All 21 questions from the Post-UTME Screening (Post-UTME) Chemistry 2018 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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H2S + Cl2 -> 2HCl + S. In the reaction above, the oxidation number of chlorine changed from
Chlorine in Cl2 has oxidation number 0; in HCl it is -1. The oxidation number changes from 0 to -1.
2K2Cr2O7(s) + 2H2O(l) + 3S(s) -> 3SO2(g) + 4KOH(aq) + 2Cr2O3. The oxidizing agent in the chemical reaction above is
K2Cr2O7 (dichromate) is reduced (Cr goes from +6 to +3) in this reaction, meaning it is the oxidizing agent.
The criterion that a given chemical reaction can occur spontaneously is given by
A reaction is spontaneous when the Gibbs free energy change (delta G) is negative.
H2O(s) -> H2O(l). The enthalpy change for the transition above is called the enthalpy of
The enthalpy change for solid converting to liquid is the enthalpy (latent heat) of fusion.
Which of the following processes is endothermic?
Dissolving ammonium chloride in water is a classic endothermic process, absorbing heat from the surroundings.
2H2(g) + O2(g) -> 2H2O(g). The entropy change, delta S, for the above reaction is
The reaction reduces the number of gas moles from 3 to 2, decreasing disorder, so the entropy change is negative.
A catalyst increases the rate of a reaction by
A catalyst speeds up a reaction by providing an alternative pathway with lower activation energy.
Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + SO2(g) + S(s). The time required for a white deposit of S to appear in the action above was measured at various temperatures. These results suggest that the rate of reaction
The classic thiosulphate-acid 'clock reaction' shows reaction time roughly halving (rate roughly doubling) for each 10C rise in temperature.
N2O4(g) <=> 2NO2(g). If the pressure in the above equilibrium is decreased, it is observed that the
Decreasing pressure shifts the equilibrium towards the side with more gas moles (2NO2), which is a darker brown colour.
Which of the following oxides is amphoteric?
Aluminium oxide (Al2O3) is a classic amphoteric oxide, reacting with both acids and bases.
In the preparation of ammonia, which of the following is used as a drying agent?
Calcium oxide (quicklime) is used to dry ammonia gas, since ammonia (a base) would react with acidic drying agents like conc. H2SO4 or form a complex with CaCl2.
Dilute AgNO3 is added to a solution, X, and a white precipitate soluble in excess ammonia is formed. X, therefore, contains
AgCl forms a white precipitate that dissolves in excess ammonia (forming a soluble silver-ammine complex), indicating chloride ions are present.
The decomposition of potassium trioxochlorate (V) in the presence of manganese (IV) oxide is represented by
MnO2 catalyses the decomposition of potassium trioxochlorate(V) (KClO3) into potassium chloride and oxygen gas: 2KClO3 -> 2KCl + 3O2.
When carbon (IV) oxide is bubbled into lime water, a white precipitate is first formed, which later dissolves in excess CO2 to form a colourless solution. The white precipitate and the soluble compound formed in the reactions are respectively
CO2 first forms insoluble CaCO3 with limewater; with excess CO2, this converts to soluble calcium hydrogencarbonate, Ca(HCO3)2.
The process whereby chloride ions are removed from an aqueous solution with silver ions is known as
Silver ions react with chloride ions to form insoluble AgCl, a process known as precipitation.
The normal properties of some metals are modified for more practical use by
Alloying metals with other metals or non-metals modifies their properties (e.g. hardness, corrosion resistance) for practical applications.
Which of the underlisted metals can be extracted from its ores by reduction of its oxide with carbon(II) oxide?
Iron is extracted in the blast furnace by reduction of its oxide with carbon monoxide (carbon(II) oxide).
Which of the following metals exhibits allotropy?
Tin exhibits allotropy, existing as white (beta) tin and grey (alpha) tin.
A salt soluble in water, gives a black precipitate with hydrogen sulphide and a deep blue colour with aqueous ammonia. The salt contains
Cu2+ forms a black CuS precipitate with H2S, and a characteristic deep blue complex ion with excess aqueous ammonia.
The IUPAC name for C(CH3)4 is
C(CH3)4 (neopentane) is correctly named 2,2-dimethylpropane.
Using the table above (CH4 = -74.8, O2 = 0, C = -393.5, H2O = -286), calculate the delta H for the following reaction at 25C: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g).
delta H = [(-393.5)+2(-286)] - [(-74.8)+2(0)] = -965.5 - (-74.8) = -890.7 kJ.
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