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 \(\left.P\right)=\Delta H=H_{\text {products }}-H_{\text {reactants }}\) (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 the validity of the following statements based on the concept of enthalpy: (a) Enthalpy is a state property. (b) The heat of reaction at constant pressure is equal to the enthalpy change of the process. (c) The magnitude of enthalpy change is not affected by the amount of reactant in a process. (d) The enthalpy of the system remains unchanged in an exothermic process. Answer: (a) True (b) True (c) False (d) False

Step-by-step solution

(a) State property

Enthalpy is a state property because its value only depends on the initial and final states of a system and not on the path followed during the process. So, this statement is true.

(b) Heat of reaction at constant pressure

The equation \(q_{\text {reaction }}\) (at constant \(\left.P\right)=\Delta H=H_{\text {products }}-H_{\text {reactants }}\) represents the enthalpy change \(\Delta H\) during a reaction (at constant pressure). The enthalpy change is equal to the heat of the reaction and is calculated as the difference between the enthalpy of products and reactants. This statement is true.

(c) Magnitude of \(\Delta H\) and amount of reactant

The magnitude of enthalpy change \(\Delta H\) is proportional to the amount of reactant in a process. If the amount of reactant changes, the enthalpy change will also change. Therefore, this statement is false.

(d) Exothermic process and enthalpy of the system

In an exothermic process, the energy (in the form of heat) is released by the system to the surroundings, causing the enthalpy of the system to decrease. So, the enthalpy of the system does not remain unchanged in an exothermic process. This statement is false. Based on our analysis, the given statements are: (a) True (b) True (c) False (d) False

Key concepts

State Property

The concept of a state property is a cornerstone in understanding thermodynamics and its applications in chemistry. A property is classified as a state property when its value does not depend on the path taken to arrive at a particular state, but rather on the current state itself. Enthalpy, denoted by the symbol \( H \), is quintessential to this category.

When analyzing the characteristics of a substance or a chemical reaction, state properties can offer valuable insights without worrying about how the system arrived at a particular point. Indeed, enthalpy is a state property. It is relevant to comprehend that whether you're melting ice in a warm room or under a heat lamp, the enthalpy change involved will ultimately be the same provided the initial and final states are identical.

Understanding this helps in the calculations of energy changes in chemical reactions, where only the starting and ending points are required, making it more manageable for students and scientists to predict the heat involved or released in a process.

Heat of Reaction

In the world of chemistry, the heat of reaction, often represented as \( q_{\text{reaction}} \), is a term that refers to the thermal energy change during a chemical reaction at a constant pressure. It is synonymous with enthalpy change \( (\Delta H) \), making the equation \( q_{\text{reaction}}(\text{at constant }P)=\Delta H=H_{\text{products}}-H_{\text{reactants}} \) immensely important.

Why Is Heat of Reaction Important?

The heat of reaction allows scientists to classify reactions as either endothermic or exothermic. It essentially measures the energy either absorbed or released when a chemical reaction takes place. In an educational setting, simplifying this concept means explaining that the heat of reaction is like a balance sheet for energy during a chemical process, where we tally up the energy content involved at the beginning and end, with the difference giving us the net heat change.

To accurately capture this concept, imagine you’re cooking, and your recipe calls for a certain amount of heat to convert raw ingredients (reactants) into a cooked dish (products). Similarly, the heat of reaction represents the 'heat recipe' that tells you how much energy would be released or required to transform the reactants into products.

Exothermic Process

An exothermic process is a chemical reaction or physical change that releases energy, usually in the form of heat, to its surroundings. This is demonstrated by the fact that during an exothermic reaction, the enthalpy of the system decreases as it transfers energy outward.

Exothermic vs. Endothermic

Contrary to its counterpart, the endothermic process which requires energy input, an exothermic process is like a warm-hearted person, giving away energy. This phenomenon is characterized by a negative \( \Delta H \), indicating that the total energy of the products is less than that of the reactants. In simpler terms, it's like your body feeling warmer when you're exercising; the energy from the calories you burn is emitted as heat, making you feel hot.

Imagine lighting a campfire; it feels warm because the combustion of wood is an exothermic reaction releasing heat. This is why statement 'd' in the exercise is false. The enthalpy of a system definitely changes during an exothermic process because energy is leaving the system and going into the environment, reducing the system’s total enthalpy.