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​These are chemistry questions and answers categorized according to topics, papers i.e. Paper 1 and 2, Levels i.e. form 1 to form 4, kcse year the examination was done and section A or B
Select topic/category to open topical questions from that particular option provided. ​Chemistry Topics
Above is a simplified diagram of a Down’s cell used for the manufacture of Sodium metal. Study it and answer the questions that follow.
a) Name the substance the anode is made of (1mk)
Graphite
b) Explain your answer in (a) above (1mk)
Reject carbon
Graphite does not react with chlorine liberated at the anode
c) What is the role of the diaphragm in Down’s cell (1mk)
To prevent metal and chlorine gas from recombining
d) In Down’s cell for the manufacture of Sodium metal, Calcium chloride salt is added to lower the melting point from 8000C to 6000C. Explain why it is necessary to lower the melting point (1mk)
To reduce the cost of energy used to melt the sodium metal
e) Calculate the mass of sodium metal produced if a current of 50 amperes is passed through the molten Sodium chloride for two (2) hours (Na = 23, F = 96500C) (2mks)
f) Below is a list of potential differences obtained when metal P, Q, R, S and T are used in the following electrochemical cell
|
| Molten M | Conduct electric current and is not decomposed |
| Molten N | Conduct electric current and a gas is formed at one of the electrodes |
Suggest the type of bonding present in;
a) Substance M (1mk)
b) Substance N (1mk)
(ii) Name the gas law illustrated in Figure 3.
(c)Use the standard electrode potentials in Table 2 to answer the questions that follow.
(ii) Arrange the elements in order of reducing power, starting with the weakest reducing agent.
(iii) I Select two half cells which combine to give a cell with the least e.m.f.
II Calculate the e.m.f of the half cells identified in (iii) I.
(i) An ore of a metal was roasted in a stream of oxygen. A gas with a pungent smell was formed which turned acidified potassium dichromate(VI) green.
(ii) The residue left after roasting was dissolved in hot dilute nitric(V) acid. Crystals were obtained from the solution.
(iii) Some crystals were dried and heated. A brown acidic gas and a colourless gas were evolved and a yellow solid remained. (iv) The solid was yellow when cold.
(v) The yellow solid was heated with powered charcoal. Shiny beads were formed.
Name the:
(a) gas formed when the ore was roasted in air.
(b) gases evolved when crystals in step (iii) were heated.
(c) yellow solid formed in step (iii).
(d) shiny beads in step (iv).
(e) The yellow solid from procedure (iii) was separated, dried, melted and the melt electrolysed using graphite electrodes.
I. Describe the observations made at each electrode.
II. Write the equation for the reaction that took place at the anode.
(f) Some crystals formed in step (ii) were dissolved in water, and a portion of it reacted with potassium iodide solution. A yellow precipitate was formed. Write an ionic equation for this reaction.
(g) To another portion of the solution from (f), sodium hydroxide solution was added drop by drop until there was no further change. Describe the observation made.
(h) To a further portion of the solution from (f), a piece of zinc foil was added.
I. Name the type of reaction taking place.
II. Write an ionic equation for the above reaction.
(b) State the products formed when brine is electrolysed using inert electrodes.
Anode:
Cathode:
(b)Anode - chlorine;
Cathode - Hydrogen;
(b) The part labelled A is a paste. Give a reason why it is not used in dry form.
(c) What is the purpose of the zinc container?
(b)To allow movement of ions / to have it as an electrolyte. When dry, the ions are immobile.
(c)It is the cathode / negative electrode.
(b)Write the equation for the reaction at the anode.
(c) Give a reason why the electrolytic process is not carried out below 950°C.
(d)Give a reason why the production of aluminium is not carried out using reduction process
(e)Give two reasons why only the aluminium ions are discharged
(f)State two properties of duralumin that makes it suitable for use in aircraft industry.
(g)Name two environmental effects caused by extraction of aluminium.
(ii)Describe how aqueous sodium hydroxide is formed in setup above.
(iii)One of the uses of sodium hydroxide is in manufacture of soaps. State one other use of sodium hydroxide.
(b) Study the information given in the table below and answer the questions that follow.
(ii) Calculate the emf of the cell constructed in (i) above.
(iii)Why is it not advisable to store a solution containing E+ ions in the container made of H?
(i) State whether acidified MnO4- can oxidise M2+. Give a reason. (2 marks)
(ii) Select two half-cells which when combined will give the highest e.m.f. (1 mark)
(iii) Write the cell representation for the cell formed in b (ii). (I mark)
(iv) Calculate the E0 value for the cell formed in b (iii). (2 marks)
(c) A mass of 1.24g of a divalent metal was deposited when a current of 6A was passed through a solution of the metal sulphate for 12 minutes. Determine the relative atomic mass of the metal.
(d) State two applications of electrolysis. (I mark)
i) Carbon electrode?
ii) The electrolyte?
ii) One of the substances used in a dry cell is manganese (IV) oxide. State two roles of manganese (IV) oxide in the dry cells
b) Below is simplified electrolytic cell used for purification of copper. Study it and answer the questions that follows.
ii) Write the equation for the reaction at the anode
iii) What name is given to L?
iv) A current of 0.6 A was passed Through the electrolyte for 2 hours.
Determine the amount of copper deposited (Cu=63.5; 1 Faraday = 96,500 coulombs)
v) St ate two uses of copper metal
II G
(ii) - Manganese (IV) oxide oxidises hydrogen to water /depolariser .
- It increases the surface area of the electrolyte
(b) (i) Cathode J.
- Electrical cables and alloys - coins, ornaments/lightening arrestors/ diodes/
- calorimeters.
Given that the E° of Cu(s) /Cu2+(aq) is +0.34 V and that of Zn (s)/Zn2+(aq) is —0.76 V, draw a labelled diagram of zinc and copper electrochemical cell. (3 marks)
(b) What observations will be made if a silver coin was dropped into an aqueous solution of copper (II) sulphate? Explain.
(b) No observable change silver is below copper in the reactivity series so it cannot displace it.
Name the product formed at the anode and give a reason for your answer.
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Maurice Atika is a teacher and a netprenuer.