Question

In a reaction, catalyst changes (a) Qualitatively (b) Physically (c) Quantitatively (d) Chemically

Short answer

A catalyst remains unchanged chemically during a reaction (d).

Step-by-step solution

Understanding a Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without undergoing any permanent chemical change itself. This means it participates in the reaction but is not consumed by it.

Analyzing the Options

Examine what each option suggests: (a) Qualitatively - This refers to changes in quality or nature. (b) Physically - This suggests changes in physical properties like state or appearance. (c) Quantitatively - This refers to changes in quantity or amount. (d) Chemically - This implies a change in chemical composition or identity.

Identifying the Correct Choice

Since a catalyst speeds up a reaction without altering its chemical identity, it does not change chemically (d). It often remains unchanged after the reaction, not affecting the quantity, and typically doesn't alter its qualitative or physical property. Therefore, it undergoes an unchanged process in terms of chemical composition.

Key concepts

Chemical Reaction

A chemical reaction is the process in which substances, known as reactants, interact to form new substances, called products.
During this process, the reactants undergo a transformation that changes them into different molecules or compounds.
Chemical reactions are a crucial part of chemistry because they explain how different substances interact at a molecular level. Reactions can be simple or complex, but they always involve the breaking and forming of bonds.

• Breaking bonds in reactants requires energy.
• Forming bonds in products releases energy.

This energy change is often a driving force for reactions.
Catalysts play a key role in chemical reactions by lowering the activation energy needed for reaction, thus speeding up the process.

Reaction Rate

The reaction rate is a measure of how quickly a chemical reaction occurs.
It is determined by the change in concentration of reactants or products over time.
Understanding and controlling reaction rates is important in many industries, from pharmaceuticals to energy production. Several factors affect reaction rates:

• Concentration: Higher concentrations of reactants can lead to faster reactions.
• Temperature: Increasing temperature usually increases the rate because particles have more energy to react.
• Surface area: Larger surface areas allow more collisions between reactants, speeding up reactions.

Catalysts are another factor that can significantly increase the rate of reaction without being consumed in the process.

Chemical Change

A chemical change occurs when the chemical composition of a substance is altered.
This means that the original substances transform into new substances with different properties.
For instance, burning wood is a chemical change because it turns into ash, carbon dioxide, and water vapor. Several key indicators of chemical change include:

• Color change
• Gas production
• Temperature change
• Formation of a precipitate

Unlike physical changes, chemical changes result in substances that cannot easily be converted back into the original materials.
Catalysts do not undergo chemical changes, as they help reactions proceed without themselves being altered permanently.

Reaction Mechanism

A reaction mechanism is a detailed step-by-step description of how a chemical reaction occurs.
It outlines the molecular events leading from reactants to products, often through several intermediate steps.
By understanding the mechanism, we can gain insight into which bonds are broken and formed, and in what order. Reaction mechanisms are essential for predicting reaction outcomes and for designing new chemical processes.

• Elementary steps: Simple reactions with single steps.
• Intermediates: Temporary species formed during the reaction.
• Transition states: High-energy states during the reaction pathway.

Catalysts often work by providing an alternative mechanism that requires less energy, thus speeding up the reaction while remaining unchanged.