What does not increase the number of reactive particles in a chemical reaction

To determine which of the following scenarios does not increase the number of reactive particles in a chemical reaction, we need to analyze the effects of temperature, pressure, and concentration on particle interaction:

Increasing the temperature

Increasing the temperature provides particles with more kinetic energy, thereby increasing their movement and collision frequency. As a result, more particles become reactive.

Increasing the pressure

Increasing the pressure, primarily in gaseous reactions, forces particles closer together. This leads to an increase in the collision rate. Hence, the number of reactive particles increases as well.

Decreasing the concentration

Decreasing the concentration of reactants means there are fewer reactive particles in a given volume. This makes collisions between particles less frequent, so the number of reactive particles does not increase. In fact, it decreases.

Increasing the concentration

Increasing the concentration of reactants means more particles are present in the same volume, which leads to an increased collision rate. Therefore, the number of reactive particles increases.

Thus, the scenario that does not increase the number of reactive particles in a chemical reaction is:

Explanation of Collision Theory

Collision theory is a framework used to understand how chemical reactions occur and why reaction rates differ for different reactions. It is based on the idea that for a reaction to take place, reactant molecules must collide with sufficient energy and proper orientation.

Key Concepts

1. Effective Collisions: Not all collisions between molecules result in a chemical reaction. For a collision to be effective, the molecules must collide with the proper orientation and possess the minimum energy required, known as the activation energy $E_a$.

2. Activation Energy ($E_a$): This is the minimum energy required for a chemical reaction to occur. If the colliding molecules do not have enough energy to overcome this barrier, the reaction will not proceed.

   $$E_a \geq \frac{1}{2} mv^2$$

   Where $m$ is the mass of the reactant molecules and $v$ is their velocity.

3. Orientation Factor: Even if molecules collide with sufficient energy, they must also be oriented in a specific way to form the desired products. This factor is often represented in probabilistic terms.

4. Rate of Reaction: The rate of reaction can be understood using the rate constant $k$, which incorporates the probability of successful collisions. The Arrhenius equation relates the rate constant to the activation energy:

   $$k = A e^{-\frac{E_a}{RT}}$$

   Where:

   • $A$ is the pre-exponential factor (frequency of collisions),
   • $E_a$ is the activation energy,
   • $R$ is the gas constant (8.314 J/mol·K),
   • $T$ is the temperature in Kelvin.

Important Points

• Temperature Dependency: As the temperature increases, the average kinetic energy of the molecules increases, leading to more collisions with sufficient energy to overcome the activation energy barrier.

• Concentration Effect: A higher concentration of reactants leads to an increased frequency of collisions, thus increasing the reaction rate.

In summary, collision theory provides a comprehensive explanation of the factors affecting the rate of chemical reactions, focusing on the frequency, energy, and orientation of molecular collisions.

Understanding the Effect of Temperature on Reaction Rate

The temperature of a system plays a crucial role in determining the rate at which chemical reactions occur. This relationship can be understood through a few key concepts and mathematical equations.

Collision Theory

According to collision theory, chemical reactions occur when reactant molecules collide with sufficient energy and proper orientation. Increasing temperature increases the kinetic energy of the molecules, leading to more collisions and hence a higher reaction rate.

Arrhenius Equation

The most important mathematical relationship that quantifies the effect of temperature on reaction rate is the Arrhenius equation:

Where:

• $k$ is the reaction rate constant,
• $A$ is the pre-exponential factor (frequency of collisions with proper orientation),
• $E_a$ is the activation energy,
• $R$ is the gas constant,
• $T$ is the absolute temperature.

Increasing the temperature ($T$) generally increases the rate constant $k$, leading to a faster reaction.

Activation Energy

The activation energy ($E_a$) is the minimum energy required for a reaction to occur. The higher the activation energy, the more sensitive the reaction rate is to temperature changes.

Van't Hoff Equation

Another useful equation is the Van't Hoff equation, which relates the change in the equilibrium constant $K$ to temperature:

Here:

• $\Delta H^\circ$ is the standard enthalpy change,
• $\Delta S^\circ$ is the standard entropy change.

This equation helps to understand how the position of equilibrium shifts with temperature.

Practical Implications

In practical terms:

• Refrigeration slows down spoilage by decreasing the rate of microbial and enzymatic reactions in food.
• Catalysts are often used with temperature adjustments to optimize industrial chemical processes.

Understanding these principles aids in manipulating reaction rates for desired outcomes in both experimental and industrial settings.