CIE iGCSE Co-Ordinated Science C12.5 Identification of ions and gases Exam Style Questions Paper 4
Question
Fig. 8.1 shows a toy car that is powered by hydrogen gas.
(a) (i) The hydrogen gas is made by the electrolysis of water.
During the electrolysis, hydrogen ions, H+, gain electrons.
Hydrogen gas, H2, is made.
Construct the ionic half-equation for this reaction.
Use e– to represent an electron.
(ii) Oxygen gas is also made during the electrolysis of water.
4OH– → 2H2O + O2 + 4e–
This is an example of oxidation.
Explain why.
(b) The equation for the breakdown of water by electrolysis is shown.
2H2O → 2H2 + O2
45g of water makes 40g of oxygen gas.
Calculate the volume occupied by 40g of oxygen gas at r.t.p.
The volume of one mole of any gas is 24dm3 at room temperature and pressure (r.t.p.).
[Ar : H, 1; O, 16]
volume of oxygen gas = ………………………………………….. dm3
(c) Hydrogen, oxygen and water are all covalent molecules with low melting and boiling points.
Explain why these covalent molecules have low melting and boiling points.
(d) At very high temperatures, oxygen reacts with silicon to form silicon(IV) oxide (silicon dioxide), SiO2.
Fig. 8.2 shows part of the structure of silicon dioxide.
Describe the structure of silicon dioxide.
(e) Oxides are classified as acidic, basic, amphoteric or neutral.
Silicon dioxide is an acidic oxide.
Draw a line to classify each of the oxides in the diagram. One has been done for you.
▶️Answer/Explanation
Ans :
8(a)(i) 2H+ + 2e– → H2 ;;
8(a)(ii) loss of electrons / AW ;
8(b) Mr of O2 = 32 ;
moles of O2 = 40 ÷ 32 = 1.25 ;
volume of O2 = 1.25 × 24 = 30 dm3 ;
8(c) weak intermolecular forces / weak forces between molecules ;
(weak forces) which require little energy to break ;
8(d) giant covalent OR covalent lattice ;
each oxygen atom forms bonds with 2 silicon atoms / each silicon atoms forms bonds with 4 oxygen atoms ;
8(e) 