Can a chemical reaction go backwards? Target 6-1.

Quick Review What happens to PARTICLES to make a chemical reaction happen? What is ACTIVATION ENERGY? What is the difference between ENDOTHERMIC and EXOTHERMIC reactions? What are the five things that can change the RATE of a chemical reaction? What is the difference between REACTANTS and PRODUCTS in a chemical reaction?

Quick Review What happens to PARTICLES to make a chemical reaction happen? Collision Theory Particles must collide – break bonds and make new bonds Particles need kinetic energy Particles need to be in the correct orientation

Quick Review What is ACTIVATION ENERGY? Energy needed to break bonds and start the reaction

Quick Review What is the difference between ENDOTHERMIC and EXOTHERMIC reactions? Endothermic – absorb energy Exothermic – release energy

Quick Review What are the five things that affect the RATE of a reaction? Temperature (kinetic energy) Concentration (amount) Pressure (squeezing) Surface Area (powders) Catalysts (activation energy)

Quick Review What is the difference between REACTANTS and PRODUCTS in a chemical reaction? Reactants = starting substance(s) Products = ending substance(s)

Can a chemical reaction go backwards? What We Think Our Questions

Why do substances WANT to react? Energy Chemical processes naturally want to RELEASE energy (exothermic) Once the activation energy allows bonds to break, they re-form very very quickly Examples Reaction of hydrogen + oxygen (exploding balloon) Burning phosphorous (“lightbulb” flask) So…why do endothermic reactions happen?

Why do substances WANT to react? Entropy Chemical processes naturally want to INCREASE entropy Remember: entropy = disorder/chaos in a system How to increase entropy: More molecules = more entropy Solid  Liquid  Gas Examples: Barium hydroxide and ammonium chloride powders Melting ice

Energy vs. Entropy Basically, substances want to have a reaction if it decreases energy or increases entropy. If it does both, they will definitely react. But what if energy and entropy disagree?

Does the reaction WANT to happen? Energy vs. Entropy Does the reaction WANT to happen? Energy Increases Energy Decreases Entropy Increases ??? YES Entropy Decreases NO

Energy vs. Entropy Sometimes energy wants the reaction to happen, but entropy does not (or vice versa) These reactions will go both ways. They are called reversible reactions.

Reversible Reactions Reversible Reaction: a reaction that can occur in both directions at the same time Forward and Reverse reaction are OPPOSITES One direction favored by entropy One direction favored by energy

Reversible Reactions Example: N2 + 3 H2 ↔ 2 NH3 + heat Forward Reaction: N2 + 3 H2  2 NH3 + heat Reverse Reaction: 2 NH3 + heat  N2 + 3 H2 Reactants Products Endothermic or Exothermic? Favored by entropy or energy?

Reversible Reactions Example: N2 + 3 H2 ↔ 2 NH3 + heat Forward Reaction: N2 + 3 H2  2 NH3 + heat Reverse Reaction: 2 NH3 + heat  N2 + 3 H2 Reactants N2 + 3 H2 2 NH3 + heat Products Endothermic or Exothermic? Exothermic (heat is a product!) Endothermic (heat is a reactant!) Favored by entropy or energy? Energy Entropy