Reaction Rate

• Measure of the rate of disappearance of one of the reactants or appearance of one of the products

• Measured as Molarity/time (M/s)

What has to happen for the reaction to take place?

• Reactants have to meet

• Reactants have to be positioned correctly

• Reactants have to have sufficient energy to produce product

How does concentration and temperature impact reaction rate?

Reaction Profile

Time

En

erg

y

Reactants

Products

Activated Complex – Transitional structure

Activation Energy

Is reaction endothermic

or exothermic?

What is the effect of a catalyst?

change in conc

change in timeRate =

•Slope of the line represents rate

•Reaction rate varies

•Initially faster, then slows down… why???

Rxn: A -> Products

What will happen to the concentration of reactant A versus

time?

Rate Law… what is it? How do I find it?

• Mathematical expression of• Reaction rate as a function of reactant

concentration • Experimentally determined• Can use the “Method of Initial Rates”

to find the initial rate

Method of Initial Rates

Rxn: A + 2 B -> 2 C

Rate Law: Rate = k[A]x[B]y

k is the rate law constantWhat is factored into the constant?

Exp Initial [A] Initial [B] Initial Rate

1 0.100 M 0.100 M 2.73 M/s

2 0.150 M 0.100 M 6.14 M/s

3 0.100 M 0.200 M 5.42 M/s

Do Now –1. In your own words, what is the rate law?

2. How does it capture all the factors that impact reaction rate?

3. Monday we used the method of initial rates. What happens initially that allows us to calculate the rate law?

4. How do you find the rate law?Exp Initial [A] Initial [B] Initial

Rate

1 0.100 M 0.100 M 2.73 M/s

2 0.150 M 0.100 M 6.14 M/s

3 0.100 M 0.200 M 5.42 M/s

The rate law for the reaction of A + 2B -> C is

rate = 2730/M2s[A]o2[B]o

• What is the order of A? B? the overall order?• Which reactant has a greater effect on the reaction

rate?• What happens to the rate if the concentration of A is

doubled? If the concentration of B is doubled? If the concentration of A is tripled?

• What is the rate if [A]o= .0500 M and [B]o = .0256 M (oh and what do the little circles mean?

Do Now1. What is reaction rate?

2. What does the term “equilibrium” signify? Can you describe physical changes in the chemistry lab where equilibrium is achieved?

3. When calculating the mass of product produced using stoichiometry, what assumption do you make about the amount of reactant you started with?

4. What is meant by chemical equilibrium?

5. Which components of the reaction (reactants/products) are present at this point?

6. What happens to the concentrations of those species once equilibrium is established?

• Methylene Blue, (MB) reacts with basic, sugar solution, and is reduced (gains electrons)

MB + electrons -> MB- (yellowish)

• Shaking, dissolves oxygen into solution, and oxidizes (removes electrons) from MB-

MB- -> MB + electrons (blue)

What is dynamic equilibrium?

• Two opposing chemicals reactions occur at the same rate:

H2(g) + I2(g) <-> 2HI(g)

Chemical Equilibrium

Forward: H2(g) + I2(g) -> 2HI(g)

Reverse: 2HI(g) -> H2(g) + I2(g)

• Will we able to use stoichiometry to predict amount of materials that will be produced?

• No! Nothing is consumed. Everything will be present!!! We will need another means to solve.

N2(g) + 3H2(g) <-> 2NH3(g)

N2 H2 NH3

Exp1: Initially 1.0 M1.0 M 0

Exp 2: Initially 0 0 1.0 M

Exp 3: Initially 2.0 M 1.0 M 3.0M

At equilibrium 0.921M 0.763M 0.157M

At equilibrium 0.399M 1.197M 0.203M

At equilibrium 2.59M 2.77M 1.82M

Nothing is consumed!!!

Law of Mass Action

• The ratio of product concentrations to reactant concentrations at equilibrium is constant.

• For reaction aA + bB <-> cC + dD• Equilibrium Expression is

K = [C]c[D]d

[A]a[B]b

Equilibrium Constant

Do Now

• What is K

Do Now:1. Write balanced chemical reactions and equilibrium expressions

for:a. Hydrogen and oxygen gases reversibly produce

water vapor

b. Tetratomic solid phosphorus reacts reversibly with gaseous nitrogen monoxide to produce solid P4O6 and nitrogen gas

c. Solid carbon reacts with carbon dioxide gas to reversibly produce carbon monoxide gas

2. What is K? What factors will affect K? Where does it come from?

The equilibrium constant, K for the reaction H2(g)+F2(g)<->2HF(g) has the value of 2100 at a particular

temperature.

• When the system is analyzed at equilibrium at this temperature , the concentrations of H2 and F2 are both found to be 0.0021 M. •What is the concentration of HF in the equilibrium system under these conditions?

What does the size of K indicate

• Extent to which a reaction proceeds to completion

• 2O3 <-> 3O2 K=1 x 1055

• 2HF <-> H2 + F2 K=1 x 10-13

• N2 +3H2 <-> 2NH3 K=10

1.00 moles of dinitrogen trioxide is placed into a 2.0 L flask at 25ºC and 1 atm and it

decomposes into nitrogen dioxide and nitrogen monoxide reversibly.

• Write an equation to describe this reversible gas phase reaction

• What is the composition of the reaction mixture at equilibrium if it contains 0.30 moles of nitrogen dioxide?

