Question

Determine whether the statements given below are true or false. Consider enthalpy \((H)\). (a) It is a state property. (b) \(q_{\text {reaction }}(\) at constant \(P)=\Delta H=H_{\text {products }}-H_{\text {reactants }}\) (c) The magnitude of \(\Delta H\) is independent of the amount of reactant. (d) In an exothermic process, the enthalpy of the system remains unchanged.

Short answer

Question: Determine whether the following statements about enthalpy (H) are true or false: a) It is a state property. b) \(q_{\text {reaction }}(\) at constant \(P)=\Delta H=H_{\text {products}}-H_{\text {reactants }}\) c) The magnitude of \(\Delta H\) is independent of the amount of reactant. d) In an exothermic process, the enthalpy of the system remains unchanged. Answer: a) True, b) True, c) False, d) False

Step-by-step solution

Statement (a) - It is a state property.

Enthalpy (H) is considered a state property because it depends only on the current state of the system and not on the path taken to reach that state. So, statement (a) is True.

Statement (b) - \(q_{\text {reaction }}(\) at constant \(P)=\Delta H=H_{\text {products}}-H_{\text {reactants }}\)

At constant pressure, the heat exchanged during a chemical reaction (q_reaction) is equal to the change in enthalpy of the system, which can also be represented as the difference between the enthalpy of products and the enthalpy of reactants, \(\Delta H=H_{\text {products}}-H_{\text {reactants }}\). Therefore, statement (b) is True.

Statement (c) - The magnitude of \(\Delta H\) is independent of the amount of reactant.

Enthalpy change, \(\Delta H\), is an extensive property, which means it depends on the size of the system or the amount of substance reacting. When more reactants are present, a larger amount of energy will be transferred as heat, leading to a greater change in enthalpy. Thus, statement (c) is False.

Statement (d) - In an exothermic process, the enthalpy of the system remains unchanged.

In an exothermic process, energy is released in the form of heat. This implies that the enthalpy of the products is less than the enthalpy of the reactants, so the enthalpy of the system changes during the process. Therefore, statement (d) is False.

Key concepts

State Property

Enthalpy, often represented with the symbol \(H\), is what scientists call a 'state property'. This means that its value is fixed when the temperature, pressure, and composition of the system are specified, without considering how the system arrived at that condition. A state property doesn't depend on the route taken to achieve that state. For example, whether you climb a hill by a steep path or a gentle slope, the height you reach is the same. Similarly, in chemistry, no matter the path a reaction takes, the initial and final enthalpy values are unique and specific to their respective states.

In the context of the textbook exercise, recognizing enthalpy as a state property helps us understand that the final energy content of a system is independent of the chemical pathway. This is why we can confidently say that the statement (a) in our exercise is true.

Enthalpy Change

The term 'enthalpy change' refers to the quantity of energy released or absorbed when a chemical reaction occurs at constant pressure, and it is denoted as \(\Delta H\). It is calculated by the formula \(\Delta H=H_{\text{products}}-H_{\text{reactants}}\). The enthalpy change is a key factor in determining the heat flow of a reaction as well as being an essential aspect of Hess's Law, which states that the total enthalpy change during the course of a reaction is the same, no matter how the reaction occurs in multiple steps.

During a reaction at constant pressure, the heat that is either taken in or given off is equivalent to the enthalpy change, making statement (b) true in our given exercise. Understanding \(\Delta H\) helps in predicting whether a reaction will be exothermic (releasing energy) or endothermic (absorbing energy) and is crucial for engineers and scientists in optimizing industrial and laboratory processes.

Extensive Property

The concept of an 'extensive property' is essential in understanding the scale of physical quantities. Unlike state properties, extensive properties are additive and dependent on the system's size or the amount of matter in the system. Mass, volume, and as noted in our textbook exercise, enthalpy change (\(\Delta H\)), are all examples of extensive properties.

To clear any confusion, the fact that enthalpy is an extensive property implies that the magnitude of \(\Delta H\) does change with the amount of reactant you have. Therefore, if you double the amount of reactant in a chemical reaction, the enthalpy change will also double. This dependency refutes statement (c), marking it as false and providing a crucial piece of information for students when scaling reactions or calculating the energy involved in industrial-sized processes.

Exothermic Process

In chemistry, a process is said to be an 'exothermic process' when it releases heat, leading to an increase in the temperature of the surroundings. This type of reaction often gives off light or heat, making it tangible and easy to understand. When bonds are formed in a chemical reaction, energy is often released, and this is the essence of an exothermic process. The enthalpy change for these processes is negative (\(\Delta H < 0\)), since the total energy of the products is less than that of the reactants.

Returning to our textbook exercise, it's important to note that because energy is being released during an exothermic process, the enthalpy does indeed change, contrary to what statement (d) implies. It decreases, which is why statement (d) is false. This concept is particularly visible in everyday phenomena such as combustion, where fuel burns and releases heat energy to the environment.