Chemistry 2009 Jamb Examination

Gibbs free energy determines spontaneity. A negative value of $$\Delta G$$ indicates the reaction can proceed without external work. Here, $$\Delta G = -43\ \text{kJ}$$ at 1300 K, which shows the process is thermodynamically spontaneous (feasible) under those conditions. Note: exothermic refers to enthalpy change, not directly to $$\Delta G$$.

Mixing two different ideal gases increases the number of accessible microstates and therefore increases disorder. Entropy is a measure of that disorder. When ammonia and nitrogen mix spontaneously, randomness rises and the system entropy increases in accordance with the second law of thermodynamics.

Decomposition of potassium chlorate produces oxygen gas gradually until the reactant is consumed. The oxygen volume rises with time and then levels off as the reaction nears completion. Curve Z best matches that kinetic profile: a steady increase followed by a plateau when the solid is exhausted.

Dilute sulfuric acid, $$H_2SO_4$$, is commonly used to increase the conductivity of water for electrolysis. Sulfate ions are stable under conditions used and do not produce interfering gases the way chloride can; nitric acid and acetic acid are less commonly used for routine electrolysis of pure water.

Use the relation between moles, concentration and volume. First compute volume in litres, then convert to cm³.
$$n = 0.045\ \text{mol}$$$$C = 1.5\ \text{mol L}^{-1}$$$$V = \dfrac{n}{C} = \dfrac{0.045}{1.5} = 0.03\ \text{L}$$Converting to cubic centimetres: $$0.03\ \text{L} = 30.0\ \text{cm}^3$$. Thus the correct volume is 30.00 cm³.

Citric acid is a triprotic weak acid and sodium hydroxide is a strong base. Neutralization yields the conjugate base (a citrate salt) which, because it comes from a weak acid and a strong base, produces a basic solution due to hydrolysis of the anion. For example:

C₆H₈O₇ + 3NaOH → Na₃C₆H₅O₇ + 3H₂O

The citrate anion hydrolyzes slightly to produce OH⁻, making the solution basic.

Acidified potassium dichromate ($K_2Cr_2O_7$) contains chromium in the +6 oxidation state (orange). Reducing agents convert Cr(VI) to Cr(III), which is green in solution. Therefore the observable colour change when a reducing agent is present is from orange to green.

Calculate oxidation state by assigning known charges and solving for chromium. Potassium is +1 and oxygen is -2.

$$2(+1) + 2(x) + 7(-2) = 0$$

$$2 + 2x -14 = 0$$

$$2x -12 = 0 \Rightarrow x = +6$$

So chromium is in the +6 oxidation state in dichromate.

Efflorescence is the loss of water of crystallization from a hydrated salt when exposed to air, causing the crystalline solid to become powdery or to lose mass. Deliquescence is absorption of moisture to dissolve, hygroscopic describes substances that absorb moisture, and effervescence is bubbling due to gas release.

Combustion of hydrogen with oxygen yields only water as product when stoichiometric amounts are used. For example, the balanced equation is:
$$2H_2 + O_2 \rightarrow 2H_2O$$This reaction produces water exclusively (no carbon-containing products), unlike hydrocarbon combustion which also yields CO₂.