Thermochemistry: Study Notes (General Chemistry, Unit 5)

Thermochemistry

Introduction

Thermochemistry is the study of energy changes, particularly heat, that occur during chemical reactions and changes of state. It is a fundamental topic in general chemistry, providing insight into how energy is transferred and conserved in chemical processes.

Energy in Chemistry

Definition and Types of Energy

• Energy: The potential or capacity to move matter. It exists in various forms and is essential for all physical and chemical processes.

• Units of Energy:

  • calorie (cal)

  • kilocalorie (kcal)

  • joule (J)

  • kilojoule (kJ)

• Conversion Factor: $1 ext{ cal} = 4.184 ext{ J}$

• Law of Conservation of Energy: Energy is neither created nor destroyed, but can be converted from one form to another.

• Potential Energy: Stored energy due to position or composition.

• Kinetic Energy: Energy of motion.

Energy Unit Conversion Factors

Energy can be measured in different units. The following table summarizes common conversion factors:

Unit	Conversion
calorie (cal)	4.184 joules (J)
kilocalorie (kcal) or Calorie (Cal)	1000 cal = 4184 J
watt-hour (kWh)	$3.60 imes 10^6$ J

Energy Transfer

System and Surroundings

• System: The part of the universe being studied (e.g., the contents of a reaction vessel).

• Surroundings: Everything outside the system.

• Energy transfers between the system and surroundings until thermal equilibrium is reached (no temperature difference).

Examples

• Ice cube melting: The ice (system) absorbs energy (gets warmer), and the surroundings lose energy (get cooler).

• Water freezing: The water (system) loses energy (gets cooler), and the surroundings (e.g., freezer) absorb energy (get warmer).

Heat of Reaction

Definition of Heat ($q$)

• Heat ($q$): Energy that flows into or out of a system due to a temperature difference between the system and surroundings.

Exothermic and Endothermic Reactions

• Exothermic Reaction: Releases heat to the surroundings. The reaction vessel warms. Example: $CH_4(g) + 2 O_2(g) ightarrow CO_2(g) + 2 H_2O(g)$ $q = -890$ kJ (heat given off)

• Endothermic Reaction: Absorbs heat from the surroundings. The reaction vessel cools. Example: $NH_4NO_3(s) ightarrow NH_4^+(aq) + NO_3^-(aq)$ $q = 36$ kJ (heat taken up)

Enthalpy ($riangle H$)

Definition and Properties

• Enthalpy ($riangle H$): The heat of a reaction at constant pressure. $riangle H = q$

• Meaning of Heats of Reaction:

  • If the reaction equation is doubled, $riangle H$ is doubled.

  • If the reaction equation is reversed, the sign of $riangle H$ is reversed.

Examples

• $N_2(g) + 3 H_2(g) ightarrow 2 NH_3(g)$ $riangle H = -92$ kJ (heat given off)

• $2 N_2(g) + 6 H_2(g) ightarrow 4 NH_3(g)$ $riangle H = -184$ kJ (double equation, double $riangle H$)

• $2 NH_3(g) ightarrow N_2(g) + 3 H_2(g)$ $riangle H = +92$ kJ (reverse equation, reverse sign)

Stoichiometry of $riangle H$

• Enthalpy change is given per mole of reaction as written.

• Example: $CH_4(g) + 2 O_2(g) ightarrow CO_2(g) + 2 H_2O(g)$ $riangle H = -890$ kJ

• To calculate heat evolved for a given mass:

  • Convert mass to moles.

  • Multiply by $riangle H$ per mole.

Example Calculation

• Calculate the heat evolved when 7.33 g of $CH_4$ burns: $7.33 ext{ g CH}_4 imes rac{1 ext{ mol CH}_4}{16.05 ext{ g CH}_4} imes rac{-890 ext{ kJ}}{1 ext{ mol CH}_4} = -406 ext{ kJ}$

Calorimetry

Measuring Heat Changes

• Calorimetry: The measurement of heat changes in chemical reactions.

• Specific Heat ($s$): The amount of heat required to raise the temperature of 1 g of a substance by 1°C at constant pressure.

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

Formulas

• For specific heat: $q = s m riangle T$ where:

  • $q$ = heat (J)

  • $s$ = specific heat (J/g°C)

  • $m$ = mass (g)

  • $riangle T = T_{final} - T_{initial}$ (°C)

• For heat capacity: $q = C riangle T$ where:

  • $q$ = heat (kJ)

  • $C$ = heat capacity (kJ/°C)

  • $riangle T = T_{final} - T_{initial}$ (°C)

When to Use Each Formula

• Use $q = s m riangle T$ if the problem mentions specific heat or specific heat capacity.

• Use $q = C riangle T$ if the problem mentions heat capacity of the calorimeter.

Example Calculations

• Calculate the heat needed to raise 19.0 g of $H_2O$ from 11°C to 88°C (specific heat of water = 4.184 J/g°C): $q = 4.184 ext{ J/g°C} imes 19.0 ext{ g} imes (88 - 11) ext{°C}$

• Calculate the heat needed to raise 104 g of Cu from 17°C to 34°C (specific heat of copper = 0.384 J/g°C): $q = 0.384 ext{ J/g°C} imes 104 ext{ g} imes (34 - 17) ext{°C}$

Summary Table: Energy Conversion Factors

Unit	Conversion to Joules (J)
calorie (cal)	4.184 J
kilocalorie (kcal)	4184 J
watt-hour (kWh)	$3.60 imes 10^6$ J

Key Concepts

• Energy is conserved in all chemical processes.

• Heat and enthalpy are central to understanding energy changes in reactions.

• Calorimetry allows quantitative measurement of heat changes.

• Unit conversions are essential for comparing energy values.

Additional info: These notes cover the foundational concepts of thermochemistry, including energy types, unit conversions, system/surroundings, heat of reaction, enthalpy, and calorimetry, as presented in a typical General Chemistry curriculum.