Question

"The resultant heat change in a chemical reaction is the same whether it takes place in one or several stages." This statement is called (a) Le-Chatelier's principle (b) Hess's law (c) Lavoisier-Laplace law (d) Joule-Thomson effect

Short answer

(b) Hess's Law.

Step-by-step solution

Analyze Statement about Heat Change

The given statement says that 'the resultant heat change in a chemical reaction is the same whether it takes place in one or several stages'. The principle that covers this phenomenon needs to be identified.

Match the Statement with the Principle

Going through the options, (a) Le-Chatelier's principle is about systems in equilibrium responding to changes to maintain the equilibrium state, (c) Lavoisier-Laplace law is connected to conservation of mass and energy, and, (d) Joule-Thomson effect is related to the temperature change of gas when it is forced through a valve. Therefore, none of these correspond completely to the statement. Only (b) Hess's law states that the total enthalpy change in a chemical reaction is the same no matter if the reaction happens in one or several steps. The statement is a description of Hess's Law.

Confirm the Correct Answer

After matching the given statement to the correct principle, the correct answer to this statement is: (b) Hess's Law.

Key concepts

Understanding Enthalpy Change

When we want to measure the energy changes in a chemical reaction, we often refer to the term enthalpy change. Simply put, enthalpy change is the difference in heat content of the products compared to the reactants under constant pressure. It's denoted by the symbol \( \Delta H \) and can either be positive or negative.

A positive \( \Delta H \) indicates that the reaction absorbs heat and is termed endothermic, while a negative \( \Delta H \) signifies that the reaction releases heat, making it exothermic.

Real-World Example

Imagine a campfire: the combustion of the wood releases heat, so the process has a negative \( \Delta H \) and is exothermic. Conversely, when you use an ice pack, the chemicals inside absorb heat to become cooler, so this has a positive \( \Delta H \) and is endothermic.
These concepts are crucial because they help us understand and predict whether a reaction will release or absorb heat, which is important in applications ranging from industrial manufacturing to preparing a hot cup of coffee.

Chemical Reactions Unveiled

Chemical reactions are the transformations that occur when substances interact to form new compounds. These reactions involve breaking bonds between atoms in the reactants and forming new bonds to create the products.

Every chemical reaction is subject to laws of thermodynamics and kinetics which govern the direction and the rate at which they occur. For instance, a reaction that emits heat, like a rusting iron nail, will progress spontaneously, while a reaction that requires heat might need a spark or energy source like the electrolysis of water.

The Role of Catalysts

In this dynamic world of reactions, catalysts are substances that speed up the reaction without being consumed themselves. They work by lowering the energy barrier for the reaction to occur, thereby increasing the rate at which products are formed. This principle is widely used in industries to enhance efficiency and productivity.

Thermochemistry: The Heat of the Matter

Thermochemistry is a branch of chemistry that deals with the study of heat energy involved in chemical reactions and physical transformations. It's grounded in the first law of thermodynamics which asserts that energy cannot be created or destroyed, only changed in form.

Thermochemistry focuses on enthalpy changes, heat capacity, and calorimetry among others to quantify the energy exchanges. A common tool used in thermochemistry is the calorimeter, an instrument that measures the heat of chemical reactions or physical changes as well as heat capacity.

Applying Thermochemistry

In practical terms, thermochemical principles are essential for processes such as calculating the energy content of food (calories), designing thermal insulation, and even developing renewable energy sources like biofuels. A solid understanding of thermochemistry is critical for managing energy resources and creating sustainable practices for the future.