In exothermic and endothermic reactions, heat can be considered to be either a product or reactant, respectively. Increasing the temperature of the an exothermic reaction is effectively the same as adding heat as a product, causing the reaction to shift to the left. This will cause the reverse reaction to increase in rate. 

From the question stem, we are told that the breakdown of 2 moles of gaseous  requires an absorption of  of energy. For simplification, we can write out the reaction as:

By definition, the heat of formation of a compound is the enthalpy change that occurs for the formation () of that compound from its elements in their standard state. Since  and  are in their standard state, we know that they have a heat of formation of zero, thus simplifying the calculation. Thus, if we reverse the above reaction, we see that:

Notice that by reversing the reaction, we are also reversing the sign for the enthalpy of the reaction. Now, to calculate the  of  as kilojoules per mole, we need to multiply the reaction by one-half to obtain:

We can calculate the standard change in enthalpy for the reaction using the following equation:

 of   

 of 

 of 

Plug in known values and solve.

A negative  indicates that the reaction is exothermic. This means that the reaction will release energy (usually in the form of heat) to the surroundings.

A positive  indicates that the reaction is endothermic. This means that the reaction will absorb energy (usually in the form of heat) from the surroundings.

Therefore, this overall reaction is endothermic.

Negative enthalpy change() indicates that heat is on the product side of the reaction, or, is released by the reaction. This is also the definition of an exothermic reaction.

When a chemical reaction is represented graphically, we see that the enthalpy change is reversed between the forward and reverse reactions. If a reaction produces energy in a forward process, it will require an input of energy in the reverse process, and vice versa.

A catalyst only affects the rate of a chemical reaction; it does not affect the equilibrium. Finally, exothermic reactions are not always spontaneous, but will have lower activation of energies compared to endothermic reactions.

The change in enthalpy is calculated by:

When  cannot be measured, it can be calculated from known enthalpies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in enthalpy of a reaction.

Hess's law states that the change in enthalpy for a total reaction can be considered equal to the sum of the enthalpy changes for every step involved in the reaction. In other words, we can determine the enthalpy change for nitrogen dioxide by adding the enthalpy changes for both steps involved in its formation.

This gives us the total change in enthalpy for the listed reaction, . Because the question asks for the enthalpy change for four moles of nitrogen dioxide, the value must be doubled. The reaction only produces two moles of nitrogen dioxide.

ΔH = Σ ΔHf products - Σ ΔHf reactants

ΔHf O₂ or any element is 0

First step is to balance the equation:

CH_{4 (g)} + O₂ (g)  ⇌  CO_{2 (g)} + 2H₂O (l)

ΔH = Σ ΔHf products - Σ ΔHf reactants

= [-393.5 kJ/mol + 2(-285.8) kJ/mol] - (-74.8 kJ/mol)

= -890.3 kJ/mol

The change in enthalpy is calculated by:

When  cannot be measured, it can be calculated from known reactions. In this case the known reactions are given.

Since the reactions are in the correct order, adding all the  values together can be used to calculate the  of the reaction.

The change in enthalpy is calculated by:

When  cannot be measured, it can be calculated from known enthalpies of formation.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in enthalpy of a reaction.