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Reaction Rates . Chapter 18 CP Chemistry. Reactions can be…. FAST!. Liquid hydrogen and oxygen reacting to launch a shuttle. ..or. S L O W. Concrete hardening. ..or. S L O W. Watching paint dry. Expressing rxn rates in quantitative terms. Reaction Rates.

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Reaction rates

Reaction Rates

Chapter 18

CP Chemistry


Reactions can be
Reactions can be…

  • FAST!

Liquid hydrogen and oxygen reacting to launch a shuttle


..or

  • S

    L

    O

    W

Concrete hardening


..or

  • S

    L

    O

    W

Watching paint dry



Reaction rates1
Reaction Rates

Red Blue


Collision theory
Collision Theory

  • Atoms, ions and molecules must collide in order to react.

  • Reacting substances must collide with the correct orientation.

  • Reacting substances must collide with sufficient energyto form the activated complex.


Orientation and the activated complex
Orientation and the activated complex

  • Analogy: if you start with two separate paperclips (reactants) and you wish to link them together (products), not only must the paperclips come into contact, but they also must collide with a specific orientation.


Activation energy and reaction
Activation energy and reaction

  • Only collisions with enough energy to react form products


Factors affecting reaction rates
Factors affecting reaction rates

  • Temperature

  • Concentration

  • Particle Size

  • Catalysts


Temperature
TEMPERATURE:

  • Generally, ↑ temp = ↑ rate

    • Why?

    • Higher temp = faster molecular motion = more collisions and more energy per collision = faster rxn

Analogy: imagine that you are baby-sitting a bunch of 6 year olds. You put them in a yard and you let them run around. Every now and then a couple of kids will run into each other. Now imagine that you decide to feed them some sugar. What happens? They run around faster and of course there are many more collisions. Not only that, the collisions are likely to be a lot harder/more intense.


Concentration
CONCENTRATION

  • As concentration ↑, frequency of collisions ↑, and therefore rxn rate ↑


Surface area
SURFACE AREA

  • As surface area ↑, rxn rate ↑

← slow

fast


Catalyst pg 547

Provides an easier way to react

Lowers the activation energy

Catalyst: a substance that speeds up the rate of a reaction without being consumed in the reaction. (remains unchanged)

CATALYST pg. 547


Catalyst
CATALYST

  • Adding a catalyst speeds up the rxn by lowering the activation energy




Consider a glass of water1
Consider a glass of water…

Now, put a lid on it….


Consider a glass of water2
Consider a glass of water…

Evaporation continues, but condensation also occurs...


Consider a glass of water3
Consider a glass of water…

The rates equalize, and the system reaches equilibrium.


Chemical equilibrium1
Chemical Equilibrium

Consider the following reaction(s):

H2O (liquid)  H20 (gas)

H2O (gas)  H2O (liquid)

H2O (liquid)  H2O (gas)

Equilibrium Symbol


Chemical equilibrium occurs when opposing reactions are proceeding at equal rates. The rate at which the products are formed from the reactants equals the rate at which the reactants are formed from the products.

For equilibrium to occur, neither reactant nor products can escape from the system.


N proceeding at equal rates. 2 + 3H2 2NH3 + 22 KCal

Forward Reaction


N proceeding at equal rates. 2 + 3H2 2NH3 + 22 KCal

Reverse Reaction


Reversible reactions
Reversible Reactions proceeding at equal rates.

  • REVERSIBLE REACTIONS do not go to completion and can occur in either direction:

    aA + bB  cC + dD


Chemical equilibrium exists when the forward reverse reactions occur at exactly the same rate
CHEMICAL EQUILIBRIUM proceeding at equal rates. exists when the forward & reverse reactions occur at exactly the same rate

EQUILIBRIUM

R

Fig. 18.10 pg. 550

concentration

P

Time (reaction progress )

There is no net change in the amounts of reactants and products at equilibrium PAGE 550 (key point)


Checkpoint pg 550
Checkpoint ? Pg. 550 proceeding at equal rates.

  • Why is chemical equilibrium called a dynamic state?


Answer
Answer proceeding at equal rates.

  • Because both the forward and reverse reactions continue


At equilibrium
At equilibrium: proceeding at equal rates.

  • If there are more products than reactants, the products are said to be favored.

  • If there are more reactants than products, the reactants are said to be favored.


