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Reaction Energy Profile

Sophie Anderson

Sophie Anderson

7 min read

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Study Guide Overview

This study guide covers energy in reactions, focusing on elementary reactions, endothermic vs. exothermic reactions (illustrated with potential energy diagrams), and the progress of a reaction (reactants, activated complex, products). It also explains activation energy (Ea), its relationship to reaction rate, and the Arrhenius equation. Finally, it provides practice questions and exam tips.

AP Chemistry: Energy in Reactions - Your Ultimate Study Guide 🚀

Hey there, future AP Chem master! Let's dive into the exciting world of reaction energies. This guide is designed to be your go-to resource, especially the night before the exam. We'll break down complex topics into easy-to-digest pieces, ensuring you feel confident and ready to ace that test! Let's get started!

⚛️ Review: The Collision Model

Remember, for a reaction to happen, molecules need to collide with:

  1. Sufficient energy
  2. Correct orientation
Key Concept

This guide focuses on the energy aspect of reactions. If you need a refresher on the collision model, check out our previous guide.

⚡ Energy in Reactions

What are Elementary Reactions?

An elementary reaction is a single-step reaction involving one or a few molecules. Think of them as the basic building blocks of more complex reactions. They can be:

  • First-order: Rate depends on one reactant.
  • Second-order: Rate depends on two reactants.
Quick Fact

Elementary reactions involve bond breaking and bond formation, which directly relate to energy changes.

Endothermic vs. Exothermic Reactions

Reactions either absorb or release energy. We visualize this using potential energy diagrams (reaction coordinate diagrams).

Potential Energy Diagrams

Endothermic vs Exothermic
*Image Courtesy of Labster Theory*
  • Endothermic Reactions:
    • Reactants have lower potential energy than products.
    • Energy is absorbed from the surroundings.
    • Think: Reactants + Energy → Products
    • The potential energy of the products is higher than the reactants.
  • Exothermic Reactions:
    • Reactants have higher potential energy than products.
    • Energy is released into the surroundings (often as heat).
    • Think: Reactants → Products + Energy
    • The potential energy of the products is lower than the reactants.
Memory Aid

ENDOthermic reactions absorb energy (think ENergy IN), while EXOthermic reactions release energy (EXit).

Exam Tip

Be able to identify endothermic and exothermic reactions from potential energy diagrams. Look at the relative energy levels of reactants and products.

🛤️ The Progress of a Reaction

Every reaction has three main stages:

  1. Reactants: The starting materials (left side of the PE diagram).
  2. Activated Complex (Transition State): The highest energy, most unstable point (peak of the PE diagram). Bonds are partially broken and formed.
  3. Products: The final substances (right side of the PE diagram).
Reaction Progress

⛰️ Activation Energy (Ea)

What is Activation Energy?

Activation energy (Ea) is the minimum energy required to start a reaction. It's the energy difference between the reactants and the transition state.

  • Think of it as an energy barrier that must be overcome.
  • Lower Ea = faster reaction.
  • Higher Ea = slower reaction.

Activation energy is a crucial concept for understanding reaction kinetics. It directly affects the rate of a reaction.

The Arrhenius Equation

The Arrhenius equation describes how the rate constant of a reaction changes with temperature. It relates the rate constant to activation energy:

k=Ae−Ea/RTk = Ae^{-E_a/RT}

  • Where:
    • k = rate constant
    • A = frequency factor
    • Ea = activation energy
    • R = gas constant
    • T = temperature (in Kelvin)
Exam Tip

Good news! You won't need to perform calculations with the Arrhenius equation on the AP exam. However, understand the relationship between temperature, activation energy, and the rate constant.

Common Mistake

Don't confuse activation energy with the overall energy change of a reaction (ΔH). Activation energy is the energy required to reach the transition state, while ΔH is the difference between the energy of the reactants and products.

🎯 Final Exam Focus

  • Key Concepts:

    • Elementary reactions
    • Endothermic vs. Exothermic reactions (and their PE diagrams)
    • Activated complex/transition state
    • Activation energy (Ea) and its effect on reaction rate
    • Relationship between temperature, activation energy, and rate constant (Arrhenius Equation)

  • Common Question Types:

    • Identifying endothermic/exothermic reactions from PE diagrams.
    • Relating activation energy to reaction rate.
    • Conceptual questions about the transition state.

  • Last-Minute Tips:

    • Quickly sketch PE diagrams to visualize energy changes.
    • Focus on understanding the concepts rather than memorizing formulas.
    • Read questions carefully and pay attention to the wording.

Practice Questions

Practice Question

Multiple Choice Questions

  1. Which of the following statements is TRUE regarding an endothermic reaction?

    (A) The potential energy of the reactants is greater than the potential energy of the products. (B) The potential energy of the reactants is less than the potential energy of the products. (C) The activation energy is negative. (D) The reaction releases heat into the surroundings.

  2. The activation energy of a certain reaction is 45 kJ/mol. If a catalyst is used, the activation energy will:

    (A) Increase (B) Decrease (C) Remain the same (D) Become zero

  3. According to the Arrhenius equation, an increase in temperature will:

    (A) Decrease the rate constant (B) Increase the rate constant (C) Have no effect on the rate constant (D) Decrease the activation energy

Free Response Question

Consider the following reaction:

2NO(g)+O2(g)→2NO2(g)2NO(g) + O_2(g) \rightarrow 2NO_2(g)

The reaction is exothermic with a ΔH = -114 kJ/mol. The activation energy for the forward reaction is 54 kJ/mol.

(a) Sketch a potential energy diagram for the reaction, clearly showing the reactants, products, activation energy, and ΔH.

(b) What is the activation energy for the reverse reaction?

(c) If a catalyst is added to the reaction, how would it affect the:

(i) Activation energy?
(ii) ΔH of the reaction?

(d) Explain how an increase in temperature would affect the rate of the reaction.

Answer Key and Scoring Rubric

Multiple Choice Answers

  1. (B)
  2. (B)
  3. (B)

Free Response Scoring Rubric

(a) Potential Energy Diagram (4 points)

  • 1 point: Correctly labeled axes (Reaction Progress or Reaction Coordinate vs. Potential Energy).
  • 1 point: Reactants (2NO(g) + O2(g)) at a higher energy level than products (2NO2(g)).
  • 1 point: Transition state (peak) shown with appropriate activation energy.
  • 1 point: ΔH indicated as a negative change from reactants to products.

(b) Activation Energy for Reverse Reaction (2 points)

  • 1 point: Calculation: 54 kJ/mol + 114 kJ/mol = 168 kJ/mol
  • 1 point: Correct answer: 168 kJ/mol

(c) Effect of Catalyst (2 points)

(i) 1 point: Activation energy will decrease.
(ii) 1 point: ΔH of the reaction will remain the same.

(d) Effect of Temperature (2 points)

  • 1 point: Increased temperature will increase the rate of reaction.
  • 1 point: Explanation: Higher temperature increases the kinetic energy of molecules, leading to more frequent and effective collisions.

Let's crush this exam! You've got this! 💪

Question 1 of 11

For a chemical reaction to occur, molecules must collide with sufficient ⚡️:

Volume

Energy

Density

Mass