• What is the value of the equilibrium constant, K?’

Do Now

• What is the law of mass action? What do we use it for?

• What is an ICE table? What is it’s purpose?

The formation of hydrogen fluoride from hydrogen and fluorine has an equilibrium constant of 115 at a certain temperature.• Write a reversible reaction and an

equilibrium expression.• In a particular experiment 3.00 moles

of each component is added to a 1.50 L flask. Calculate the equilibrium concentrations of all species.

Do Now

• Have new HW ready.

• Take a look at the old HW. I will go over any problems (from either HW) that you would like.

• If you are good, work on the next HW set.

Do Now:1.What is an ice table used for? How do we know if we need it?2.What is the quadratic equation? When do we have to use it?

For the reaction: 2NO(g)<->N2(g)+O2(g)

the equilibrium constant is 2400 at a certain temperature.

• For which of the following sets of conditions is the system at equilibrium? For those that are not at equilibrium, in which direction is the reaction proceeding toward equilibrium.

• A) 1.0 L flask contains 0.24 mole NO, 2.0 mol N2, and 2.6 mole O2

• B) 2.0 L flask contains 0.032 mole NO, 0.60 mol N2, and 4.1 mole O2

• C) 1.0 L flask contains 0.060 mole NO, 4.4 mol N2, and 3.7 mole O2

Reaction quotient, Q

• A tool used to determine if a reaction is at equilibrium

• Insert any concentrations you have and find the quotient, Q

• Compare to the K, the equilibrium constant.

What is Le Châtelier’s Principle?

• If a stress is applied to an equilibrium system, the reaction will shift in a direction that tends to reduce the change.

N2(g) + 3H2(g) <-> 2NH3 H =-92 kJStress Shift [N2] [H2] [NH3]

Add N2

Remove NH3

Increase Temperature

Increase Pressure(by

decreasing V)

Add catalyst

Increase Volume

Add acid (NH3 is a base)

Do Now• Bring me your HW when I call you.• Work on review problems.• Prepare any questions you have.• Test is on Friday. Get prepared. Some of you were very unprepared last

time based on your responses to the multiple choice and problems which were very similar if not identical to the problems we did in class/HW!!!

Do Now:1.Describe LeChatelier’s principle in your own words.Looking at the boxed table of equations in your lab…2. What types of equations are presented in the table?3. In Part A: If solutions of FeCl3(aq) and KSCN(aq) were

added to a container, which ions would they provide in reference to the equation in the table.

4. If the reaction had already reached equilibrium, and more FeCl3 was added, what would happen? Apply LeChatelier’s principle and write it out… in sentence(s) in proper English!!!

5. In Part B: If solutions of K2CrO4(aq) or K2Cr2O7(aq) were added… what would they provide in terms of the equation in the table?

6. What type of substances are HCl and NaOH?

Do Now• In your own words, describe

LeChatelier’s principle.• What are the two reaction systems we

will investigate today?• From your experience, describe what

you know about a solution of HCl.• From your experience, describe what

you know about a solution of NaOH.• What is H3O+? When is it present in

large amounts?

Do Now:

• Take out Hw and lab. Have answers highlighted. Bring them to me FOUR students at a time.

• If you are not at my desk, please read the lab for tomorrow (left on your desk) or work on the additional worksheets I left for you or that I gave you on Monday.

• Also prepare questions for our review which may include going over any HW problem or topic.

What is stability???

• Thermodynamically stable - the reaction will not take place spontaneously, overall energy change for the reaction is positive

• Kinetically stable – the reaction will take place spontaneously, however the reaction proceeds so slowly we can not detect a change

Do NowHighlight, IN COLOR, the answers…•31. mol H2•33. concs. at eq•53. init and eq conc of HI•81. K•21. Pressure changes and why•69. “at eq” or direction rxn proceeds

•Then, bring up to me no more than 3 students at a time. Work on next HW!

Check answers

• 15.31 (.173 mol)

• 15.33 ( [H2]=[Br2]= 0.00018M; [HBr] = 0.267M)

• 15.53 ([HI] Initial= 0.35M; Eq = 0.53 M)

• 15.81 (1.7)

• 21. PCl3 and Cl2 increase, PCl5 decreases because Q<K

• 69. a. eq b. proceeds right c. proceeds left

• 15.39 , 41

• 15.45( a. increases, b. decreases, c. no change)

• 15.49

Do Now … Mark up your HW, in color, as follows…

• The practice exercise problem – highlight the direction reaction proceeds

• 45 – highlight increase/decrease/no change in your response

• 69 – highlight if at eq or direction reaction proceeds

• 21 – highlight what will happen to PCl3, PCl5, and Cl2

After you have your HW checked you may do one part of 49 on the board for a mole$

Given the reaction, 2NH3(g)<-> N2(g)+ 3H2(g)

• Write the equilibrium expression• The following equilibrium concentrations

were observed at 127ºC[NH3]= 0.031 M

[N2] = 0.85 M

[H2] =0.0031 M

• Find the value of the equilibrium constant

Do Now• What is the law of mass action? • How do you write an equilibrium expression and

what is it used for?• Consider the following equilibrium process:

2NO2(g) <-> 2NO(g) + O2(g)

Kc = 1.58 at 1000 K. The equilibirum concentrations of oxygen and nitrogen dioxide are 0.25 M and 0.36 M, respectively. Determine the equilibrium concentration of nitrogen monoxide.