Question

The false statement among the following is (1) The heat liberated during the neutralization of \(\mathrm{a}\) strong acid and a strong base in an aqueous solution is constant. (2) The heat of combustion is always an exothermic change. (3) The enthalpies of formation of organic substances can be conveniently determined from heat of combustion data. (4) Heat of fomation of a compound is equal in magnitude to heat of combustion.

Short answer

Statement (4) is false.

Step-by-step solution

Understand thermochemistry concepts

Review the basic concepts of thermochemistry including enthalpy changes, heat of combustion, and heat of formation.

Analyze Statement (1)

Statement (1) states that the heat liberated during the neutralization of a strong acid and a strong base in an aqueous solution is constant. This is true because the neutralization of strong acids and strong bases releases a constant amount of heat.

Analyze Statement (2)

Statement (2) says that the heat of combustion is always an exothermic change. This is true because combustion reactions release energy, making them exothermic.

Analyze Statement (3)

Statement (3) indicates that the enthalpies of formation of organic substances can be conveniently determined from heat of combustion data. This is true as the heat of combustion can be used to find the enthalpy of formation.

Analyze Statement (4)

Statement (4) claims that the heat of formation of a compound is equal in magnitude to heat of combustion. This is false because the heat of formation and the heat of combustion generally involve different processes and therefore have different values.

Identify the false statement

Based on the analysis of each statement, it is clear that Statement (4) is false.

Key concepts

enthalpy change

Enthalpy change is a vital concept in thermochemistry. It refers to the heat change at constant pressure during a chemical reaction. We denote it as \(\Delta H \) and it can either be positive or negative. When \(\Delta H > 0\), the reaction absorbs heat, making it endothermic. If \(\Delta H < 0\), the reaction releases heat, hence it's exothermic. To determine enthalpy change, consider whether bonds are being broken or formed. Breaking bonds requires energy (endothermic), while forming bonds releases energy (exothermic). In chemical reactions, summing up the enthalpies of the reactants and products helps determine the overall enthalpy change. This is key to understanding energy transfer in reactions.

heat of combustion

The heat of combustion is always an exothermic process. It measures the amount of heat released when a substance completely burns with oxygen. We express it in kilojoules per mole (kJ/mol). Combustion reactions are exothermic because they release energy as heat. The general formula for a combustion reaction is:

\[ \text{Fuel} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Heat} \]
Key examples include burning hydrocarbons like methane or butane. The significant heat released in such reactions is what makes fuels useful for energy production. Measuring the heat of combustion helps in comparing the energy content of different fuels.

heat of formation

The heat of formation, also known as enthalpy of formation, is the heat change when one mole of a compound is formed from its elements in their standard states. Represented as:
\[ \Delta H_f^\text{o} \]
To calculate the heat of formation, we need the heats of combustion of the components involved. We use the equation:
\[ \Delta H_f^\text{o} = \sum \Delta H_\text {Products} - \sum \Delta H_\text {Reactants} \]
This makes it convenient to derive \Delta H_f^\text{o} from heat of combustion data. For instance, forming water from hydrogen and oxygen involves a specific enthalpy change, and knowing the heats of combustion of H\textsubscript{2} and O\textsubscript{2} assists in this calculation.

neutralization

Neutralization is a process where an acid reacts with a base to form water and a salt, typically releasing heat. For strong acids and strong bases, the heat of neutralization is nearly constant, about -57 kJ/mol. The general neutralization reaction is:
\[ \text{H}_\text{A} + \text{B}\text{OH} \rightarrow \text{AB} + \text{H}_2\text{O} \]
Here, \( \text{H}_\text{A} \) represents the acid and \( \text{B}\text{OH} \) the base. Strong acids and bases fully dissociate in water, which is why the heat change remains consistent. The released heat comes from the formation of water molecules, which is always exothermic. Understanding neutralization helps in fields like chemistry and environmental science for managing acids and bases safely.