Thermochemistry and Energy: Study Notes

Thermochemistry and Energy

Introduction

Thermochemistry is the study of energy changes, particularly heat, that occur during chemical reactions and physical changes. Understanding the flow and transformation of energy is essential for analyzing chemical processes and predicting their outcomes.

Exothermic and Endothermic Processes

• Exothermic Reaction: A process that releases energy, usually in the form of heat, to its surroundings. Example: Combustion of methane.

• Endothermic Reaction: A process that absorbs energy from its surroundings. Example: Melting of ice.

• Thermochemistry: The branch of chemistry concerned with the quantities of heat evolved or absorbed during chemical reactions.

Energy Exchange: Work and Heat

• Energy: The capacity to do work or produce heat.

• Work (w): Energy used to move an object against a force.

• Heat (q): Energy transferred between objects due to a temperature difference.

• Equation: $\Delta E = q + w$

• Example: When a gas expands against a piston, it does work on the surroundings.

Types of Energy and Conservation of Energy

• Kinetic Energy: Energy due to motion. $KE = \frac{1}{2}mv^2$

• Potential Energy: Energy due to position or composition.

• Chemical Energy: Energy stored within the bonds of chemical substances.

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed from one form to another.

• Example: In a chemical reaction, the total energy before and after the reaction remains constant.

Energy Transfer and Conversion

• Energy can be transferred as heat or work and converted between different forms.

• Example: Electrical energy can be converted to thermal energy in a resistor.

System and Surroundings

• System: The part of the universe under study (e.g., reactants and products in a reaction vessel).

• Surroundings: Everything outside the system.

• Energy exchange occurs between the system and its surroundings.

• Example: In a calorimeter, the reaction mixture is the system, and the water is the surroundings.

Thermodynamics

• Thermodynamics: The study of energy and its transformations.

• Focuses on the relationships between heat, work, and energy.

First Law of Thermodynamics

• Statement: Energy of the universe is constant; energy can be transferred and transformed, but not created or destroyed.

• Equation: $\Delta E_{system} = q + w$

• Application: Used to calculate energy changes in chemical reactions.

Internal Energy and State Functions

• Internal Energy (E): The sum of all kinetic and potential energies of the system's components.

• State Function: A property that depends only on the current state of the system, not on the path taken to reach that state (e.g., internal energy, pressure, volume).

• Example: Change in internal energy ($\Delta E$) depends only on initial and final states.

Calculating Energy Changes

• Energy change in a system can be calculated by measuring heat and work exchanged with the surroundings.

• Equation: $\Delta E = q + w$

• Example: If a system absorbs 100 J of heat and does 40 J of work, $\Delta E = 100\,J + (-40\,J) = 60\,J$.

Temperature vs. Heat

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

• Heat: Energy transferred due to temperature difference.

• Comparison: Temperature is an intensive property; heat is an extensive property.

Direction of Heat Transfer

• Heat flows from a region of higher temperature to a region of lower temperature.

• Sign Convention: Heat absorbed by the system ($q > 0$); heat released by the system ($q < 0$).

Heat Capacity and Temperature Change

• Heat Capacity (C): The amount of heat required to raise the temperature of a substance by 1°C.

• Specific Heat Capacity (c): The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Equation: $q = mc\Delta T$

• Application: Used to calculate energy changes in heating or cooling substances.

Calorimetry

• Calorimetry is the measurement of heat flow in a chemical or physical process.

• Equation: $q_{system} = -q_{surroundings}$

• Example: Determining the heat of reaction by measuring temperature change in water.

Pressure-Volume Work

• Pressure-Volume Work: Work done when the volume of a system changes against an external pressure.

• Equation: $w = -P_{ext}\Delta V$

• Application: Used in reactions involving gases.

Summary Table: Key Thermochemistry Concepts

Concept	Definition	Equation
Internal Energy ($E$)	Total energy of a system	$\Delta E = q + w$
Heat ($q$)	Energy transferred due to temperature difference	$q = mc\Delta T$
Work ($w$)	Energy used to move an object against a force	$w = -P_{ext}\Delta V$
Specific Heat Capacity ($c$)	Heat required to raise 1 g of substance by 1°C	$q = mc\Delta T$
First Law of Thermodynamics	Energy cannot be created or destroyed	$\Delta E_{system} = q + w$

Key Equations to Memorize

• $\Delta E = q + w$

• $q = mc\Delta T$

• $w = -P_{ext}\Delta V$

Applications

• Predicting energy changes in chemical reactions

• Calculating heat required for temperature changes

• Determining work done by expanding gases