Energy Changes in ReactionsChapter 7 Section 3

Chemical Bonds and Energy

• Chemical energy is the energy stored in the chemical bonds of a substance

• Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products

Chemical Bonds and Energy

• Breaking Bonds– Breaking bonds requires energy– There has to be enough energy to break bonds of

reacting molecules to get the reaction started• Forming Bonds– The formation of chemical bonds releases energy– Heat and light are examples of what happens

when new chemical bonds are formed

Exothermic and Endothermic Reactions

• During a chemical reaction, energy is either release or absorbed

• Exothermic Reactions– A chemical reaction that releases energy to its

surroundings– The energy released as the products form is greater

than the energy required to break the bonds in the reactants

– Combustion is an example of an extremely exothermic reaction

Exothermic and Endothermic Reactions

• Endothermic Reactions– A chemical reaction that absorbs energy from its

surroundings – More energy is required to break the bonds in the

reactants than is released by the formation of the products

– Energy of products is greater than the energy of the reactants

Exothermic and Endothermic Reactions

Exothermic Endothermic

Conservation of Energy

• The total amount of energy before and after the reaction is the same. This is known as the law of conservation of energy

Reaction Rate

Chapter 7 Section 4

Reactions Over Time

• A reaction rate is the rate at which reactants change into products over time.

• Reaction rates tell you how fast a reaction is going.

• Some reactions happen almost instantaneously, while other happen gradually

Factors Affecting Reaction Rates

• The reaction rate depends on how often the particles collide. If the collisions occur more frequently, then the reaction rate increases.

• Factors that affect reaction rates include temperature, surface area, concentration, stirring and catalysts

Factors Affecting Reaction Rates

• Temperature– Increase in temperature will generally increase the

reaction rate.– Increasing the temperature of a substance causes

its particles to move faster, on average. Particles that move faster are both more likely to collide and more likely to react.

Factors Affecting Reaction Rates

• Surface Area– The smaller the particle size of a given mass, the

larger is its surface are.– An increase in surface area increases the exposure

of reactants to one another. – The greater the exposure, the more collisions

Factors Affecting Reaction Rates

• Stirring– Stirring the reactants will generally increase the

reaction rate• Concentration– Concentration refers to the number of particles in a

given volume. The more reacting particles that are present in a given volume, the more opportunities there are for collisions

– For gas, concentration changes with pressure. The greater the pressure of a gaseous reactant, the greater is its concentration, and the faster the reaction rate.

Factors Affecting Reaction Rates

• Catalysts– a substance that affects the reaction rate without

being used up in the reaction.– Since the catalyst is neither a reactant nor a

product, it is written over the arrow

Equilibrium

Chapter 7 Section 5

Types of Equilibria

• Equilibrium is a state in which the forward and reverse paths of a change take place at the same rate

1. Physical Equilibrium– When a physical change does not go to

completion, a physical equilibrium is established between the forward and reverse changes

Types of Equilibria

2. Chemical Equilibrium– When a chemical reaction does not go to

completion, a chemical equilibrium is established between the forward and reverse reactions

• A reversible reaction is a reaction in which the conversion of reactants into products and the conversion of products into reactant can happen simultaneously

Factors Affecting Chemical Equilibrium

• When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change- aka Le Chatelier’s principle

1. Temperature– Heat written as a product, then forward reaction

is exothermic; in the reverse reaction, heat is a reactant

– When heat is added to a system, the equilibrium will shift in the direction that removes heat from the system

Factors Affecting Chemical Equilibrium

2. Pressure– If pressure is increased, the equilibrium would

shift in the direction that decreases the pressure of the system

– To decrease pressure, the system would favor the reaction that produces fewer gas molecules

3. Concentration– Change in concentration of the reactants or

products also affects equilibrium