Unit 11 Rates and Equilibrium

Unit 11

Rate of Reaction

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Hello!
Sorry I could not be with you again today but I wanted to make sure we stayed on pace. Please take out your notes and take note on this presentation. You will also receive a grade for answering the questions you will find embedded.

Recall….

Activation energy is the minimum energy required for the reaction to proceed.

Activated Complex – The point in the reaction where bonds that will
form begin forming, and bonds that will break begin breaking.

Catalysts – a substance that increases the rate of a reaction by lowering the activation energy. Catalysts are not consumed in the process.

What determines how quickly or slowly a reaction occurs?

The speed of a chemical reaction is called its reaction rate. The reaction rate
tells us how quickly reactants are converted to products per unit of time.

Explosion of thermite happens very fast

Rusting of a car takes a very long time

molding food takes a little while but not too long

Reaction Rates Examples

sodium and water, burning a
match, combustion of fuel, etc.

Food spoiling, rusting of iron,
baking a cake, food digestion,
cooking,

Forming diamonds, weathering of
rocks, setting concrete, silver
tarnishing, etc.

In chemistry, the rate
of a reaction is
expressed as the
amount of reactant
changing per unit
time.
(Example: mol/sec)

Categorize

Options (5)

C_(graphite) ---> C_(diamond)

Digesting food

Spoiling food

Lighting a match

Combusting gasoline

Organize the following reactions by rate. 1 = very slow 2 = medium speed 3 = immediately

1 = very slow

2 = medium

3 = immediately

Factors Affecting the Rate of a Reaction

Increasing any of these factors will increase the rate of the reaction. Decreasing the factor will decrease the rate.

• Concentration
  (How much)

• Temperature
  (How hot)

• Surface area
  (How big)

• Catalysts
  (Rxn helpers)

• Pressure
  (how squished)

• Agitation
  (stirring)

Multiple Choice

Which of the following is NOT a factor that affects reaction rates?

• Pressure
• Concentration
• Catalysts
• Temperature
• Volume

Collision Theory

Collision theory tells us that a reaction will only take place if three conditions are met:

1.

The reactants come into contact.

2.

The reactants collide with a certain minimum energy. If they don’t have enough kinetic energy they will just bounce apart.

3.

The collision has the correct molecular orientation in 3-dimensional space.

If these three conditions are not met, no reaction will occur.

Reaction Rate Changes over Time

• As time goes on, the rate of a reaction generally slows down because the concentration of the reactants decreases.

• At some time, the reaction stops, either because the reactants run out or because the system has reached equilibrium.

Multiple Choice

How do catalysts speed up a chemical reaction?

• Catalysts increase the temperature of the reaction
• Catalysts decrease the concentration of reactants
• Catalysts change the stoichiometry of the reaction
• Catalysts lower the activation energy barrier for the reaction

Multiple Choice

When you increase the temperature of a reaction, what happens to the rate at which it occurs?

• The rate of the reaction becomes negative.
• The rate of the reaction remains constant.
• The rate of the reaction increases.
• The rate of the reaction decreases.

Multiple Choice

When you decrease the surface area of a reaction, what happens to the rate at which it occurs?

• The rate of the reaction decreases.
• The reaction stops completely.
• The rate of the reaction increases.
• The reaction becomes more efficient.

Unit 11

Chemical
Equilibrium

Reversible Reactions

Many chemical changes are reversible, in other words, there is a forward and reverse reaction.

For example, All phase changes are reversible:

H2O(l) ⇌ H2O(s)

Gas can be absorbed in Soda, and then be desorbed out of the solution

2Na(s) + Cl2(g) ⇌ 2NaCl(s)

Multiple Choice

What is a reversible reaction?

• A reversible reaction only occurs in one direction
• A reversible reaction involves only solid substances
• A reversible reaction cannot reach equilibrium
• A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions.

Dynamic Equilibrium

• A reversible reaction might look like it has stopped even though it has not.

• At equilibrium, the Rates of the forward and reverse reactions are equal

• The Concentration of the products and reactions do not change.

• However, the concentration of the products and reactions do not have to be equal, they just have to remain constant.

Multiple Choice

What is dynamic equilibrium?

• A state in which no reactions occur
• A state in which reverse reactions occur faster than forward reactions
• A state in which forward and reverse reactions occur at the same rate.
• A state in which only forward reactions occur

Multiple Choice

True or False: Equillibrium is reached when the concentrations of products and reactants are equal

• True

• False

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