Reaction Rates and Equilibrium Chapter 12 (page 366)

transcript

Reaction Rates and Equilibrium

Chapter 12(page 366)

Essential Question!!

• What determines how rapidly chemicals will react?

Vocabulary: Section 1• Rate• Reaction rate• Molarity• Stoichiometry• Collision theory• Activation energy• Transition state• Reaction profile• Activated complex

Some reactions are fast Others are slow

C(s) + O2(g) → CO2(g) 4Fe(s) + 3O2(g) → 2Fe2O3(s)

What is a rate? How is it measured?

The idea of rate

• Why are some chemical reactions fast and some so slow?

• What some foods burn faster than others?• What determines how fast some foods spoil?• What is homeostasis and how is that similar to

chemical equilibrium?

1.25( )

0.625( )

distance milesspeed rate

time mi

distancespeed rate

Average speed traveled by a typical race horse:

The idea of rate

Beginning:High concentration of ANo C present

A → C

At time 0 seconds

End:High concentration of CLow concentration of A

A → C

Factors Affection Reaction Rates

• Concentration of reacting chemicals

• The temperature of reacting particles

• Surface area – size of reacting particles

• Addition of a catalyst or inhibitor

• Pressure of reactant gas or products (if gas)

Reaction Rate

• Reaction rate explains and calculates how a reaction takes place over time

• As the concentration of the reactants (a and b) decrease, the concentration of the concentration of products (c and d) increases (page 370)

Rate = Change in Concentration = ∆ C Change in time ∆ t

rate = -[A]t

rate = [B]t

Data for Reaction of A → C

Time (min)Time (min) Time (sec)Time (sec) Moles AMoles A Moles BMoles B

00 00 1.001.00 0.000.00

1010 600600 0.740.74 0.260.26

2020 12001200 0.540.54 0.460.46

3030 18001800 0.400.40 0.600.60

4040 24002400 0.300.30 0.700.70

Sample problem

• Calculate the Average Rate C between 10 minutes and 20 minutes.

• Rate = Change in Concentration = ∆ C Change in time ∆ t

= [moles of C (at 20 min) – Moles of C (at 10 min)] 20 min – 10 min

= 0.2 / 10 = 0.02 moles/min

Assignment

• Take a new sheet of paper and fold it into three sections

• Write your name, the title of the chapter and the number

• On the first section from the sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Practice Problems

Time Time (min)(min)

Time Time (sec)(sec)

Moles Moles A A

Moles Moles BB

00 00 1.001.00 0.000.00

1010 600600 0.740.74 0.260.26

2020 12001200 0.540.54 0.460.46

3030 18001800 0.400.40 0.600.60

4040 24002400 0.300.30 0.700.70

• Calculate rate of change in C between 20 minutes and 30 minutes.

• Calculate the rate of change in A between 1200 seconds and 1800 seconds.

Stoichiometry and Rate• The decomposition of N2O5 is shown below:

2N2O5 (g) → 4NO2 (g) + O2 (g)

• The rate of decomposition of N2O5 was measured after 25 seconds and found to be 5.60 x 10-6 M/s. What is the rate of appearance of NO2?

• Solve:

4 mol NO2

5.06 x 10-6 x --------------- = 1.12 x 10-5 2 mol N2O5

Practice Problem

• The decomposition of KClO3 is shown below:– 2KClO3 → 2KCl + 3O2,

• The rate of decomposition of KClO3was measured after 25 seconds and found to be 2.60 x 10-3 M/s. What is the rate of appearance of O2, ?

• 2KClO3 → 2KCl + 3O2, how many moles of oxygen are produced when 34g of KClO3 decompose completely?

Collision theory

Chemical reactions take place at the molecular level,

where molecules of reactants are colliding with each

A + B → Products

Collision theory

But not all collisions are successfulCollision alone does not guarantee success

The same is true in chemistry

The Collision Theory• Chemical reactions take place at the level where molecules of

reactants are colliding with each other (imagine a game of pool)

• This cannot be seen but can be observed, since reactions only take place when molecules or atoms collide with each other (very few collisions result in the actual formation of a product)

• In the game of pool, the balls must be hit hard enough in order for the balls to move at the right angle to go into the pocket; in reactions the right amount of energy must be achieved at the right angle, in the initial hit in order, for the bonds in the molecule of the compound to break

• The minimum amount of energy required for a reaction to take place is referred to as the activation energy (energy of activation) Ea

