(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:
Zn(^{2+})(_{(aq)}) + 2e(^-) Zn(s) Eᶿ = – 0.76 V
Cu({2+})(_{(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.
(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.
(d) Using chemical equations, explain briefly what would happen when hydrogen peroxide is added to:
(i) silver oxide;
(ii) chlorine gas.
(e) List three physical properties of nitrogen.
Explanation
(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) → Zn(^{2+})(_{(aq)}) + 2e(^-) / Zn(s) – 2e(^-) → Zn(^{2+})(_{(aq)})
Cathode/Cu electrode
Cu(^{2+}) (_{(aq)}) + 2e(^-) → Cu(s)
(ii) Zn(s) + Cu(^{2+}_{(aq)}) → Zn(^{2+}_{(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 + H(_2)O(_2)(_{(aq)}) → 2Ag(_{(s)}) + H(_2)O(_{(l)}) + O(_{2(g)})
(ii) Hydrogen peroxide reduces chlorine to hydrochloric acid/H(_2)O(_2) is oxidized to O(_2)
Cl(_{2(g)}) + H(_2)O(_{2(aq)}) → 2HCl(_{(aq)}) + O(_{2(g)})
(e) – colourless
– odourless
– slightly lighter / less dense than air
– slightly soluble in water
– low melting (-210(^{o})C/ 63 K)/low boiling point (-196(^{o})C/ 77 K)