9.1 The Nature of Energy Part 2

Exothermic reaction:

results in the evolution of heat

• feels hotter

How does energy as heat flow in an exothermic reaction

out of the system, so ∆E is negative

Where does energy come from in an exothermic reaction?

energy comes from the difference in potential energy of the reactants and products

Energy gained by the surroundings =

energy lost by the system

Exothermic reaction energy:

potential energy in the chemical bonds is converted to thermal energy (random kinetic energy) via heat

• more potential energy in reactants, less in products

• bonds in products are stronger than those in reactants

• more energy is released in forming new bonds in products than is consumed in breaking bonds in reactants

• Net result is that the quantity of energy ∆(PE) is transferred to the surroundings through heat 

For an exothermic reaction, do the reactants or products have more potential energy?

the reactants have more potential energy

For an exothermic reaction, are the bonds stronger in the reactants or products?

bonds in products are stronger

For an exothermic reaction, more energy is…

released in forming new bonds in the products than breaking bonds in the reactants

Net result of an exothermic reaction

• Net result is that the quantity of energy ∆(PE) is transferred to the surroundings through heat 

Endothermic reaction

absorbs energy from the surroundings

• gets colder

How does energy as heat flow in an endothermic reaction?

into the system so ∆E is positive

For an endothermic reaction, do the reactants or products have more potential energy?

products have more potential energy

For an endothermic reaction, do the reactants or products have stronger bonds?

reactants have stronger bonds

Combustion of Methane Example

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + energy (heat)

• exothermic = energy flows out of system as heat

• system = reactants and products

• reactants have more potential energy than products

Pressure equation

P = F/A

• force per unit area