Question

The correct thermochemical cquation is (1) \(\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2} ; \Delta H=94 \mathrm{kcal}\) (2) \(\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2} ; \Delta H=+94 \mathrm{kcal}\) (3) \(\mathrm{C}_{(n)}+\mathrm{O}_{(\mathbb{B})} \longrightarrow \mathrm{CO}_{2 \mathrm{~g}} ; \Delta H=94 \mathrm{kcal}\) (4) \(\mathrm{C}_{(8)}+\mathrm{O}_{\mathrm{cg}} \longrightarrow \mathrm{CO}_{2 \mathrm{~m}} ; \Delta H=+94 \mathrm{kcal}\)

Short answer

The correct thermochemical equation is \( \mathrm{C}+\backslashmathrm{O}{2}\longrightarrow \mathrm{CO}_{2} ;\Delta H=-94 \mathrm{kcal}\

Step-by-step solution

Understand the Thermochemical Equation

A thermochemical equation represents a chemical reaction along with the enthalpy change, \( \Delta H \). \Delta H is positive when the reaction absorbs heat (endothermic), and negative when it releases heat (exothermic). The standard notation uses \( \Delta H \) without a sign for exothermic reactions by convention.

Identify Correct Reactants and Products

In the given reactions, the correct reactants should be \( \mathrm{C} \) and \( \mathrm{O}_{2} \), and the correct product should be \( \mathrm{CO}_{2} \). Verify the states of the elements: Reactants are solid carbon and gaseous oxygen, while the product is gaseous carbon dioxide.

Determine Enthalpy Change (Delta H)

For an accurate thermochemical equation, determine if \( \Delta H \) indicates exothermic or endothermic reaction. The combustion of carbon to form carbon dioxide (\

Key concepts

enthalpy change

Enthalpy change, represented by \(\text{ΔH}\), is the heat exchanged in a chemical reaction at constant pressure. It signifies whether a reaction absorbs or releases heat.

If \(\text{ΔH}\) is positive, the reaction is endothermic, meaning it absorbs heat from the surroundings. Conversely, if \(\text{ΔH}\) is negative, the reaction is exothermic, and it releases heat.

In the context of our problem, \(\text{ΔH} = 94 \text{kcal}\) for the reaction: \(\text{C} + \text{O}_{2} \rightarrow \text{CO}_{2}\). The notation for \(\text{ΔH}\) doesn't have a sign, which by convention implies an exothermic reaction. Therefore, it suggests the reaction releases 94 kcal of heat.

Understanding enthalpy change is crucial as it helps predict the heat flow direction in reactions and their energy efficiency. Factors like initial reactants' enthalpies and final products determine \(\text{ΔH}\). Notice in the equation, reactants must be in specific states, such as gaseous form for \(\text{O}_{2}\) and solid for \(\text{C}\), ensuring correct enthalpy calculations.

exothermic reaction

An exothermic reaction releases heat into the surroundings. This kind of reaction has a negative \(\text{ΔH}\) value.

For example, burning carbon in the presence of oxygen to form carbon dioxide, shown in our problem, is exothermic: \(\text{C} + \text{O}_{2} \rightarrow \text{CO}_{2}\) with \(\text{ΔH} = 94 \text{kcal}\), implies it releases 94 kcal of energy.

The energy given off generally manifests as heat, causing the temperature of the surroundings to rise. Common examples include combustion reactions, like burning fossil fuels or metabolic processes in living organisms.

Recognizing exothermic reactions is valuable because they often indicate reactions that are heat-generating, like those used in heat packs. In industrial settings, they can be harnessed for energy production.

Note that, in thermochemical equations, the states of matter (solid, liquid, gas) are crucial as they affect the energy released or absorbed.

endothermic reaction

An endothermic reaction is one that absorbs heat from its surroundings. Here, the value of \(\text{ΔH}\) is positive.

Endothermic reactions require energy to proceed. This energy intake is reflected in a temperature drop in the surroundings. For instance, photosynthesis in plants absorbs sunlight to convert carbon dioxide and water into glucose and oxygen.

These reactions are less intuitive than exothermic ones since they feel like they consume energy. Why would you need such reactions? They are essential for processes like chemical cold packs used for injuries or endothermic industrial processes.

For such reactions, thermochemical equations will show a positive \(\text{ΔH}\), indicating the energy absorbed. The reactants and products' states are critical, affecting the enthalpy calculations, ensuring accuracy in predicting energy changes.

Remember, the correct identification of surroundings in such reactions helps in heat management strategies, important in both lab and real-world applications.