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

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:
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:
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:
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:
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 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:
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:
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:
Four $C-H$ bonds and two $O=O$ bonds are broken. Two $C=O$ and four $O-H$ bonds are formed.
ΔH =$\Sigma$ΔHBEθ(bonds broken) - ΣΔHBEθ(bonds formed).

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

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

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.