Reaction Rates & Equilibrium - mrgssciencepage

Rapid Learning Printable Tutorial - 20

Rapid Learning Center Chemistry :: Biology :: Physics :: Math

Rapid Learning Center Presenting …

Teach Yourself High School Chemistry in 24 Hours

1/54

http://www.RapidLearningCenter.com

Reaction Rates & Equilibrium Rapid Learning Core Tutorial Series

Wayne Huang, PhD Kelly Deters, MA Russell Dahl, PhD

Rapid Learning Center www.RapidLearningCenter.com/ © Rapid Learning Inc. All rights reserved.

© Rapid Learning Inc. All rights reserved.

1


Objectives By completing this tutorial you will learn… What Kinetics studies Collision Theory How factors affect rate Dynamic equilibrium Equilibrium constants Reaction Quotients Le Chatelier’s Principle

3/54

Rates & Equilibrium Concept Map Previous Previouscontent content

Chemistry Chemistry

New Newcontent content

Kinetics Kinetics studies

Reaction Reaction Rate Rate uses

Collision Collision Theory Theory

Studies Can be studied with

Forward = reverse

Matter Matter

Equilibrium Equilibrium

Undergo

Equation with Ratio of products : reactants

Equilibrium Equilibrium Constant Constant Expression Expression

Chemical Chemical reactions reactions When it’s disturbed, follow

Le LeChatelier’s Chatelier’s Principle Principle

4/54


2


Kinetics and Reaction Coordinate Diagrams

5/54

Definition - Kinetics

Kinetics – Study of the rates of reactions. Reaction Rate – Rate at which reactants produce products. How fast a reaction takes place.

6/54


3


Definition - Collision Theory Collision Theory – Defines 3 circumstances to be met for a collision to occur.

1

Reactants must collide

2

Collision must be at the correct orientation

3

Collision must have minimum energy for reaction to occur Only a small fraction of collisions meet the requirements and results in a successful reaction.

7/54

Collisions Must Occur In order for two molecules to react, they must come into contact with one another.

FF FF

NN

O O O O

There’s no way they’ll ever react if they don’t run into one another!

8/54


4


Collision with Correct Orientation For a collision to result in a chemical reaction, it must occur with the correct orientation.

FF FF

FF FF

NN

O O

NN

O O

O O O O

This is not the correct orientation. The reaction will not happen. This is the correct orientation. The reaction will happen.

9/54

Collision with Enough Energy For a collision to result in a chemical reaction, it must occur with the minimum energy for reaction.

FF FF

FF FF

NN

O O O O

NNNN

O O O O O O O O

The collision does not have enough energy to produce a reaction. This collision had more energy (faster moving molecules). A reaction will occur. 10/54


5


Definition - Activation Energy

Activation Energy – Energy that must be overcome for reaction to occur.

11/54

Reaction Coordinate Diagram Reaction coordinate diagrams show the energy changes throughout the reaction. Activated complex (Also called the transition state)

Energy

Activation Energy

Products

Reactants Reaction proceeds

Energy change for reaction

12/54


6


Activated Complex What is an “activated complex”? Energy

Reactants

F2 + NO2

Activated Complex

F2…NO2

Products F + FNO2

Reaction proceeds

FF FF

FF FF

NN

O O O O

NN

O O O O

13/54

Definition - Endo & Exothermic

Endothermic Reaction – The reaction takes in energy…the products have more energy than the reactants. Exothermic Reaction – The reaction gives off energy…the products have less energy than the reactants.

14/54


7


Endo- and Exothermic Diagrams

Energy

Reaction coordinate diagrams show whether a reaction is endothermic or exothermic.

Endothermic Products are higher energy than reactants

Energy

Reaction proceeds

Reaction proceeds 15/54

Exothermic Products are lower energy than reactants

Factors Affecting Reaction Rates

16/54


8


Surface Area of Reactants How does the surface area of the reactants affect the reaction rate?

Reactants must collide in order to react

Larger surface area means more particles can come in contact with each other at the same time

More reactants can collide at the same time and a fraction of those will result in reaction

As surface area increases, reaction rate increases. 17/54

Concentrations of Reactants How does the concentration of reactants affect the reaction rate?

Only a small fraction of the collisions meet the requirements and result in a reaction

More reactants mean more collisions will occur

If more collisions occur, more will meet the requirements and result in a reaction

As reactant concentration increases, reaction rate increases. 18/54


9


Temperature How does temperature affect the reaction rate?

Reactants must collide with at least energy equal to the activation energy

If molecules are at a higher temperature, they have a higher average kinetic energy

With higher energy molecules, collisions will have higher energy and more often result in reaction

For most reactions, as temperature increases, reaction rate increases. 19/54

Definition--Catalysts Catalysts – Substance that increases the rate of reaction without being used up. A+B+CÆD+C “C” is the catalyst…it is present in the beginning and in the end.

Energy

Without Catalyst

With Catalyst Reaction proceeds

Enzymes are catalysts in the body.

20/54


10


Catalysts How do catalysts help speed up the reaction without being used? They increase the chances that a collision will successfully produce a reaction.

For example, catalysts hold one or more of the reactants in place to allow collisions to occur with the correct orientation.

Once the reaction has occurred, the catalyst releases the molecule(s) and finds another one to help.

21/54

Equilibrium

22/54


11


Definition - Equilibrium Reversible Reaction – A chemical reaction that can proceed in both directions (represented by a “'”). Equilibrium – When the rate of the forward reaction equals the rate of the reverse reaction.

23/54

Establishing Equilibrium It takes time to establish equilibrium. At first, there are only reactants present. Only the forward reaction is possible.

