1.1 Formulae and Equations

Question 1

What is an element?

Answer 1

An element is a substance made up of only one type of atom.

Question 2

What is a compound?

Answer 2

A compound is made up of at least two different elements chemically bonded together. This means compounds are made up of more than one type of atom.

Question 3

What are the chemical formulas for nitrate and carbonate ions?

Answer 3

Nitrate: NO3 (-)
Carbonate: CO3 (2-)

Question 4

What are the chemical formulas for oxide and hydroxide ions?

Answer 4

Oxide: O(2-)
Hydroxide: OH(-)

Question 5

What are the chemical formulas for sulfate and ammonium ions?

Answer 5

Ammonium: NH4 (+)
Sulfate: SO4 (2-)

Question 6

What are the chemical formulas for sulfate and ammonium ions?

Answer 6

Ammonium: NH4 (+)
Sulfate: SO4 (2-)

Question 7

What is an ionic compound?

Answer 7

Ionic compounds are made up of positive and negative ions, held together by electrostatic attraction.

Question 8

What is the chemical formula for the ionic compound sodium chloride?

Answer 8

Sodium is in Group 1 so forms the ion Na+.
Chlorine is in Group 7 so forms the ion Cl-.
Sodium chloride has the formula NaCl as the charges on the ions must cancel to give a compound with no overall charge.

Question 9

What is the chemical formula for the ionic compound aluminium oxide?

Answer 9

Aluminium forms the ion Al(3+). The oxide ion is O(2-).
For the ionic compound to have no overall charge, two aluminium ions must bond with three oxygen ions, giving the formula Al2O3.

Question 10

What is the chemical formula for the ionic compound zinc nitrate?

Answer 10

Zinc forms the ion Zn2+. The nitrate ion is NO3 (-)
For the ionic compound to have no overall charge, two nitrate ions must bond with the zinc ion. This gives the formula Zn(NO3)2.

Question 11

What is an oxidation number?

Answer 11

A number assigned to an atom or an ion which represents how many electrons have been lost or gained by the atom.

Question 12

What is the oxidation number of an element?

Answer 12

Zero

Question 13

What is the overall oxidation number of the ionic compound CaCO3?

Answer 13

Zero.
The oxidation numbers of a neutral compound add up to zero.

Question 14

What is the overall oxidation number of
HCO3 (-)?

Answer 14

-1.
The overall oxidation number of a charged compound is simple as it is the charge shown on the molecule.

Question 15

What are the oxidation states of hydrogen and oxygen ions?

Answer 15

Hydrogen: +1 (except in metal hydrides where it is -1).
Oxygen: -2 (except in peroxides and F2O where it is -1).

Question 16

What are the common oxidation states of the halogens and Group one metals?

Answer 16

Halogens: -1
Group one metals: +1

Question 17

What is the oxidation number of sulfur in Na2SO4?

Answer 17

Na → +1
O → -2
The overall charge on the compound is zero so sulfur must have an oxidation number of +6 to balance the charges.

Question 18

What is the oxidation number of chromium in K2Cr2O7?

Answer 18

K → +1
O → -2
The overall charge on the compound is zero so chromium must have an oxidation number of +6 to balance the charges.

Question 19

What are the state symbols used in chemical equations?

Answer 19

(g) - gas
(aq) - aqueous
(l) - liquid
(s) - solid

Question 20

What is an ionic equation?

Answer 20

Ionic equations only show the reacting ions and can be written for any reaction involving ions in solution.

Question 21

Write an ionic equation for the reaction between nitric acid (HNO3) and sodium hydroxide (NaOH). Include state symbols.

Answer 21

Reaction equation: HNO3 + NaOH → NaNO3 + H2O
Rewrite the equation with the aqueous ions:
H(+) + NO (-) + Na(+) + OH(-) → Na(+) + NO(-) + H2O
Cancel any ions appearing on both side to get the ionic equation and add in state symbols:
H(+)(aq) + OH(-)(aq) → H2O(l)