Explanation​:
Combining the first equation and the reverse of the second equation gives the equation for the enthalpy change in question. (+33.9 kJ)−(−56.5 kJ)=+90.4 kJ

Explanation:​
Condensation (gas to liquid) requires making bonds and therefore, releases energy. Sublimation (solid to gas) requires breaking bonds and therefore absorbs energy.

Explanation​:
The silver in silver oxide has a $+1$ charge, resulting in Ag2O as the product. When arriving at the sum of the coefficients, it is important to remember to add a 1 for the O2​. Hence, the sum of the coefficients is 4+1+2=7

4Ag(s)+1O2​(g) →2Ag2​O(s)

This question is categorized as medium as the student must ensure the silver's correct charge and add a 1 for the O2.

Explanation:​
Average bond enthalpies give data for species in the gaseous phase. In choice C, water is a liquid and so the enthalpy change for the condensation of water vapour would also be needed.

Explanation​:
The standard enthalpy of formation of a pure element in its standard state is zero; not for a compound.

Explanation:
Atomisation is defined as the formation of one mole of gaseous atoms.
Ionization is the process in which one mole of electrons is removed from one mole of gaseous atoms/ions.
Electron affinity is the addition of one mole of electrons to gaseous atoms/ions.

Explanation​:
ΔG=−RT ln(K).
If $\Delta G$ is positive, $ln(K)$ must be negative. $ln(K)$ is negative for values smaller than 1.
A positive $\Delta G$ value indicates that the reaction is not spontaneous. Therefore, the equilibrium concentrations of the reactants are larger than the ones of the products; the equilibrium constant is less than 1.

Explanation:
Coal is an impure form of fossil fuel and therefore contains many impurities that will also combust. This will result in many byproducts from the impurities combusting, so more oxygen is required to combust the hydrocarbons completely.
Propane, natural gas, and crude oil are all cleaner fuels (have fewer impurities). So, they are more likely to undergo complete combustion with a comparable amount of oxygen per mole of fuel.

Explanation​:
Endothermic reactions absorb energy from the surroundings. This decreases the temperature of the surroundings and increases the energy in the bonds (the system).

C6H12O6+yeast →2CH3CH2OH+2CO2​

To eliminate the incorrect answers, students must recognize that carbon fixation is the process where inorganic carbon (such as CO2COX2​) is converted to organic compounds during photosynthesis.
Combustion of hydrocarbons would be the process that would take place when the biofuel is combusted.

This question is categorized as easy since the student needs only to recall that ethanol for biofuel is produced via the fermentation of plant material.

Explanation​:
When very little oxygen is present in the combustion of a hydrocarbon, the result is carbon soot (C solid) as opposed to carbon monoxide or carbon dioxide.

Sufficient oxygen $\to$ complete combustion resulting in CO2 and water;

Limited oxygen $\to$ incomplete combustion resulting in CO and water;

Very limited oxygen $\to$ incomplete combustion resulting in C soot and water.

For this question, the student must recognize that when there is limited oxygen, there are fewer oxygens in the products. This question is categorized as easy as the student needs only to recognize that the amount of oxygen in the combustion directly relates to the number of oxygens in the products.

Explanation:​
Energy is absorbed when breaking bonds (endothermic). The amount of energy required is dependent on the strength of the bond.
Statement III is incorrect because average bond enthalpies are averaged over a range of compounds.

Explanation​:
Increased temperature increases the kinetic energy of the gas particles and therefore increases the disorder. Gases are more disordered than liquids and solids. The greater the pressure, the lower the disorder.

Explanation​:
The endothermic reaction is favoured when the temperature increases. The concentration of products increases at t=10 mins; therefore, the forward reaction must be endothermic.
Increasing the volume decreases the pressure and causes the side with more moles of gas to be favoured. The concentration of reactants increases at $t$; therefore, the reactants must have more moles of gas than the products.

Explanation​:
The reaction equation is N2​O4​ (l) $\to$ 2NO2​ (g).

Drawing an enthalpy cycle reveals that ΔHfΘ​ (N2​O4​) = 2ΔHfθ​ (NO2​) − ΔHrθ​=2×33.1−85.8=−19.6kJ mol−1.

The alternative options are obtained if the wrong signs are given to the enthalpy changes in the equation and/or ΔHfθ (NO2​) is not multiplied by 2.

This question is categorised as very hard, as the student must deduce the stoichiometry of the reaction, sum the enthalpy changes with the correct signs, and obtain the correct value for the unknown quantity, without the aid of a reaction equation or enthalpy cycle.

Explanation:
D is a thermal decomposition reaction and so requires energy from the surroundings through heating in order to take place. A, B, and C are all combustion reactions and so exothermic.

Explanation:​
The products are at a lower energy level than the reactants meaning that the total average bond enthalpy of formation of bonds in the products is greater than the total average bond enthalpy of breaking of bonds in the reactants.

Explanation​:
Crude oil is non-renewable since there is a finite amount of this resource available on Earth. Crude oil has a high energy density meaning that it stores considerable energy in a smaller quantity than other energy sources.
Combusting crude oil releases sulfur and nitrogen oxides (thus contributing to acid precipitation) and carbon dioxide. Importantly, carbon dioxide is always a product of hydrocarbon combustion.

The question difficulty is categorized as a medium because there are a number of facts that the student must remember to answer this question correctly, but there is nothing that the student must solve to arrive at the correct answer.

Explanation​:
At equilibrium, Gibbs free energy is at a minimum.

Explanation:
As temperature increases, the equilibrium shifts to the left hand side (more blue) to oppose the change.
The backward reaction is therefore endothermic and absorbs energy, whereas the forward reaction is exothermic.
As equilibrium shifts to the left, Kc​ decreases as the concentration of reactants increases.