PreviewPreview Key IdeasKey Ideas BellringerBellringer Factors Affecting Reaction RatesFactors Affecting Reaction Rates CatalystsCatalysts Equilibrium SystemsEquilibrium Systems

Section 4: Section 4: Reaction Rates and EquilibriumReaction Rates and Equilibrium

〉 What kinds of things speed up a What kinds of things speed up a reaction?reaction?

〉 What does a catalyst do?What does a catalyst do?

〉 What happens when a reaction goes What happens when a reaction goes backward as well as forward?backward as well as forward?Key IdeasKey Ideas

Not all reactions happen at the same speed. Atoms, Not all reactions happen at the same speed. Atoms, ions, and molecules can only interact when they are in ions, and molecules can only interact when they are in close contact with each other. Below is a sample of zinc close contact with each other. Below is a sample of zinc arranged in three different ways.arranged in three different ways.

BellringerBellringer

1. 1. In the reaction Zn + 2HCl → ZnClIn the reaction Zn + 2HCl → ZnCl22 + H + H22, which sample do , which sample do you think would react the fastest? you think would react the fastest?

2. 2. When you want to start a bonfire, why do you use many When you want to start a bonfire, why do you use many small sticks as kindling to start the larger logs?small sticks as kindling to start the larger logs?

3. 3. Which do you think will react faster with hydrochloric acid Which do you think will react faster with hydrochloric acid (HCl(HCl), ), atoms of liquid zinc at its melting point or atoms of atoms of liquid zinc at its melting point or atoms of solid zinc at its melting point? (solid zinc at its melting point? (Hint:Hint: Which situation allows Which situation allows more contact among the particles?)more contact among the particles?)

Bellringer, Bellringer, continuedcontinued

〉 What kinds of things speed up a reaction?What kinds of things speed up a reaction?

〉 Anything that increases contact between Anything that increases contact between particles will increase the rate of a particles will increase the rate of a reaction.reaction.

For any reaction to occur, For any reaction to occur, the particles of the reactants the particles of the reactants must collide with one another.must collide with one another.

Factors Affecting Reaction Factors Affecting Reaction RatesRates

Most reactions go faster at higher temperatures.Most reactions go faster at higher temperatures.

A large surface area speeds up reactions.A large surface area speeds up reactions.

Higher concentrations of reactants react faster.Higher concentrations of reactants react faster.

Reactions are faster at higher pressure.Reactions are faster at higher pressure.

Massive, bulky molecules react more slowly.Massive, bulky molecules react more slowly.

Factors Affecting Reaction Rates, Factors Affecting Reaction Rates, continuedcontinued

Factors Affecting Reaction Factors Affecting Reaction RateRate

〉 What does a catalyst do?What does a catalyst do?

〉 A catalyst speeds up or slows down a A catalyst speeds up or slows down a reaction but is not changed by the reaction.reaction but is not changed by the reaction.

catalyst:catalyst: is substance that is substance that changes the rate of a chemical changes the rate of a chemical reactionreaction without being consumed or changed significantly without being consumed or changed significantly

inhibitorsinhibitors:: substances that slow reactions substances that slow reactions

Catalysts are often used in industryCatalysts are often used in industry to make reactions go to make reactions go faster.faster.

CatalystsCatalysts

Visual Concept: CatalystVisual Concept: Catalyst

Enzymes are biological catalysts.Enzymes are biological catalysts. Enzyme:Enzyme: a type of protein that speeds up metabolic a type of protein that speeds up metabolic

reactions in plants and animals without being reactions in plants and animals without being permanently changed or destroyedpermanently changed or destroyed Each enzyme controls one reaction or one set of similar Each enzyme controls one reaction or one set of similar

reactions.reactions. Most enzymes are fragile,Most enzymes are fragile, and stop working above certain and stop working above certain

temperatures.temperatures.

substrate:substrate: the reactant in reactions catalyzed by the reactant in reactions catalyzed by enzymesenzymes Example:Example: hydrogen peroxide is the substrate for catalase hydrogen peroxide is the substrate for catalase

Catalyst, Catalyst, continuedcontinued

catalase2 2 2 22H O 2H O + O

Visual Concept: EnzymeVisual Concept: Enzyme

Visual Concept: InhibitorsVisual Concept: Inhibitors

〉 What happens when a reaction goes What happens when a reaction goes backward as well as forward?backward as well as forward?

〉 Some processes may go in both Some processes may go in both directions, which results in an directions, which results in an equilibrium system.equilibrium system.

EquilibriumEquilibrium can be described as a balance that is can be described as a balance that is reached by two opposing processes.reached by two opposing processes.

Equilibrium SystemsEquilibrium Systems

Some changes are reversible.Some changes are reversible. A reversible change is indicated by the A reversible change is indicated by the ⇆⇆ sign sign

in a chemical equation.in a chemical equation. Example:Example: the physical change of carbon dioxide the physical change of carbon dioxide

dissolvingdissolving

Equilibrium results when rates balance.Equilibrium results when rates balance. Example: Molecules of COExample: Molecules of CO22 are coming out of are coming out of

solution and dissolving back into the liquid at solution and dissolving back into the liquid at the same rate.the same rate.

Equilibrium Systems, Equilibrium Systems, continuedcontinued

increase pressure

2 2decrease pressureCO (gas above liquid) CO (gas dissolved in liquid)

EquilibriumEquilibrium

EquilibriumEquilibrium

Equilibrium Systems, Equilibrium Systems, continuedcontinued• chemical equilibrium: a state of balance in which the

rate of a forward reaction equals the rate of the reverse reaction and the concentrations of products and reactants remain unchanged

• Systems in equilibrium respond to minimize change.• Le Châtelier’s principle predicts changes in equilibrium.

Visual Concept: Factors Affecting Visual Concept: Factors Affecting EquilibriumEquilibrium

Le ChLe Châtelier’s principle:âtelier’s principle:If a change is made to a system in chemical If a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is the change until a new equilibrium is reached.reached.

Le ChLe Châtelier’s principle can be used to control âtelier’s principle can be used to control reactions.reactions. The Haber process is used to make The Haber process is used to make

ammonia. ammonia.

The most ammonia is produced when the reaction is run at The most ammonia is produced when the reaction is run at a high pressure and a low temperature.a high pressure and a low temperature.

Equilibrium Systems, Equilibrium Systems, continuedcontinued

Visual Concept: Le Visual Concept: Le ChChatelier’s Principleatelier’s Principle