Chapter 18 Notes Reaction Rates and Equilibrium

Honors Chemistry

Chapter 18 Notes –Reaction Rates and Equilibrium

(Student’s edition)

Chapter 18 problem set: 49-52, 55, 56, 70, 75, 76, 78, 79, 81, 83, 86

18.1 Rates of Reaction

Kinetics: the study of how and how

Reaction rate: the with which a reaction takes place

rate =  [R]

Time

[R] represents the

Reaction mechanism: the in a reaction

Thermodynamics: the study of in chemical reactions

Collision Theory: In order for molecules to react, they must , but doesn’t

guarantee reaction.

For a reaction to take place,

1. the collisions need enough

2. the particles need proper

Reaction Mechanism: The series of in a reaction.

We have learned the following:

H2+I2 2 HI

H = + 26.5 kJ

However, we now know that it is really:

Step 1I22 I

Step 2I+H2H2I

Step 3H2I+ I2 HI

H2+I22 HI

Reaction Intermediate: a species that appears in but not in the

reaction. It is relatively . So, in the above

example, is the reaction intermediate.

What affects reaction rate?

- nature of reactants

- temperature

- concentration of reactants

- pressure (gases only)

- a catalyst

- surface area (when reactants are in multiple phases)

The Nature of Reactants

Reaction rates are determined by the type of .

Reactants that have ionic bonds react ______than reactants that have covalent bonds.

Temperature and Reaction Rate

For every 10 C0 increase, the rate of reaction approximately (souring milk).

This is easily explained by . Since particles move faster, there are more and the molecules have more .

Concentration of Reactants

An Increase in [R] increases reaction rate if it is a homogeneous reaction (all reactants are in the same phase).

Ex.2 H2(g)+O2(g)2 H2O(g)

NaCl(aq)+AgNO3(aq)NaNO3(aq)+AgCl(ppt)

Heterogeneous reaction: when the reactants are in more than one .

Ex.4Fe(s)+3O2(g)2Fe2O3(s)

Again…. [R]leads to rate. This is explained by .

Ex. A+BC

If we double [A], the rate .

If we double [B], the rate .

If we double the [ ] of both reactants, the rate .

Pressure and Reaction Rate

Pressure only affects .

For gases, if pressure increases, the amount of reactants in an area . So, the reaction rate .

Catalysts and Reaction Rate

Catalyst: A substance that a reaction, but is not in the reaction.

Inhibitor: A substance that the rate of reaction.

Catalysts work by the mechanism of a reaction and the

activation energy (Ea).

Activation Energy and the Activated Complex:

______

Not enough ______to make new products

Activated complex: a structure existing when old bonds are broken and new

bonds are being formed.

Energy of activation: energy needed to transform .

Reactants must have sufficient .

Energy Diagrams:

h 40

a 30 reactants

products

time

Energy of the reactants = Energy of the products =

Energy of the activated complex = Activation energy =

Change in the heat = The reaction is .

h 40

a 30products

reactants

time

Energy of the reactants = Energy of the products =

Energy of the activated complex = Activation energy =

Change in the heat = The reaction is .

Activation Energy – Temperature and Concentration:

An increase in temperature equals an increase in .

So, the number of particles that can reach the appropriate Ea .

kinetic energy

Activation Energy and Catalysts:

Catalysts lower Ea by changing the .

Lower activation energy means more can reach the more easily, thereby forming more quickly.

18.2 Reversible Reactions

_Fe3O4(s)+ H2(g) Fe(s)+ H2O(g)

This reaction is also possible in .

If water is removed from the vessel, the reaction shifts to the , but if in a closed

container…

_Fe3O4(s)+ H2(g) Fe(s)+ H2O(g)

The double arrows represent .

Equilibrium: when the rate of the reaction equals rate of the reaction.

r r r

a a a

t t t

e e e

time time time

Forward reaction:Reverse reaction:Equilibrium:

Reactants combiningProducts combiningReactants forming and

to form to form products forming

Mass Action Expression: It is used to describe a system undergoing a .

xA + yB  zC + aD

[C]z[D]a  products

the mass action expression  [A]x[B]y  reactants

so for…

2A + B  3C + 2D

the mass action expression 

Equilibrium Constant: It is essentially the same thing as the .