Factors affecting equilibrium

Factors Affecting Equilibrium proceeding at equal rates.



Le chatelier s principal
Le Chatelier’s Principal something changes this condition.

when a change (“stress”) is applied to a system at equilibrium, the system will shift its equilibrium position to counter act the effect of the disturbance.


Factors affecting equilibrium include changes in
Factors affecting equilibrium include changes in: something changes this condition.

  • Concentrations of reactants or products

  • Temperature

  • Pressure (gases)


Changes in concentration
Changes in Concentration: something changes this condition.

  • Consider this reaction at equilibrium:

    H2(g) + I2(g)  2HI(g)

  • What will happen to the equilibrium if we:

    • add some H2?

Reaction shifts to the right

(forms more product)


Changes in concentration1
Changes in Concentration: something changes this condition.

  • Consider this reaction at equilibrium:

    H2(g) + I2(g)  2HI(g)

  • What will happen to the equilibrium if we:

    • remove some H2?

Reaction shifts to the left

(forms more reactants)


Changes in concentration2
Changes in Concentration: something changes this condition.

  • When a substance is added, the stress is relieved by shifting equilibrium in the direction that consumes some of the added substance.

  • When a substance is removed, the reaction that produces that substance occurs to a greater extent.


Changes in temperature
Changes in Temperature: something changes this condition.

  • Consider this reaction at equilibrium:

    2SO2(g) + O2(g)  2SO3(g) + 198 kJ

  • What will happen to the equilibrium if we:

    • increase the temperature?

Reaction shifts to the left

(forms more reactants)


Changes in temperature1
Changes in Temperature: something changes this condition.

  • Consider this reaction at equilibrium:

    2SO2(g) + O2(g)  2SO3(g) + 198 kJ

  • What will happen to the equilibrium if we:

    • decrease the temperature?

Reaction shifts to the right

(forms more products)


Changes in temperature2
Changes in Temperature: something changes this condition.

  • Increasing the temperature always favors the reaction that consumes heat, and vice versa.


Changes in pressure
Changes in Pressure: something changes this condition.

  • Consider this reaction at equilibrium:

    2NO2(g)  N2O4(g)

  • What will happen to the equilibrium if we:

    • increase the pressure?

Reaction shifts to the right

(forms more product)


Changes in pressure1
Changes in Pressure: something changes this condition.

  • Consider this reaction at equilibrium:

    2NO2(g)  N2O4(g)

  • What will happen to the equilibrium if we:

    • decrease the pressure?

Reaction shifts to the left

(forms more reactant)


Changes in pressure2
Changes in Pressure: something changes this condition.

  • Increasing the pressure favors the reaction that produces the fewer moles of gas, and vice-versa.


Catalysts
Catalysts something changes this condition.

A catalyst increases the rate at which equilibrium is reached, but it does not change the composition of the equilibrium mixture.


Action of a catalyst
Action of a Catalyst something changes this condition.

Activation Energy

Without a catalyst


Action of a catalyst1
Action of a Catalyst something changes this condition.

Lower Activation Energy

With a catalyst: A catalyst lowers the activation energy.


Example consider the rxn at equilibrium n 2 g 3h 2 g 2nh 3 g 94 kj
Example: consider the rxn at equilibrium: something changes this condition.N2(g) + 3H2(g)  2NH3(g) + 94 kJ

  • How would the equilibrium be influenced by:

    • Increasing the temp:

    • Decreasing the temp:

    • Increasing the pressure:

    • Adding more H2:

    • Removing some NH3:

    • Decreasing the pressure:

    • Adding a catalyst:

rxn shifts to the left

rxn shifts to the right

rxn shifts to the right

rxn shifts to the right

rxn shifts to the right

rxn shifts to the left

no change in equilibrium position


Example how will an increase in pressure affect the equilibrium in the following reactions
Example: How will an something changes this condition.increase in pressure affect the equilibrium in the following reactions:

  • 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

    • RXN SHIFTS LEFT

  • 2H2(g) + O2(g) 2H2O(g)

    • RXN SHIFTS RIGHT


Example how will an increase in temperature affect the equilibrium in the following reactions
Example: How will an something changes this condition.increase in temperature affect the equilibrium in the following reactions:

  • 2NO2(g) N2O4(g) + heat

    • RXN SHIFTS LEFT

  • H2(g) + Cl2(g) 2HCl(g) + 92 KJ

    • RXN SHIFTS LEFT

  • H2(g) + I2(g) + 25 kJ  2HI(g)

    • RXN SHIFTS RIGHT


To close let s talk about fritz haber and the process that made him famous
To close, let’s talk about Fritz Haber and the “process” that made him famous…

Let’s say you want to manufacture ammonia gas (NH3). What are the optimum conditions?