Reactants ' Products

24/54

But once there are products as well, they can begin to reform reactants. The reverse reaction becomes possible. Forward rate slows and reverse rate increases until they are the same. Once the rate of the forward and reverse process are equal, it is at equilibrium. When equilibrium is established, the number of products and reactants doesn’t change…but the reaction keeps going.


12


Definition - Dynamic Equilibrium

Dynamic Equilibrium – The reaction continues to proceed in both directions, but at the same rate. The number of products and reactants no longer change, it may look as thought the reaction has stopped… But the reaction continues! 25/54

Equilibrium Constants

26/54


13


Definition - Equilibrium Constant Equilibrium Constant Expression – Equation showing the ratio of the concentrations of products to reactants at equilibrium. Concentration is symbolized with brackets “[A]”.

Equilibrium Constant (K) – The number calculated from the equilibrium constant expression. “K” is different for every reaction at every temperature! 27/54

Writing Equilibrium Constant Expressions To write an equilibrium constant expression:

1

Write the concentration of products on the top— take each one to a power of the coefficient in the balanced equation.

2

Write the concentration of reactants on the bottom—also take each to the power of the balanced equation coefficient.

Example: Write the equilibrium constant expression for the following: 2 H2 (g) + O2 (g) ' 22 H2O (g)

K=

[H2O] [H2] [O2]

28/54


14


Definition: Homo- and Heterogeneous Equilibrium

Homogeneous Equilibrium – All of the species are the same state of matter. 2 H2 (g) + O2 (g) ' 2 H2O (g)

Heterogeneous Equilibrium – There are at least 2 states of matter. 2 H2 (g) + O2 (g) ' 2 H2O (l) 29/54

Concentrations of Solids and Liquids Pure solids and pure liquids have constant “concentrations”. If concentration (Molarity) = mole liters And Density = grams liters And Molar Mass = grams mole Then for a pure solid or liquid, Molarity = Or, Molarity =

30/54

grams / liters grams / mole

Density . Molar Mass

Both Density and Molar Mass are constants—they don’t change. Therefore, “concentration” of a pure solid or liquid is a constant.


15


“K” Expressions with Solids or Liquids How does this affect the writing of Equilibrium Constant Expressions? If the “concentration” of a pure solid or liquid is constant, then it will not change during equilibrium and it is not written in the “K” expression. 2 H2 (g) + O2 (g) ' 2 H2O (g)

K=

[ H 2 O ]2 [ H 2]2 [O2 ]

2 H2 (g) + O2 (g) ' 2 H2O (l)

K=

1 [ H 2]2 [O2 ]

H2O is not included in this “K” expression because it’s a liquid.

Only gases and solutions are included in “K” expressions! 31/54

Calculating “K” Example Example: Solve for equilibrium constant for Fe2O3 (s) + 3 H2 (g) ' 2 Fe (s) + 3 H2O (g) if the following are concentrations at equilibrium: [H2] = 0.45 M and [H2O] = 0.18 M [H2]eq = 0.45 M [H2O]eq = 0.18 M

K=

[ H 2O]3 [ H 2 ]3

K=

[0.18]3 [0.45]3

K=?

Note that Fe2O3 and Fe were not included in the K expression as they are solids!

K = 0.064

Most instructors and textbooks do not require units for “K” as each one would be different 32/54


16


Meaning of Equilibrium Constant What general meaning can you get from the magnitude of the equilibrium constant?

[Products]

If K is very large…

[Reactants]

There is a much larger ratio of products to reactants at equilibrium. The reaction is said to “lie to the right” (products are on the right). If K is very small…

[Products]

[Reactants] There is a much smaller ratio of products to reactants at equilibrium. The reaction is said to “lie to the left”. 33/54

Using “K” to Find Equilibrium Concentration Example: Find the equilibrium concentration for NO if the equilibrium constant for N2 (g) + O2 (g) ' 2 NO (g) is 1.24×10-4, and the other equilibrium concentrations are [N2] = 0.166 M and [O2] = 0.145 M [N2]eq = 0.166 M

K=

[O2]eq = 0.145 M K = 1.24×10-4 [NO]eq = ? M

1.24 ×10 −4 =

[ NO ]2 [ N 2 ][O2 ]

[ NO ]2 (0.166 M )(0.145M )

(1.24 ×10 )(0.166M )(0.145M ) = [ NO] −4

[NO]eq = 0.00173 M 34/54


17


Reaction Quotient

35/54

What is the Reaction Quotient? Reaction Quotient is “Q” K

Q

Equilibrium Constant

Reaction Quotient

Expression is ratio of products to reactants with balanced equation coefficients as powers

Expression is ratio of products to reactants with balanced equation coefficients as powers

Only includes gases and solutions

Only includes gases and solutions

To solve for K, plug in concentrations at equilibrium

To solve for Q, plug in concentrations at any time

36/54


18


The Difference between K and Q What exactly is the difference? 2 H2 (g) + O2 (g) ' 2 H2O (g)

K=

[ H 2 O ]2 [ H 2]2 [O2 ]

2 H2 (g) + O2 (g) ' 2 H2O (g)

Q=

[ H 2 O ]2 [ H 2]2 [O2 ]

The expressions for K and Q are the same. To solve for “K”, plug in concentrations at equilibrium only. To solve for “Q”, plug in concentrations at any time.

37/54

Using Reaction Quotient Reaction Quotient is used to determine if a system is at equilibrium…and if it’s not, which way does it need to go to get there. [products now]

= Q

K =

[reactants at equilibrium]

[reactants now] Q=K

Q>K

Q