Thermochemistryis the study of heat change in chemical

reactions.

Chapter 5

Energy & Rates of Reaction

Unit 3

Chapter 5 Key Concepts: After completing this chapter you will be able to…..

• compare the energy changes of different substances, perform

calorimetry calculations, and describe the relationship between

reactions using enthalpy terms.

• represent thermochemical equations using different methods, and determine whether a reaction is exothermic or endothermic

• calculate and use bond energies to estimate the enthalpy change

of a reaction.• define and solve problems using Hess’s law. • write formation reactions and calculate enthalpy changes using

standard enthalpy of formation values.• describe current and future energy sources and explain their

advantages and disadvantages

5.1: Energy: is the capacity to do work

• Thermal energy is the energy associated with the random motion of atoms and molecules

• Chemical energy is the energy stored within the bonds of chemical substances

• Nuclear energy is the energy stored within the collection of neutrons and protons in the atom

• Electrical energy is the energy associated with the flow of electrons

• Potential energy is the energy available by virtue of an object’s position

5.1

Heat is the transfer of thermal energy between two bodies that are at different temperatures.

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.

Temperature = Thermal Energy

900C400C

greater thermal energy5.1

thermal energy: the total quantity of kinetic and potential energy in a substance

Law of Conservation of Energy: Energy cannot be created or destroyed

The system is the specific part of the universe that is of interest in the study.

open

mass & energyExchange:

closed

energy

isolated

nothing

SYSTEM

SURROUNDINGS

5.1

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.

Endothermic process is any process in which heat has to be supplied to the system from the surroundings.

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

5.1

energy + H2O (s) H2O (l)

Nuclear Energy• All nuclear reactions are exothermic. • Per unit of mass, nuclear reactions release much more energy than

exothermic chemical reactions.

• Two main types of Reactions: Fusion and Fission• A fusion reaction occurs when nuclei of small atomic mass

combine to form larger molecule.2

1H + 31H → 4

2He + 10n

In Fission, large nuclei with high atomic mass are split to smaller, lighter mass by collision with a neutron:

23592U + 1

0n → 9236Kr + 141

56Ba + 310n + 1.9 x 10 10 kJ/mol

See P. 290 Table 15.1

Chapter 5.1 Assignment

• Read page# 286 -291.• Answer Questions 1 to 7 (page 291)

5.1

The specific heat (c) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.

5.2: Calorimetry and Enthalpy

The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius.

C = mc Heat (q) absorbed or released:

q = mcDt

q = CDt

Dt = tfinal - tinitial

5.2

Calorimetry: the measurement of heat into or out of a system for chemical and physical processes.

the heat released = the heat absorbed

The device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”

5.2

Enthalpy (H): Total amount of thermal energy in a substance

Enthalpy Change (∆H) : Energy changes in Physical and Chemical Reactions

The enthalpy change of the chemical system is equal to the flow of thermal energy in and out of the system,

∆H surrounding = |q system|

∆H surrounding > 0, Endothermic reaction

∆H surrounding < 0, Exothermic reaction

5.2