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).

Explanation​:
ΔG= ΔH−TΔS, and reactions are spontaneous if $\Delta \mathrm{G}$ is negative.
In endothermic reactions, $\Delta \mathrm{H}$ is positive, and if entropy increases, $\Delta \mathrm{S}$ is also positive.
At high temperatures, $-$ resulting in a negative $\Delta \mathrm{G}$ value.

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

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. (+33.9 kJ)−(−56.5 kJ)=+90.4 kJ

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:
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:
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:​
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:
To answer this question, the student must first recognize that oxygen gas is reduced at the cathode, and the fuel (methanol) is oxidized at the anode.
Next, it is essential to select the answer choice where the amount of electrons produced in the oxidation is equivalent to the electrons used in the reduction.
Finally, the question states the reaction is under acidic conditions, and therefore, the correct answer is the one that includes H+ as opposed to OH−.

This question is categorized as hard because there are many parameters to consider in order to select the correct answer:

• Cathode = site of reduction
• Anode = site of oxidation
• Electrons must be equal in each half-reaction
• Acidic conditions require H+ to be present in half-reactions

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

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:
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​:
The reactants are at a lower energy level to the products and are therefore more stable. The products are at a higher energy level to the products meaning the enthalpy change is positive (endothermic reaction).