Chapter 20: Chemical Reactions and Energy

Thermodynamics

What is Energy?

2 Main Types: Potential and Kinetic

Potential – energy of composition or position

Kinetic – energy of motion

Chemical Systems contain both Potential and Kinetic Energy Energy can change form or flow but its always conserved. Takes energy to break bonds and energy is released when

bonds are formedLaw of Conservation of Mass and Energy – energy is neither

created or destroyed in a chemical reaction. Also known as the 1st Law of Thermodynamics Heat Energy – symbol is q– always flows from hot to cold

Units of Energy are Joules (J) or kilojoules (kJ)

When the energy stored in the products is less than the reactants, energy has been given off.

Exothermic reaction – energy given off to the environment. Products are at a lower energy level than the reactants.

Energy Diagrams are used to illustrate energy changes in chemical reactions.

Endothermic Reactions

When the products are at a higher energy level (system absorbed energy) than the reactants.

In Exothermic reactions the energy stored in products is less than in reactants

∆ H is negative

The difference in energy between products and reactants in a chemical reaction is given the symbol ∆ H (delta H)

∆ symbol means difference or change. Also known as Enthalpy∆ H = enthalpy = heat energy lost or gained

Formula: ∆H reaction = ∆ H products - ∆ H reactants

Formula: ∆Hrxn = ∆Hprod - ∆Hreact

Endothermic reactions energy stored in products is greater than that of the reactants.

∆ H is positive

Input of energy is called Activation Energy (Ea)Required in both Endothermic and Exo Reactions

A= reactantsB= Activation energyC= Activated complexD= ΔHE= products

B = Activation Energy difference in energy between the reactants and the maximum energy

Two curves are shown in the following energy diagram. Which represents the catalyzed reaction?

Red curve

Catalysts lower the activation energy

How do catalysts effect the rate of a reaction?

Reaction without a catalyst

Reaction with a catalyst

Exhaust gases from an automobile pass through a two-stage catalytic converter. In one stage, carbon monoxide and unburned hydrocarbons are converted to carbon dioxide and water. In the second stage, NO is converted to nitrogen gas and oxygen gas.

A Catalytic Converter

Forces that Drive Chemical Reactions

Why do some reactions occur spontaneously? (fast without a catalyst)

3 Factors are involved: Order, Disorder and Entropy

• Entropy – describes disorder. Not conserved. Used to describe and measure the degree of disorder.

Ice melting – classic example of entropy increasing

The mixture on the right has more disorder and a higher entropy than the mixture on the left

• Systems tend to go from a state of high energy (∆H) to a state of low energy (more stable state). Exothermic reactions more common.

• Systems tend to become more disordered – greater entropy.

• Spontaneous reactions tend to occur if energy decreases and if disorder increases (exothermic).

• The Direction of a Chemical reaction - In room temp. most exothermic reactions tend to be spontaneously in the forward direction.