N2 + 3H2 2 NH3 + Heat

We wish to favor the forward reaction, thus producing more NH3 gas.


For example
For example “process” that made him famous…

N2 + 3H2 2 NH3 + Heat

By cooling the reaction, the reactions counters by producing heat. It does this by shifting to the right, producing heat, and more NH3 gas.


For example1
For example “process” that made him famous…

N2 + 3H2 2 NH3 + Heat

4 moles of gas

2 moles of gas

By increasing the pressure, the reaction tries to reduce the pressure. It does this by shifting to the side with the fewest moles of gas. This is the product side with 2 moles of gas. Thus the reaction shifts to the right reducing pressure, and producing more NH3 gas.


For example2
For example “process” that made him famous…

N2 + 3H2 2 NH3 + Heat

By adding additional N2 and H2, the reactions tries to use them up. In doing so, the reaction shifts to the right.

By removing NH3 as soon as its formed, the reactions tries to produce more. Shifting the reaction to the right.


For example3
For example “process” that made him famous…

N2 + 3H2 2 NH3 + Heat

So if you want to produce maximum ammonia gas you should

Cool the reaction

Conduct the reaction under high pressure

Add N2 and H2

Remove NH3


The equilibrium constant k eq page 556
The Equilibrium Constant, K “process” that made him famous…eqpage 556

  • For the reaction: aA + bB  cC + dD at equilibrium, the constant, kc or keq:

  • Keq is a measure of the extent to which a reaction occurs; it varies with temperature.


Example a write the equilibrium expression for
Example (a): “process” that made him famous…Write the equilibrium expression for…

PCl5 PCl3 + Cl2


Example b write the equilibrium expression for
Example (b): “process” that made him famous…Write the equilibrium expression for…

4NH3 + 5O2 4NO + 6H2O


Ex: One liter of the equilibrium mixture from example (a) was found to contain 0.172 mol PCl3, 0.086 mol Cl2 and 0.028 mol PCl5. Calculate Keq.

PCl5 PCl3 + Cl2


What does k eq 0 53 mean to me
What does k was found to contain 0.172 mol PCleq=0.53 mean to me???

  • When Keq > 1, most reactants will be converted to products.

  • When Keq < 1, most reactants will remain unreacted.


The equilibrium constant allows us to …. was found to contain 0.172 mol PCl

Predict the direction in which a reaction mixture will proceed to achieve equilibrium.

Calculate the concentrations of reactants and products once equilibrium has been reached.


Ex: was found to contain 0.172 mol PClPCl3(g) + Cl2(g)  PCl5(g)Keq=1.9 In a system at equilibrium in a 1.00 L container, we find 0.25 mol PCl5, and 0.16 mol PCl3. What equilibrium concentration of Cl2 must be present?


Reaction quotient q
Reaction Quotient (Q) was found to contain 0.172 mol PCl

  • Reaction Quotient (Q) is calculated the same as Keq, but the concentrations are not necessarily equilibrium concentrations.

  • Comparing Q with Keq enables us to predict the direction in which a rxn will proceed if it is NOT at equilibrium.


Comparing q to k eq

When Q < K was found to contain 0.172 mol PCleq:

When Q = Keq:

When Q > Keq:

Forward rxn predominates – “reaction proceeds to the right”(until equil. is reached)

System is at equilibrium

Reverse reaction predominates – “reaction proceeds to the left” (until equilibrium is reached)

Comparing Q to Keq


Ex: was found to contain 0.172 mol PClH2(g) + I2(g)  2HI(g)Keq for this reaction at 450 C is 49. If 0.22 mol I2, 0.22 mol H2, and 0.66 mol HI are put into a 1.00-L container, would the system be at equilibrium? If not, what must occur to establish equilibrium.

Q < Keq; therefore forward reaction predominates until equilibrium is reached.


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