• When two molecules collide with sufficient activation energy, a new high energy state can be reached and is called the transition state

• The transition state is a high energy state where old bonds are broken, atoms or molecules are rearranged, and new bonds are formed (also known as the activated complex, Ac)

• To show what happens in a reaction, scientists show a graph that shows the progress of a reaction with respect to the energy changes that occur during a collision

Assignment

• On the second section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Reaction profile

Energy is released as a result of the reaction

Reactants

Products

Reaction: A + B → C + D ∆H < 0

Reactants ProductsEnergy Energy

Reaction profile

activation energy, Ea: the minimum amount of energy required for molecules to react.

Reactants

Products

A + B C + D

Exothermic Reaction Endothermic Reaction

The activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction.

Reaction profile

activated complex, Ac: a high- energy state where bonds are being broken and reformed; also referred to as the transition state.

Reactants

Products

Reaction profile

Reactants

Products

Ac is unstable and can:

proceed to C + D (products)

go back to A + B (reactants)

activated complex, Ac

A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed.

uncatalyzed catalyzed

ratecatalyzed > rateuncatalyzed

C + D → A + B ∆H > 0A + B → C + D ∆H < 0

Exothermic process Endothermic process

Ea is larger!

Exothermic reactions tend to be more common than endothermic reactions because the energy barrier is lower

Factors Affecting the Rate

• Concentration• Increase or decrease in temperature• Surface area of the particles involved• Catalysts and inhibitors• Change in volume or pressure

More collisions occur when the concentration of reactants is higher

More collisions means more reactions are possible

The reaction rate is higher when more reactants are present

Temperature is a measure of the average kinetic energy of molecules

The higher the kinetic energy, the higher the number of molecules that

successfully overcome the energy barrier

Energy barrier between reactants and products

Temperature

Increased surface area means more particles are available for collisions

Increased surface area leads to a higher reaction rate

A A A A

More particles are exposed andavailable to collide with

other particles to have a reaction

Surface area

Assignment

• Write a three dollar summary on what has been learned in this section, be sure to use the vocabulary covered also

• Complete # 1 – 4 on page 404

• Honors Chemistry Homework– Page 405 # 21 - 31

Vocabulary: Section 2

• Equilibrium• Dynamic• Closed system• Equilibrium position• Le Chateliers principle• Equilibrium expression• Equilibrium constant• Low of mass action

traffic intraffic in

traffic outtraffic out

There is a balance or equilibrium between cars that go into the city and cars that leave the city

Concept of equilibriumPhysical equilibriumChemical equilibrium

Physical equilibriumChemical equilibrium

There is also a “balance” in a chemical system

Reactants Products

N2(g) + 3H2(g) 2NH3(g)

Changes of State• Equilibrium - dynamic condition in which two opposing physical or chemical

changes occur at equal rates in a given closed system (constant mass ); chemical reactions are reversible

• When chemical equilibrium is attained, there is not net change in concentration of products and reactants – the reaction is still dynamic and not static (like a bridge connecting drivers in and out of the city; the number of people leaving the city is equal to the number going into the city)

• Equilibrium - does not mean that there is the same number of products and reactants, just that a balance has been reached

• Boiling water in a closed container is a good example of physical equilibrium, because it is an equilibrium between both phases of the same substance and the changes that take place are physical changes not chemical

• Even though changes cannot be seen with the naked eye (macroscopic level), changes are always dynamic on the microscopic level

Equilibrium and Changes of State• System – anything being observed• Closed system – a system with no interaction between inside

and outside the reaction • Open system – a system with interaction between inside and

outside the reaction • Phases – typical forms of matter like solids, liquids, gases, and

plasma • Condensation – going from gas to liquid by losing energy• Concentration – how much solute is present based on the

amount of solution • Equilibrium position is the favored direction of a reversible

reaction (and it is determined by each set of concentrations for the reactants and the products at equilibrium

Le Chatelier’s Principle• When a physical or chemical system at equilibrium is disturbed by

application of a stress (such as a change in concentration, pressure, or temperature), it attain a new equilibrium position that minimizes the stress.

