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

Explanation:
Combining the first equation and the reverse of the second equation gives the equation for the enthalpy change in question.
ΔH∘=−393.5−(−283.0)=−110.5

Explanation​:
By applying Hess’s law, regardless of the route by which a chemical reaction proceeds, the enthalpy change will always have the same, provided that the initial and final states of the system are identical.

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:
Many products can be made from the same reactants by more than one route. Regardless of the route by which a chemical reaction proceeds, the enthalpy change will always be the same provided that the initial and final states of the system are identical.

Explanation:
An exothermic reaction is one in which heat energy is transferred from system to surroundings. Endothermic reaction is one in which heat energy is transferred from surroundings to system.

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

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 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​:
The graph shows that the maximum change in temperature is 24.9−20.0=4.9°C. However, extrapolating the linear cooling trend to the time of addition gives a theoretical maximum change of 5.4$°$C. This is the value of $\Delta T$ that should be used to obtain the best estimate of $q$, as the extrapolation accounts for heat loss to the environment during the reaction.

The other distractors correspond to the temperature change at the latest recorded time point (option A) and the extrapolated temperature change at time = 0$s$ (option D), a time point which precedes the addition of the solid and is therefore irrelevant.

This question is moderately difficult as the student must interpret a graph and apply the heat loss correction with little guidance.

Explanation:
The formation of ethanol in biofuel occurs during the fermentation of plant material in anaerobic conditions and in the presence of yeast as a biological catalyst:

C6H12O6+yeast →2CH3CH2OH+2 CO2​

To eliminate the incorrect answers, students must recognize that carbon fixation is the process where inorganic carbon (such as CO2​) 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:
In an electrochemical fuel cell, the fuel (methanol, for example) is oxidized at the anode, while oxygen gas is reduced at the cathode to form water. As is the case for any electrochemical cell, the electrons flow through an external wire from the anode to the cathode.

This question is categorized as medium because it requires the knowledge that oxygen gas is reduced at the cathode and that the product is water. The CO2​ would be a product for oxidizing a fuel such as methanol.

Explanation​:
ΔG=ΔH−TΔS.
Negative $\Delta \mathrm{G}$ values result in spontaneous reactions.
Negative $\Delta \mathrm{H}$ and positive $\Delta \mathrm{S}$ values result in a negative $\Delta \mathrm{G}$ value at all temperatures.

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:
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.

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​:
Most combustion reactions are exothermic. D is the opposite of a combustion reaction

Explanation​:
Gases have the highest entropy followed by liquids and then solids.
CO2​ has a higher entropy than O2 because it has more bonds, and therefore, more vibration modes.

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