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​:
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​:
When a metal is combusted, a tremendous amount of energy is released in the form of heat or light. Since the energy is released, the reaction is exothermic, and the enthalpy is negative.
Additionally, the product of the combustion of the metal is the synthesis of the metal oxide.

This question is a medium difficulty where the student must know that combustions are exothermic and correspond to a negative enthalpy change. Further, the student must recognize that in the product compound, magnesium oxide, the magnesium has a +2 charge, and the oxygen has a −2 charge, resulting in MgO.

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

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

Explanation:
The full, balanced equation divided into the balanced half-reactions where the hydrogen gas is oxidized, and the oxygen gas is reduced, is:

Anode half-reaction: 2H2(g) →4H^+(aq)+4 e^−

Cathode half-reaction: O2(g)+4H+(aq)+4e^− →2H2O(l)

The number of electrons required from these half-reactions can be identified as 4.

This question is categorized as hard. The student must be able to separate the hydrogen and oxygen gases to their correct electrodes and determine which is oxidized and which is reduced.
The easiest way to start this is to recognize that if the hydrogen were to be reduced, then it become negatively charged, which is not as stable as the cationic form (H+). Therefore, the hydrogen gas must be oxidized and the oxygen gas reduced.
From that point, the student must correctly place the water in the oxygen’s half-reaction.

Another strategy that is effective is to determine the oxidation state changes for the elements in each half-reaction. Oxidation will occur at the anode, and so the half-reaction must have an increased change in oxidation state. At the cathode, reduction will be occurring and so the oxidation state will decrease.

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​:
$\mathrm{G}$ becomes more negative as a spontaneous reaction proceeds until it reaches its minimum. At this point, there will be no net change, and the system has reached equilibrium. $\Delta \mathrm{G}$ will equal zero but $\mathrm{G}$ will not.

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 is taking place under standard conditions, so it is fair to use standard enthalpies of formation to calculate $\Delta$Hr​. Even if the reaction takes place under non-standard conditions, the error is typically small unless a phase transition such as boiling or melting has taken place. In their standard states, carbon is a solid and bromine is a liquid, but it is not necessary for the elements to be gases as the enthalpy of vaporisation is accounted for in the measurement of ΔHfθ​.

Bond energy calculations do not account for the enthalpy changes of boiling, melting and sublimation, but the reactants and products in this reaction are gases so such errors are not relevant. The bond energies may, however, differ from the average values; for example, the C-H bond energies in the product may be affected by the neighbouring C-Br bond.

This question is only moderately difficult as the errors and assumptions of the two calculation methods are clearly stated in the syllabus.

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).

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​:
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