Question

An exothermic reaction is carried out in a coffee-cup calorimeter. Which of the following statements is/are NOT true for the process? (a) The temperature of the water increases. (b) Heat is absorbed by the water. (c) The enthalpy of the products is higher than the enthalpy of the reactants. (d) \(q_{\mathrm{H}_{2} \mathrm{O}}=q_{\mathrm{rxn}}\) (e) \(q_{\mathrm{rxn}}>0\) (f) \(q_{\mathrm{rxn}}+q_{\mathrm{H}_{2} \mathrm{O}}=0\)

Short answer

Answer: The statements that are NOT true for the given exothermic reaction in a coffee-cup calorimeter are (c) and (e).

Step-by-step solution

Evaluate Statement (a)

The statement states that the temperature of water increases. This is true for an exothermic reaction because, as mentioned, heat is released during the reaction, which increases the temperature of the surrounding medium (water in this case). So, (a) is not the correct answer.

Evaluate Statement (b)

The statement states that heat is absorbed by water. Since the reaction is exothermic, the heat released gets absorbed by the water, which is true. So, (b) is not the correct answer.

Evaluate Statement (c)

The statement states that the enthalpy of products is higher than the enthalpy of reactants. This is NOT true for exothermic reactions, as the enthalpy of products is lower than the enthalpy of reactants. Therefore, (c) is a correct answer.

Evaluate Statement (d)

The statement states that \(q_{\mathrm{H}_{2} \mathrm{O}}=q_{\mathrm{rxn}}\). This is true since the heat absorbed by water equals the heat released by the reaction. So, (d) is not the correct answer.

Evaluate Statement (e)

The statement states that \(q_{\mathrm{rxn}}>0\). This is NOT true for exothermic reactions. The heat released by an exothermic reaction is negative, meaning \(q_{\mathrm{rxn}}<0\). Therefore, (e) is a correct answer.

Evaluate Statement (f)

The statement states that \(q_{\mathrm{rxn}}+q_{\mathrm{H}_{2} \mathrm{O}}=0\). This statement is true for any process carried out in a calorimeter since the heat released by the reaction is equal to the heat absorbed by the water. So, (f) is not the correct answer. The statements that are NOT true for the given exothermic reaction in a coffee-cup calorimeter are (c) and (e).

Key concepts

Coffee-cup Calorimeter

A coffee-cup calorimeter, often used in chemistry labs to measure heat changes, is a simple device that allows us to study the heat transfer associated with chemical reactions. These calorimeters consist of two Styrofoam cups nested together with a lid to minimize heat loss to the surroundings. Inside, the reaction occurs in aqueous solution, effectively absorbing or releasing heat into the surrounding water.

During an exothermic reaction in such a setup, heat released by the chemical reaction is absorbed by the water and the calorimeter itself. We measure the temperature change of the water to calculate the amount of heat (\( q \)) released or absorbed during the reaction using the specific heat capacity of water and mass of the water. This way, students can practically understand the concept of heat transfer in a more hands-on approach.

Enthalpy of Reaction

Enthalpy of reaction, often symbolized as \( \Delta H_{\mathrm{rxn}} \), is a thermodynamic property reflecting the heat content change during a chemical reaction. It provides information on the amount of heat absorbed or released when reactants are converted into products at constant pressure.

In the context of exothermic reactions, the enthalpy of products is lower than the enthalpy of reactants because energy is released to the surroundings, usually in the form of heat. This release shows that the system loses enthalpy, which is marked as a negative \( \Delta H_{\mathrm{rxn}} \). Understanding enthalpy changes is crucial for students because it helps predict whether a reaction is exothermic or endothermic and to what extent temperature will influence a reaction's progress.

Heat Transfer in Chemical Reactions

The heat transfer in chemical reactions is central to thermodynamics and involves the movement of thermal energy from a system to its surroundings or vice versa. Depending on whether a reaction is exothermic or endothermic, heat transfer will occur in different directions.

In an exothermic process, the heat transfers from the system, which contains the reacting substances, to the surroundings, thus increasing the temperature of the latter. The concept of heat transfer is essential as it helps predict how the energy of a system changes during a reaction. Furthermore, by applying the law of conservation of energy, we can understand that the energy lost by the system is equal to the energy gained by the surroundings, leading to equations such as \( q_{\mathrm{rxn}} + q_{\mathrm{H}_{2}O} = 0 \), reinforcing the principles that govern calorimetry and heat measurements in chemistry.