General Chemistry II Exam 3 Study Guide: Chemical Kinetics & Equilibrium

Q1. What is the collision model?

Background

Topic: Chemical Kinetics – Collision Theory

This question is testing your understanding of how molecular collisions relate to reaction rates and the requirements for a reaction to occur at the molecular level.

Key Terms and Concepts:

• Collision Model: A theory that explains how chemical reactions occur and why reaction rates differ for different reactions.

• Activation Energy ($E_a$): The minimum energy that reacting molecules must have for a reaction to occur.

• Effective Collision: A collision that results in a chemical reaction, requiring proper orientation and sufficient energy.

Step-by-Step Guidance

1. Start by considering what must happen for two molecules to react. Think about both the energy and the orientation of the molecules during a collision.

2. Recall that not every collision leads to a reaction. Identify the two main requirements for a successful (effective) collision.

3. Think about how the collision model explains the effect of temperature and concentration on reaction rates.

4. Consider how the collision model connects to the concept of activation energy and the need for molecules to overcome this barrier.

Try explaining the collision model in your own words before checking the answer!

Final Answer:

The collision model states that for a reaction to occur, reactant molecules must collide with sufficient energy (at least the activation energy) and with the correct orientation. Only a fraction of collisions are effective and lead to product formation. This model helps explain why increasing temperature or concentration increases the reaction rate: more frequent and more energetic collisions occur.

Q2. What are the factors that impact the rate of reaction? How do they impact the reaction rate?

Background

Topic: Chemical Kinetics – Factors Affecting Reaction Rate

This question is about identifying and explaining the main factors that influence how quickly a chemical reaction proceeds.

Key Terms and Concepts:

• Concentration: The amount of reactant present in a given volume.

• Temperature: A measure of the average kinetic energy of particles.

• Catalyst: A substance that increases the reaction rate without being consumed.

• Surface Area: The amount of exposed area of a solid reactant.

• Nature of Reactants: The chemical identity and physical state of the reactants.

Step-by-Step Guidance

1. List the main factors that can affect the rate of a chemical reaction (think about what you can change in an experiment to speed up or slow down a reaction).

2. For each factor, briefly describe how increasing or decreasing it would affect the reaction rate (e.g., what happens if you increase the temperature?).

3. Consider the molecular-level explanation for each factor (e.g., why does a catalyst speed up a reaction?).

4. Think about examples or evidence that support each factor's effect on reaction rate.

Try listing and explaining the factors before checking the answer!

Final Answer:

The main factors that impact reaction rate are: concentration (higher concentration increases rate), temperature (higher temperature increases rate), presence of a catalyst (increases rate), surface area (greater surface area increases rate for solids), and the nature of the reactants (some substances react faster than others). Each factor affects the frequency and/or effectiveness of molecular collisions.

Q3. How do you define the rate of a reaction based on the change in concentration of a reactant? A product? How do you account for the coefficients in a balanced chemical equation when determining the overall rate of the reaction? What does a positive or a negative reaction rate mean?

Background

Topic: Chemical Kinetics – Defining Reaction Rate

This question is about expressing reaction rate mathematically and conceptually, and understanding how stoichiometry affects rate expressions.

Key Terms and Formulas:

• Reaction Rate: The change in concentration of a reactant or product per unit time.

Key formula:

$\text{Rate} = -\frac{1}{a} \frac{d[\text{A}]}{dt} = -\frac{1}{b} \frac{d[\text{B}]}{dt} = \frac{1}{c} \frac{d[\text{C}]}{dt} = \frac{1}{d} \frac{d[\text{D}]}{dt}$

For a general reaction: $a\text{A} + b\text{B} \rightarrow c\text{C} + d\text{D}$

Step-by-Step Guidance

1. Recall that the rate of reaction can be defined in terms of the change in concentration of any reactant or product over time.

2. Remember that reactant concentrations decrease over time, so their rates are negative, while product concentrations increase, so their rates are positive.

3. To account for stoichiometry, divide the rate of change by the coefficient from the balanced equation for each species.

4. Interpret what a positive or negative rate means in the context of reactants and products.

Try writing the rate expressions for a sample reaction before checking the answer!

Final Answer:

The rate of a reaction is defined as the change in concentration of a reactant (negative sign) or product (positive sign) per unit time, divided by its stoichiometric coefficient. A negative rate means the reactant is being consumed; a positive rate means the product is being formed. The overall rate is the same regardless of which species you use, as long as you account for the coefficients.