Chapter 7 Section 3

• 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

• 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

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

Chapter 7 Section 4

• 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

• 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

• 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.

• 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

• 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.

• 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

Chapter 7 Section 5

• 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

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