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: CO₃²⁻

Question 4

what are the chemical formulas for oxide and hydroxide ions?

Answer 4

oxide: O²⁻
hydroxide: OH⁻

Question 5

what are the chemical formulas for sulphate and ammonium ions?

Answer 5

ammonium: NH₄⁺
Sulphate: SO₄²⁻

Question 6

what is an ionic compound?

Answer 6

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

Question 7

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

Answer 7

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 8

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

Answer 8

aluminium forms the ion Al³⁺. the oxide ion is O²⁻
For the ionic compound to have no overall charge, two aluminium ions must bond with three oxygen ions, giving the formula Al₂O₃.

Question 9

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

Answer 9

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 10

what is an oxidation number?

Answer 10

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

Question 11

what is the oxidation number of an element?

Answer 11

zero

Question 12

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

Answer 12

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

Question 13

what is the overall oxidation number of HCO₃⁻?

Answer 13

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

Question 14

what are the oxidation states of hydrogen and oxygen ions?

Answer 14

hydrogen: +1 (except in metal hydrides where it is -1)

oxygen: -2 (except in peroxides and F₂O where it is -1)

Question 15

what are the common oxidation states of halogens and group one metals?

Answer 15

halogens: -1
Group 1 metals: +1

Question 16

what is the oxidation number of sulphur in Na₂SO₄?

Answer 16

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

Question 17

what is the oxidation number of chromium in K₂Cr₂O₇?

Answer 17

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 18

what are the state symbols used in chemical equations?

Answer 18

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

Question 19

what is an ionic equation?

Answer 19

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

Question 20

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

Answer 20

reaction equation: HNO₃ + NaOH → NaNO₃ + H₂O
rewrite the equation with the aqueous ions:
H⁺ + NO₃⁻ + Na⁺ + OH⁻ → Na⁺ + NO₃⁻ + H₂O
cancel any ions appearing on both side to get the ionic equation and add in state symbols:
H⁺ (aq) + OH⁻ (aq) → H₂O (l)

Question 21

write an ionic equation for the reaction between silver nitrate and sodium chloride. include state symbols.

Answer 21

reaction equation: AgNO₃ + NaCl → AgCl + NaNO₃
rewrite the equation with aqueous ions (note AgCl is a solid):
Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl + Na⁺ + NO₃⁻
cancel any ions appearing on both side to get the ionic equation:
Ag⁺ (aq) + Cl⁻ (aq) → Agcl (s)