2021 WAEC Chemistry Objective Essay Question And Answer
Chemistry Essay Theory.
1. Identify the solid remaining when each of the following is heated.
(a) lithium trioxonitrate (V)
(b) potassium trioxonitrate (V)
(c) calcium trioxonitrate (V)
2. (a) When calcium oxide and coke are heated in an electric furnace, the products are carbon (ii) oxide and calcium carbide (CaC2), write the equation for this reaction.
(b) The addition of water to calcium carbide leads to the formation of calcium hydroxide and ethyne. Write the equation for the production of ethyne.
3. Calculate the percentage by mass of silicon tetrachloride. [2 marks]
4. Ammonia, NH3, and phosphine, Ph3, are the hydrides of the first two elements in group 5.
(a) Draw a dot and cross diagram for the ammonia molecule. [2 marks]
(b) Sketch and explain the shape of the ammonia molecule. [3 marks]
5. The first ionization energy of chlorine is +1260KJmol-1.
(a) Define the term first ionization energy.
(b) State and explain the general trend in the values of the first ionization energy for the elements across the period, sodium to argon in the periodic table.
6. Compound A consisting of carbon and hydrogen only. The compound was found to contain 80% carbon by mass.
(a) Calculate the empirical formula of compound A using the data above.
(b) The relative molecular mass of compound A was found to be 30.
Use this information to deduce the molecular formula of compound A.
[H = 1.00 C = 12.00]
7. State two factors other than a change in temperature or the use of a catalyst that influence the rate of a chemical reaction.
1. (a) Two elements represented by the letters Q and R have atomic numbers 9 and 12 respectively.
(i) Write the election configuration of R.
(ii) To what group does Q belong in the periodic table.
(iii) Write the formula of the compound formed when Q combines with R.
(iv) Explain briefly, why Q is a good oxidizing agent.
(v) State whether R would be expected to form acidic or basic oxide.
(b) (i) State two assumptions of the kinetic theory of gases.
(ii) When some solids are heated, they change directly into the gaseous state. What name is given to this phenomenon?
(iii) List two substances which exhibit the phenomenon mentioned in (ii).
(iv) Write an expression to show the mathematical relationship between the rate of diffusion of a gas and its vapour.
2. An aqueous solution has a pH of 4.0.
(a) (i) What is the hydrogen ion concentration of the solution?
(ii) What effect will it have on litmus paper?
(iii) Which of the following salt solutions would have the same effect on litmus? Give a reason for your answer. NH4Cl(aq); NaCl(aq) ; CH3OON(aq).
(b) (i) Differentiate between a fine chemical and a heavy chemical.
(ii) Name two sources of air pollution.
(iii) Suggest one way of reducing air pollution in cities
3. (a) (i) Explain briefly the fermentation process.
(ii) Write a balanced equation for the fermentation of glucose.
(iii) What substance must be added to glucose solution to ferment it?
(iv) Explain briefly why tightly corked glass filled to the brim with palm wine shatters on standing.
(b) State one industrial application of each of the following methods of separation:
(ii) Fractional distillation.
(c) Explain the following terms:
(d) Write a balanced equation to illustrate each of the terms in (c).
(e) i) What is hydrocarbon compound?
(ii) Name two principal sources of hydrocarbons.
The relationship between the compounds A and B is that they have similar chemical properties
– A : Trans-1,2-dichloroethene
– B : Cis-1,2-dichloroethene
Their chemical properties will be the same
The reason is because they are geometric isomers. Geometric isomers have the same chemical properties but different physical properties.
(i) Temperature remains constant during boiling
(ii) There is change of state during boiling
I – It will increase the boiling point of water.
II – It will reduce the boiling point of water.
Boiling occurs at a specific temperature While Evaporation occurs at almost all temperatures.
An unsaturated solution is one that contains less than the maximum amount of solute that can dissolve at a given temperature While A saturated solution is one that contains maximum of solute that can be dissolved in it at that temperature.
(i) By increasing the temperature of the solution.
(ii) By increasing the amount of the solute.
Mole is a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles
MgO(s)+2HCl(aq) —> MgCl(aq)+H2O(l)
From the equation of reaction; 1 mole of MgO = 2 moles of HCl
Hence n = cv/100
where c = 0.1moldm-³; v = 25.0cm³
No of mole of HCl = n = 0.1×25/1000
= 2.5 x 10-³ moles
Hence the no of moles ofMgO required = 2.5/2 x10-³
But n = m/M = reacting mass/ Molar mass —> m = nM = 1.25×10-³ x 40
= 50 x 10-³ = 0.05g of MgO.
(i) They are malleable
(ii) They are ductile
(iii) They are good conductors of heat and electricity
Draw the diagram
I. Activation Energy = (210 – 100)KJ = 110K
II. Hp – HR = ΔH
54 – 100 = -46KJ
Q2(g) + R2(g) –> 2QR(g)
The heat content of the reactant is higher than that of the product.
E7 – 1s² 2S² 2P³
F9 – 1s² 2S² 2P⁵
G12 – 1S² 2S² 2P⁶ 3S²
H13 – 1S² 2s² 2P⁶ 3S² 3P¹
I. G and H
Isotopy is the existence of an element in more than one physical form with the same atomic number but different atomic masses.
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