Question 2
(a) Explain the statement, the standard electrode potential of zinc is -0.76 v.What is meant by the term periodic property of elements?
(b) Consider the following standard electrode potentials:
Zn2+ (aq) + 2e- Zn(s) Eᶿ = - 0.76 V
Cu2+(aq) + 2e- Cu(s) Eᶿ = + 0.34 V
When the two half cells are connected:
(i) write the reaction equation at each electrode;
(ii) write the overall cell reaction equation;
(iii) state the type of reaction occurring at each electrode;
(iv) calculate the e.m.f. of the cell. [10 marks]
(c) (i) Name two chemical industries.
(ii) State two factors that should be considered when siting a chemical industry.
(iii) List two effects of a chemical industry on the community in which it is sited. [6 marks]
(d) Using chemical equations, explain briefly what would happen when hydrogen peroxide is added to:
(i) silver oxide;
(ii) chlorine gas. [6 marks]
(e) List three physical properties of nitrogen. [3 marks]
Observation
This question was not a compulsory question. Majority of the candidates did not attempt the question.
In part (a), majority of the candidates were not familiar with standard electrode potential, and hence could not explain the statement.
In part (b), majority of the candidates could not write the reaction equation at each electrode and the overall cell reaction equation.
In part (c), majority of the candidates named two chemical industries and stated two factors that should be considered when sitting a chemical industry.
In part (d), majority of the candidates could not explain what would happen when hydrogen peroxide is added to silver oxide and chlorine gas.
In part (e), majority of the candidates listed three physical properties of nitrogen.
The expected answers include:
(a) When a standard hydrogen electrode is connected to a standard zinc half-cell the emf measured is 0.76V. The negative sign implies the flow of electrons from the zinc electrode to the hydrogen electrode.
(b)(i) Anode/ Zn electrode
Zn(s) → Zn2+(aq) + 2e- / Zn(s) – 2e- → Zn2+(aq)
Cathode/Cu electrode
Cu2+(aq) + 2e- → Cu(s)
(ii) Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
(iii) Anode/Zn electrode – oxidation
Cathode/Cu electrode – reduction
(iv) Eocell = Eored – Eooxid
= (0.34 V) – (-0.76 V)
= + 1.10 V
(c) (i) - Textile
- Brewery
- Plastic
- Refinery
- Cement
- Food [e.g flour, tea, sugar, etc]
- Pharmaceutical
- Fertilizer
- Metallurgical
- Paint
- Soap and detergent
- Ceramic
- Rubber
- Glass
- Etc
(ii) - nearness to raw materials / feed stock
- nearness to market
- labour supply
- transportation
- nearness to power supply
- government policy
- away from residential area
- availability of water
(iii)- Improvement in the standard of living
- Employment opportunities
- Development in the community
- Pollution: air, noise, water, thermal
- Social effects
(d)(i) Hydrogen peroxide reduces silver oxide to silver metal/H2O2is oxidized to O2
A + H2O2(aq) → 2Ag(s) + H2O(l) + O2(g)
(ii) Hydrogen peroxide reduces chlorine to hydrochloric acid/H2O2is oxidized to O2
Cl2(g) + H2O2(aq) → 2HCl(aq) + O2(g)
(e) - colourless
- odourless
- slightly lighter / less dense than air
- slightly soluble in water
- low melting (-210 oC/ 63 K)/low boiling point (-196 oC/ 77 K)