rates of reaction and chemical equilibrium chemistry ms. piela

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Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

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Page 1: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Rates of Reaction andChemical Equilibrium

ChemistryMs. Piela

Page 2: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Rates of Reaction

A rate is any change that occurs within an interval of time

Collision theory states that particles will react to form products when they collide This requires a certain amount of kinetic energy

As an example, imagine throwing clay balls at one another

Without enough force, they will ricochet off from one another. When they have enough kinetic energy, they will smash and fuse into something entirely new.

This is the same in chemical reactions

Page 3: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Activation Energy

Activation energy (Ea) is the minimum amount of energy required for a chemical reaction to occur

Page 4: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Activation Energy Diagram

Diagram can be used to determine in reaction is exothermic (- ∆H) or endothermic (+∆H)

Activated complex (or transition state) is the arrangement of atoms at the peak of the Ea barrier Unstable and only

appears briefly

Page 5: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Activation Energy Diagram

Page 6: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Rates of Reactions

Four Major FactorsTemperature

Concentration

Particle Size

Catalysts

Page 7: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Rates of Reaction

Temperature Generally, an increase in temperature will increase

the rate of a reaction, and vice versa Explanation:

Increasing temperature speeds up particles, increasing kinetic energy

This allows a reaction to overcome its activation energy

Example Popcorn

Page 8: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Rates of Reaction

Concentration Increasing the number of particles (thereby

increasing concentration) will increase the reaction rate

Explanation: More particles will increase the collision frequency More collisions will mean more chances for a

reaction to occurExample

Wooden splint in pure oxygen

Page 9: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Rates of Reaction

Particle Size (aka Surface Area) The smaller the particle, the larger the surface area

for a given mass of particles An increase in surface area will increase the rate of

reaction Explanation:

With more exposed area to react, this increases the rate of reaction

Example Fine Dust Explosions (1902 Chicago Fire) Lycopodium dust explosion

Page 10: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Rates of Reaction

Catalysts Any substance that

increases the rate of a reaction without being consumed

Catalysts allow a reaction to occur at lower activation energy

Enzymes are biological catalysts

Page 11: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Activation Energy Diagram

Page 12: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Rates of Reaction

Page 13: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Review of Factors Affecting Rates

Temperature

• Increase causes higher kinetic energy

Concentration

• Higher concentration causes more collisions

Particle Size

• More surface area leads to more exposed spaces to react

Catalysts

• Substances that increase the rates of reaction without being spent

Page 14: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Chemical Equilibrium

Reversible reactions are reactions that simultaneously occur in both directions

Chemical equilibrium is the state in which the forward and reverse reactions take place at the same rate Sample Reaction:

2 SO2 (g) + O2 (g) 2 SO3 (g)

Page 15: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Chemical Equilibrium

Differences between rates and concentration One doesn’t imply the other! Although rates of forward and reverse reactions are

equal at chemical equilibrium, the concentrations are not necessarily the same

ExampleA B

1% 99% If A reacts to give B and the equilibrium mixture

contains 99% B and only 1% A, the formation of B is said to be favored

Page 16: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Effect of Catalyst on Equilibrium

Although catalysts speed up a reaction rate, they will not affect the concentration of reactants and products at equilibrium Catalysts can only decrease the time required to reach

equilibriumAffects speed,

not concentration! Meep meep!

Page 17: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier's Principle

When a stress is applied to a system at equilibrium, the equilibrium will shift in order to relieve the stress

Page 18: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Factors Affecting Equilibrium

Three Major Factors

Concentration

Temperature

Pressure

Page 19: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier's Principle

Concentration Changing the amount, or concentration, of any

reactant or product in a system at equilibrium disturbs that equilibrium

Example equilibrium:

2 H2CO3 (g) 2 H2O (l) + 2 CO2 (g)

Adding CO2

Shifts equilibrium to the left Concentration of H2O will decrease Concentration of H2CO3 will increase

Page 20: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier's Principle

Example equilibrium:

2 H2CO3 (g) 2 H2O (l) + 2 CO2 (g)

Removing CO2

Shifts equilibrium to the right Concentration of H2O will increase Concentration of H2CO3 will decrease

Page 21: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier's Principle

Temperature Increasing the temperature causes an equilibrium

position of a reaction to shift in the direction that will absorb the heat. Consider heat as if it were a part of the reaction

2 SO2 (g) + O2 (g) 2 SO3 (g) + heat

Adding heat Shifts equilibrium to the left. Concentration of SO2 and O2 will increase. Concentration of SO3 will decrease.

