Formulae And Calculations (1.1)

Question 1

What does a formula tell you?
Give an example

Answer 1

The formula of a compound is a set of symbols and numbers. The symbols tell you what elements are present and the numbers tell you the ratio of the number of atoms of the different elements in the compound
H2SO4 has:
2 hydrogen atoms
1sulfur atom
4 oxygen atoms

Question 2

What is meant by oxidation and reduction?

Answer 2

OILRIG
Oxidation is loss
Reduction is gain

Of electrons

Question 3

What is meant by oxidation number/state?

Answer 3

The oxidation number or state of an element is the number of electrons that need to be added to (or taken away from) that element to make it neutral.

Question 4

What are the rules for oxidation states?

Answer 4

All uncombined elements have an oxidation number of zero.
E.g. in O2 gas the oxidation number = 0

The sum of oxidation numbers in a compound is zero.
E.g. in MgO, Mg = +2, O = -2, +2-2 = 0

Group 1 metals have an oxidation number of +1
Group 2 metals have an oxidation number of +2.

Group 6 elements usually have an oxidation number of -2; Group 7 elements usually have an oxidation number of -1.

The oxidation number of oxygen is -2, except in peroxides (in H2O2 it is -1) or when combined with fluorine.

The oxidation number of hydrogen is +1, except in metal hydrides. For example, in NaH, hydrogen is -1 because Group 1 metals like Na always have an oxidation number of +1.

Question 5

What are the rules for balancing equations?

Answer 5

Remember that atoms cannot be created or destroyed in a chemical reaction.
Count the number of atoms on both sides.
Only add numbers in front of the compounds to balance the equation.
The number of atoms of each element on the reactants’ side must equal the number of atoms of each element of the products.
Check your answer.
Add state symbols if required.