College Chemistry Ch7

Energy, Rate, and Equilibrium

Thermodynamics

• the study of energy, work, and heat

• The laws of thermodynamics help us to understand why some chemical reactions occur and others do not

Thermodynamics Chemical Change

Calculate the quantity of heat obtained from combustions of one gallon of fuel oil

Thermodynamics Physical Change

Determine the energy released by boiling water

System

contains the process under study

Surroundings

the  rest of the universe

Cannot be Measured

Absolute value for energy stored in a chemical system

Can be Measured

The change in energy during these chemical changes

First Law of Thermodynamics

•energy of the universe is constant

•This law is also called the Law of Conservation of Energy

If a reaction will occur

• Bonds must break

• Breaking bonds requires energy

exothermic reaction

If the energy required to break the bonds is less than the energy released when the bonds are formed, there is a net release of energy

energy is a product of this reaction

endothermic reaction

If the energy required to break the bonds is greater than the energy released when the bonds are formed, there will need to be an external supply of energy

Enthalpy (DH)

• represents heat energy

• energy difference between the products and reactants of a chemical reaction

• when pressure is constant

Sign of Enthalpy (exothermic, negative)

If energy is released in a reaction, the reaction is ____.

The sign of DH will be ____.

Sign of Enthalpy (endothermic, positive)

•If energy is absorbed in a reaction, the reaction is ____.

The sign of DH will be ____.

Second Law of Thermodynamics

the universe spontaneously tends toward increasing disorder or randomness

Entropy (S)

a measure of the randomness of a chemical system

no such thing as negative ____.

High entropy

highly disordered system, the absence of a regular, repeating pattern

Low entropy

well organized system such as a crystalline structure

If exothermic and positive (DS)

Spontaneous

If endothermic and negative (DS)

Non spontaneous

Free energy (DG)

•represents the combined contribution of the enthalpy and entropy values for a chemical reaction

•predicts spontaneity of chemical reactions

–Negative (DG), always Spontaneous

–Positive (DG), never Spontaneous

Calorimetry

the measurement of heat energy changes in a reaction

Calorimeter

•device which measures heat changes in calories

•The change in temperature is used to measure the loss or gain of heat

Specific heat (SH)

the number of calories of heat needed to raise the temperature of 1 g of the substance 1 ^oC

SH of water is 1.00 cal/g^oC

Kinetics

•the study of the rate (or speed) of chemical reactions

Effective collision

is one that produces product molecules

Activation energy (Ea)

•the minimum amount of energy required to initiate a chemical reaction

•Picture a chemical reaction in terms of changes in potential energy occurring during the reaction

Activated complex

–an extremely unstable, short-lived intermediate complex

–Formation of this activated complex requires energy (E_a) to overcome the energy barrier to start the reaction

Factors That Affect Reaction Rate

1.Structure of the reacting species

2.Molecular shape and orientation

3.Concentration of reactants

4.Temperature of reactants

5.Physical state of reactants

6.Presence of a catalyst

Rate

•____ is related to the concentration of one or more of the reacting substances

•____ will generally increase as concentration increases

Liquid, Solid

•Reactions are fastest in the ____ state and slowest in the ____ state

Catalyst

a substance that increases the reaction rate, but has no effect on equilibrium composition

–Undergoes no net change

–Does not alter the final product of the reaction

–Interacts with the reactants to create an alternative pathway for product production

Physical equilibrium

• Two phases (ex: states of matter) of the same substance

• reversible

Reversible reaction

a process that can occur in both directions

Dynamic equilibrium

the rate of the forward process in a reversible reaction is exactly balanced by the rate of the reverse process

Chemical equilibrium

state of a reaction in which the rates for the forward and reverse reactions are equal

Equilibrium reactions

•chemical  reactions that do not go to completion

–Completion: all reactants have been converted to products

–Also called incomplete reactions

Equilibrium occurs when…

the rate of reactant depletion is equal to the rate of product depletion

LeChatelier’s Principle

•if  a stress is placed on a system at equilibrium, the system will respond by altering the equilibrium composition in such a way as to minimize the stress

•If reactants and products are present in a fixed volume and more NH₃ is added into the container, the system will be stressed

Adding NH₃ to the system causes stress

To relieve stress, remove as much of added material as possible by converting it to reactants

Adding N₂ or H₂ to the system causes stress

To relieve stress, remove as much of added material as possible by converting it to product