• Scientists are constantly trying to find ways to influence the concentrations to make the reactions go in the direction that they favor

• Four factors that affect the direction of a reaction are: concentration, temperature, pressure and volume

• Anything that stresses the equilibrium (or the balance) of a reaction will cause a disturbance until a new equilibrium is reached

• This also applies to life, people generally avoid stress or trouble and change their behavior to adapt

Concept of equilibriumPhysical equilibriumChemical equilibrium

Le Châtelier’s principleEffect of temperatureEffect of concentrationEffect of pressure / volume

Chemical equilibrium

Reactants Products

N2(g) + 3H2(g) 2NH3(g)

Reactants

Products

Equilibrium Vapor Pressure of a Liquid

• The pressure exerted by the molecules of a vapor which are in equilibrium with its corresponding liquid at a given temperature

• Water can be a liquid up to 374.10C See next slide

Physical equilibrium

An equilibrium between

two phases of water:H2O(l) H2O(g)

Equilibrium does not mean that there must be equal amounts on each side

It is an equilibrium when the level of neither tank changes over time. This occurs when the amount transferred

and the rate of transfer are the same from both sides

Physical equilibrium

50 mL 50 mL 50 mL 50 mL

Assignment

• On the third section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Triple pointCritical temperature

Critical Values

• Triple point- indicates the temperature and pressure at which the solid, liquid, and vapor of the substance can coexist.

• Critical temperature- the temperature above which the substance cannot exist in the liquid state

Critical Values

• Ct water = 374.10C

• Critical pressure - the lowest pressure required for the substance to exist as a liquid at the critical pressure.

• Cp water = 218.3 atm

Le Châtelier’s principle

N2(g) + 3H2(g) 2NH3(g)

The forward reaction is favored so there is more product

If an equilibrium has been established…

… what happens when two NH3

molecules are removed?

You no longer have an equilibrium! The equilibrium is re-established

Le Châtelier’s principle: principle that states that when a “change” is made to a system at equilibrium, the system will shift in a direction that partially offsets the “change.”

N2(g) + 3H2(g) 2NH3(g)

Favored reactionIf removed

Pressure and equilibrium

Pressure or volume only affect gaseous equilibrium systems

Higher pressure

Smaller volume

Lower pressure

Larger volume

The number of particles in the system is unchanged

2SO2(g) + O2(g) 2SO3(g)2 moles 1 mole 2 moles

3 moles 2 moles

Reactants take up more space (larger volume) than the product

If we increase the pressure, the volume will decrease

The equilibrium will shift to the right

2SO2(g) + O2(g) 2SO3(g)2 moles 1 mole 2 moles

3 moles 2 moles

Reactants take up more space (larger volume) than the product

If we decrease the pressure, the volume will increase

The equilibrium will shift to the left

C D ProductsK

ReactantsA B

The equilibrium position depends on a ratio between products and reactants, called the equilibrium expression:

aA + bB cC + dD

[A] means “molarity of A”

raise to the power of coefficients

Equilibrium constant

CH4(g) + 2H2S(g) CS2(g) + 4H2(g)

C D ProductsK

ReactantsA B aA + bB cC + dD

Write the equilibrium expression for the following reaction:

CH H S

Answer:Molarities of products

Molarities of reactants

Experimental results for: N2(g) + 3H2(g) 2NH3(g) at 500oC

Calculate the equilibrium concentration of hydrogen iodide [HI], given the following information: K = 50 at 450oC

[H2] = 0.22 M, and [I2] = 0.22 MH2(g) + I2(g) 2HI(g)

Answer: Calculate the concentration of HI at equilibrium

Given: K = 50 at 450oC, [H2] = [I2] = 0.22 M

Relationships:

Equilibrium expression KH I

Calculate the equilibrium concentration of hydrogen iodide [HI], given the following information: K = 50 at 450oC

[H2] = 0.22 M, and [I2] = 0.22 MH2(g) + I2(g) 2HI(g)

Answer: Calculate the concentration of HI at equilibrium

Given: K = 50 at 450oC, [H2] = [I2] = 0.22 M

Relationships:

Solve:

Answer: The equilibrium concentration of [HI] is 1.56 M

Equilibrium expression KH I

500.22 0.22 0.0484

50 0.04

84 2.42

.42 1.56The square root of

Assignment

• Elementary steps• Reaction mechanism• Intermedialt• Unimolecular• Bimolecular• Rate determining step

reaction mechanism: a proposed sequence of elementary steps that leads to product formation. Most reaction mechanisms must be determined using experimental evidence.

elementary steps: a series of simple reactions that represent the overall progress of the chemical reaction at the molecular level.