Question

If the value of \(\Delta \mathrm{H}\) in a reaction is positive, then the reaction is called: (a) Exothermic (b) Endothermic (c) Polymorphic (d) Polytropic

Short answer

The reaction is endothermic.

Step-by-step solution

Understand the Terms

First, let's understand what each term means. An exothermic reaction releases heat, while an endothermic reaction absorbs heat. The terms 'polymorphic' and 'polytropic' are unrelated to heat in chemical reactions.

Analyze the Question

The question states that the value of \( \Delta \mathrm{H} \) is positive, which means the enthalpy of the products is greater than the enthalpy of the reactants.

Apply the Definition of Endothermic

For a reaction where \( \Delta \mathrm{H} \) is positive, the system absorbs heat from the surroundings. Therefore, this reaction is endothermic.

Eliminate Incorrect Options

Based on the definition of an endothermic reaction, (a) Exothermic is incorrect because these reactions release heat. (c) Polymorphic and (d) Polytropic are not relevant to changes in enthalpy.

Key concepts

Enthalpy Change

Enthalpy change, denoted as \(\Delta H\), is an essential concept in thermochemistry. It refers to the difference in enthalpy between the products and reactants in a chemical reaction. Enthalpy itself is a measure of the total heat content of a system. In simpler terms, it tells us whether energy was absorbed or released during a reaction.

• If \(\Delta H\) is positive, it means the reaction absorbed heat, indicating that the products have more energy than the reactants.
• If \(\Delta H\) is negative, it means the reaction released heat, so the reactants had more energy than the products.

Understanding enthalpy change helps in predicting the heat interactions of a reaction with its surroundings. This concept is crucial in determining whether a reaction can effectively proceed under specific conditions.

Endothermic Reaction

An endothermic reaction is characterized by the absorption of heat from its surroundings. When the enthalpy change of a reaction \((\Delta H)\) is positive, it implies that the system takes in energy. This is a hallmark of endothermic processes, which require energy input to proceed.

• Endothermic reactions often feel cold to the touch, as they draw heat away from their environment.
• Common examples include ice melting or photosynthesis in plants.

The concept of endothermic reactions is important in understanding biochemical and physical processes occurring naturally or in laboratory settings. Recognizing endothermic reactions can aid in effectively controlling and utilizing these processes in practical applications.

Exothermic Reaction

Exothermic reactions are the opposite of endothermic reactions. In these reactions, heat is released into the surrounding environment, which makes them particularly interesting. They result in a negative \(\Delta H\) value, indicating energy is being expelled during the process.

• These reactions often produce noticeable heat or light, such as in combustion or rusting.
• An exothermic reaction can feel hot to the touch, due to the heat being released.

Understanding exothermic reactions is vital for fields such as engineering, environmental science, and various industries, where managing heat release and energy efficiency is crucial. Scientists and engineers carefully study exothermic reactions to harness energy in a controlled manner for constructive uses.