measured volume of one solution to a conical flask using a pipette
2) Add the other solution to the burette and record the initial reading to the nearest 0.05cm3
3) Add a few drops of indicator to the solution in the conical flask
4) Run the solution in the burette into the solution in the flask, swirling the flask to mix it. When the indicator changes colour, you have reached the end point.
5) Record the final burette reading. Calculate the titre value (final vol. - initial vol.)*
6) Repeat until two concordant results within 0.10cm3 from each other

*Usually a rough titre is done at first to find the approximate titre

Question 9

Oxidation rules

Answer 9

• In pure elements, oxidation number is 0 (e.g. H2)
• Each atom in a compound has an oxidation number
• The sign is placed before the number

Question 10

What is oxidation?

Answer 10

Loss of electrons/ gain of oxygen. Increase in oxidation number.

Question 11

What is reduction?

Answer 11

Gain of electrons/ loss of oxygen. Decrease in oxidation number

Question 12

What are some special cases for oxidation numbers?

Answer 12

Hydrogen in metal hydrides = -1
Oxygen in peroxides = -1
Oxygen bonded to Fluorine = +2