Temperature, Heat, Endothermic & Exothermic Chemical Reactions Packet #33

Introduction • Temperature • The degree of hotness or coldness of a body of mass or environment. • Measure of the random motions of components of a substance.

Consider… • Consider the following experiment in which a scientist place 1.00 kg of H2O of hot water (90°C) next to 1.00 kg H2O of cold water (10°C) in an insulated box. The water samples are separated by a thin metal plate. • What will happen? • There is a transfer of energy from the hot water to the cold water.

Heat • Heat (q) • A flow of energy due to a temperature difference between two “objects.” • Heat always flows from a warmer object to a colder object.

EXOTHERMIC AND ENDOTHERMIC PROCESSES

System & Surroundings I • Think of the following • When a match is struck, energy is released through heat as the match burns.

• To discuss this reaction, scientists divide the universe into two parts. • The system • The surroundings

System & Surroundings II • The System • Is the part of the universe on which to focus attention

• The Surroundings • Includes everything else in the universe.

• For the match striking and burning, the system is the reactants and the products. • The surroundings consist of the air in the room and anything else other than the reactants and products.

System & Surroundings III Relation to the Laws of Thermodynamics • The First Law of Thermodynamics • Energy cannot be created or destroyed but changed from one form to the next • The total amount of energy is constant!

• ∆Esystem = q + w • q = heat added to system • w = work done by the system • Remember that work is a force that has acted on an object

• ∆Esystem + ∆Esurroundings = ∆Euniverse = 0

EXOTHERMIC REACTIONS

Exothermic Reaction I • When a process results in the evolution of heat (energy), it is said to be an exothermic reaction. • Energy flows out of the system • Combustion of a match • Energy flows out as heat.

• Body loses heat to surroundings after workout

Exothermic Reaction II • What is the system in the picture shown? • What are the surroundings in the picture shown?

Exothermic Reaction III • Where does the energy, released as heat, come from? • The difference in potential energies between the reactants and the products.

• In any exothermic reaction, some of the potential energy, stored in the chemical bonds, is converted to thermal energy (random kinetic energy) via heat.

Potential Energy Diagram

ENDOTHERMIC REACTIONS

Endothermic Reaction I • When a process results in absorption of energy, it is said to be an endothermic reaction. • Energy flows into the system • Boiling water to form steam.

POTENTIAL ENERGY PHASE DIAGRAMS

Potential Energy Diagrams I • A potential energy diagram is used to illustrate the progress of a chemical reaction and to provide qualitative information about energy changes within the reaction. Potential Energy Diagram

Potential Energy Diagrams II • Activation energy, Ea, is the minimum energy needed by the reactants in order for the reaction to occur.

Potential Energy Diagram

Potential Energy Diagrams III Exothermic Reactions • In an exothermic reaction • Reactants  Products + Heat • Since more energy is released than absorbed, the products are at a lower energy state than the reactants. • The difference between the energy of the products and the energy of the reactants is the heat of reaction. • ∆H

Potential Energy Diagram

Potential Energy Diagrams IV Exothermic Reactions • Many exothermic reactions are selfsustaining • They continue to occur because the heat they liberate provides the activation energy for the reactants. • The burning of a match is a self-sustaining reaction. • When the match has been lit, or coal at a barbecue, it continues to burn until there are no more reactants available.*

Potential Energy Diagram

Potential Energy Diagrams V Endothermic Reactions • In an endothermic reaction • Heat + Reactants  Products • Less energy is released than absorbed • The energy for the products are at a higher energy state than that of the reactants.

• Remember—the heat of reaction (∆H) can still be calculated.

Potential Energy Diagram

REVIEW