AP Chemistry 6.6 Introduction to Enthalpy of Reaction Study Notes - New Syllabus Effective fall 2024
AP Chemistry 6.6 Introduction to Enthalpy of Reaction Study Notes.- New syllabus
AP Chemistry 6.6 Introduction to Enthalpy of Reaction Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Explain changes in the heat q absorbed or released by a system undergoing a phase transition based on the amount of the substance in moles and the molar enthalpy of the phase transition.
Key Concepts:
- Energy of Phase Changes
- Exothermic & Endothermic Reactions
- Energy Diagrams
- Thermal Energy & Molecular Collisions
6.6.A.1 Enthalpy Change: Heat Energy Released or Absorbed at Constant Pressure:
1. Definition of Enthalpy and Enthalpy Change (ΔH):
The sum of a system’s internal energy (U) and the product of pressure (P) and volume (V) is known as its enthalpy, or total heat content: H = U + PV
Change in Enthalpy (ΔH)
ΔH is the difference in enthalpy between the final and initial states of a system. It stands for the heat energy that is either released or absorbed during a process that is conducted under constant pressure:
ΔH > 0: Endothermic heat absorption.
ΔH < 0: Exothermic heat release.
2. Exothermic and Endothermic Reactions:
| Reaction Type | Heat Flow | ΔH | Effect on Surroundings | Example |
|---|---|---|---|---|
| Exothermic | Heat released | Negative (ΔH < 0) | Temperature increases | Combustion (e.g., burning wood) |
| Endothermic | Heat absorbed | Positive (ΔH > 0) | Temperature decreases | Photosynthesis, dissolving salts |
3. Heat Transfer at Constant Pressure:
At constant pressure, the heat added to or liberated by a system is equal to the change in enthalpy (ΔH). This is due to the fact that, at constant pressure, enthalpy includes both internal energy and work done by the system as it expands or contracts.
The equation is given as: q=ΔH
– q is the heat absorbed or liberated by the system.
– ΔH = change in enthalpy.
Therefore, under constant pressure, ΔH is actually a measure of the heat energy transferred in a reaction or process.
4. Measurement and Calculation of Enthalpy Change:
i. Calorimetry:
– Quantifies change in heat in a calorimeter.
– Formula: (where m is mass, c is specific heat, and ΔT is temperature change).
ii. Hess’s Law:
– The overall enthalpy change is the sum of the enthalpy changes of the individual steps.
– Equation: .
5. Applications of Enthalpy Change:
i. Chemistry: Forecasts heat transfer in reactions and assists reaction design.
ii. Industry:
– Maximizes energy generation (e.g., combustion).
– Essential for chemical production.
iii. Biological Systems:
– Significant in metabolism and temperature regulation.
6.6.A.2 Energy Transfer and Thermal Equilibrium in Exothermic and Endothermic Reactions:
1. Exothermic and Endothermic Reactions:
- Exothermic Reactions
– Energy released to the environment.
– ΔH is negative (ΔH < 0).
– The environment becomes warmer.
– Example: Combustion (fuels burning). - Endothermic Reactions
– Energy absorbed from the environment.
– ΔH is positive (ΔH > 0).
– The environment becomes cooler.
– Example: Photosynthesis or ice melting.
2. Thermal Equilibrium:
Thermal equilibrium is achieved when there is no net heat flow between a system and its environment, i.e., both are in the same temperature. Energy exchange takes place as follows:
– Heat Transfer: If the surroundings and the system are at different temperatures, heat flows from the hotter body to the cooler body.
– Achieving Equilibrium: This is repeated until both the surroundings and the system achieve a common temperature, whereupon no more heat transfer takes place.
– Outcome: The system and environment are in thermal equilibrium and become stabilized in terms of their temperatures.
In short, thermal equilibrium occurs when the rate of heat flow into the system is equal to the rate of heat flow out of the system.
6.6.A.3 Chemical Potential Energy and Temperature Change in Reactions:
1. Chemical Potential Energy and Bond Energy:
i. Chemical Potential Energy:
– The energy of the chemical bonds within molecules.
– It is dependent on the orientation of the atoms and the bonds between them.
ii. Bond Breaking:
– Energy is needed to break bonds (endothermic process).
– The energy supplied in this process is referred to as **bond dissociation energy**.
iii. Bond Forming:
– Energy is released upon the formation of new bonds (exothermic process).
– The energy released equals the bond energy of the new bonds.
Energy Changes:
– Overall Reaction: The net energy change of a chemical reaction (ΔH) is the difference between the energy needed to break bonds and the energy released on the formation of new bonds:
– Exothermic Reactions: Energy released during bond formation is greater than the energy used to break bonds (ΔH < 0).
– Endothermic Reactions: Energy to break bonds is greater than the energy released in forming new bonds (ΔH > 0).
2. Kinetic Energy and Temperature Change:
i. Kinetic Energy:
– Energy of the motion of the particles (atoms or molecules).
– The greater the motion of the particles, the greater their kinetic energy.
ii. Temperature:
– A quantity of the average kinetic energy of the particles in a substance.
– Greater temperature = greater average kinetic energy of particles.
Energy Differences and Temperature Change
– When energy is added (e.g., heat), particles accelerate, which leads to an increase in their kinetic energy. This causes a temperature rise.
– When energy is removed (e.g., cooling), particles decelerate, their kinetic energy decreases, and there is a temperature fall.
Changes in energy lead to a change in the kinetic energy of the particles, which has a direct impact on the temperature of a substance.
OLD Content
Introduction to Enthalpy of Reaction
- Enthalpy (H) = heat
- Is an extensive property and state function
; 
- E is the internal energy of the system, P is the pressure of the system, and V is the volume of the system
- Units of ΔH = kJ
- When a reaction and ΔH are given, changing the coefficients/amounts changes ΔH by the same factor
- Heat value written on reactant side = endothermic
- Heat value written on product side = exothermic & ΔH will be negative