Page 22: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier’s Principle

2 SO2 (g) + O2 (g) 2 SO3 (g) + heat

Removing heat Shifts equilibrium to the right. Concentration of SO2 and O2 will decrease. Concentration of SO3 will increase.

Page 23: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier's Principle

Pressure Changing the pressure only affects an

equilibrium with an uneven number of reactants and products

Increasing pressure will shift an equilibrium in the direction of the least number of moles

Be aware that a change in volume also affects pressure Reducing volume will increase the

pressure, and vice versa

Page 24: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Le Châtelier's Principle

Example Equilibrium:CO + 3 H2 (g) CH4 (g) + H2O (g)

Increasing pressure (reducing volume) will shift the equilibrium to the…

rightDecreasing pressure (increasing volume)

will shift the equilibrium to the… left

Page 25: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Haber Bosch Process & Le Châtelier’s Principle

Page 26: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Equilibrium Constant Expressions

A mathematical relationship exists between concentration of the reactants and products once equilibrium has been reached that is independent of the initial concentration of the participants

Given the symbol Keq

Can be used to determine whether products or reactants are favored in an equilibrium

Page 27: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Equilibrium Constant Expressions

Page 28: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Steps for Determining Keq

Make sure the chemical reaction is balanced and at equilibrium

Place the product concentrations in the numerator, and the reactant concentrations in the denominator

[products] [reactants]

Concentrations of any solid or liquid is left out because the concentrations never change Water is always omitted in aqueous solution. Water is

almost constant during the reaction

Keq =

Page 29: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Steps for Determining Keq

To complete the expression, raise each substance’s concentration to the power equal to the substance’s coefficient in the balanced chemical equation

For a general reaction: aA+ bB cC + dDThe equilibrium constant expression can be

written as

The equilibrium constant is unitless

Keq = [C]c[D]d

[A]a[B]b

Page 30: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Example #1

1. An aqueous solution of carbonic acid reacts to reach equilibrium described below:H2CO3 (aq) + H2O (l) HCO3

- (aq) + H3O+

(aq)

Write the equilibrium expression, Keq, for this reaction

][

]][[

32

33

COH

OHHCOKeq

Page 31: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Example #2

2. The brown gas, NO2, is in equilibrium with the colorless gas N2O5. Write the equilibrium expression for the following reaction:

N2O4 (colorless) 2 NO2 (brown)

][][

42

22

ONNO

eqK

Page 32: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Practice Problems

Practice #1

Practice #2

][][

][

23

2

23

NH

NHKeq

][][

][

22

2

23

OSO

SOKeq

Page 33: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Calculating using Keq

Example #1H2CO3 (aq) + H2O (l) HCO3

- (aq) + H3O+

(aq)

The solution contains the following solute concentrations: H2CO3; 3.3 x 10-2 M; HCO3

- ion, 1.19 x 10-4 M; and H3O+, 1.19 x 10-4 M. Determine the Keq expression, and the value of Keq

Page 34: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Example #1

][

]][[

32

33

COH

OHHCOKeq

]103.3[

]1019.1][1019.1[2

44

Mx

MxMxKeq

6101.1 x

Page 35: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Example #2

Keq for the equilibrium below is 1.8 x 10-5 at a temperature of 25 °C. Calculate [NH4

+] when [NH3] = 6.82 x 10-3 M and [OH-] = 3.45 x 10-3 M.

NH3 (aq) + H2O (l) NH4+

(aq) + OH- (aq)

Page 36: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Example #2

][

]][[

3

4

NH

OHNHKeq

]1082.6[

]1045.3][[108.1

3

345

Mx

MxNHx

Mx 51056.3

Page 37: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Practice Problems

Problem #1

Problem #2

]102.4][108.1[

][1065.1

43

23

MxMx

NOx

29 ][1025.1 NOx

]1032.5[]105.1[

]1098.1[323

22

MxMx

MxKeq

32752

][1054.3 5 NOMx

Page 38: Rates of Reaction and Chemical Equilibrium Chemistry Ms. Piela

Magnitude of Keq

The meaning of Keq

If Keq is very large, products will be favored

This implies the numerator is large in the Keq expression compared to denominator

If Keq is very small, reactants will be favored

This implies the denominator is large in the Keq expression compared to numerator

If Keq is close to 1, then roughly equal amounts of reactants and products are present at equilibrium