For…H2 + I2  2 HI

Keq=

Ex1:At the beginning of a reaction, the Hydrogen concentration is 1.00 M

(1 mole/Liter) and so is the Iodine concentration. At equilibrium, [H2] = 0.228 M,

[I2] = 0.228 M, and [HI] = 1.544 M. Calculate Keq.

Ex2:At the beginning of a reaction, the HI concentration is 1.00 M. At equilibrium,

[H2] = 0.114 M, [I2] = 0.114 M, and [HI] = 0.772 M. Calculate Keq.

Note: If K1, are favored.

If K1, are favored.

Applications of Keq

Ex1:If the concentration of HI at equilibrium is 0.158 M, what are the concentrations

of Hydrogen and Iodine? (Keq = 45.9)

H2 + I2  2 HI

Ex2:If the concentration of HI at equilibrium is 1.423 M, what are the concentrations

of Hydrogen and Iodine if twice as much Hydrogen was added as Iodine?

H2 + I2  2 HI

Ex3:For the reaction….N2O42 NO2

If 1.00 mol of Dinitrogen tetroxide is in a 5.00 dm3 container at 100 C0, it

decomposes into Nitrogen dioxide. At equilibrium, 1.00 mole of the product is

present in the container. Calculate Keq.

Le Chatelier’s Principle:

We’ve learned before, Keq is a – however, conditions in a reaction can .

Le Chatelier’s Principle: When a system at equilibrium is , the system shifts to that stress.

Types of change:

 [ ]

for the reaction:

A+BC+D

If we increase the amount of A or B, the system shifts to the

If we increase the amount of C or D, the system shifts to the

If we decrease the amount of C or D, the system shifts to the

(a good way to increase !)

Shift away from an and towards a .

Please note: Keq

2. T

The shift depends on whether the reaction is or .

A+B+heatC

For an endothermic reaction, if we increase the amount of heat, the system

shifts to the Treat heat like a reactant. It is the

for an exothermic reaction.

For exothermic reactions, removing heat helps increase .

Shift away from an and towards a .

Please note: Keq

P

Pressure only affects .

An increase in pressure shifts the reaction towards the .

For the reaction:

A(g)+3B(g)2C(g)

An increase in pressure will shift the reaction to the

A decrease in pressure will shift the reaction to the

Please note: Keq

Catalysts

It has effect on equilibrium.

It’s just that the reaction proceeds to equilibrium faster.

18.3Solubility Equilibrium

Equilibrium is also reached when solids dissolve in .

AgCl(s)+H2O(l)

We don’t count solids or liquid water in K expressions, so…

Ksp =

Ksp = solubility product constant (only for products)

K = soluble

For…Ag2SO4(s)+H2O(l)

Ksp =

Ex1: If Ksp for CdS = 1.0 x 10 –28, what is the concentration of each ion?

Ex2: If the Ksp for Ag2SO4 = 1.10 x 10-12, calculate the [ ] of each ion.

Ex3: If the solubility of BaSO4 is 9.09 g in 100.0 cm3 of water, find Ksp.

The Common Ion Effect

When a slightly soluble solid is dissolved in solution, and another salt with a

is added, equilibrium shifts , causing .

Ex:AgCl (s)Ag+1(aq) + Cl-1(aq)

Ksp = 1.7 x 10 –11 (very )

If we add NaCl….

NaCl(s)

(adding a )

Since we add Cl-1, we are increasing the , and the reaction shifts

and AgCl .

18.4 Entropy and Free Energy

In regard to enthalpy, is more favorable (- H).

For example;C8H18+O2CO2+H2O+ heat

However, this is not the only driving force.

Entropy (S): the measure of in a system. The higher disorder (more +S), the

likely the reaction is to occur (messy room, leaves on trees).

Systems tend to go towards energy (-H) and randomness (+S).

General trends in entropy:

slg=+ S

gg + g + g=+ S

 temperature=+ S

Spontaneous reactions take place outside influence (they can be fast or slow).

Ex1:2 C8H18(l) +25 O2(g) 16 CO2(g) + 18H2O(g) + heat

pieces and heat

Entropy (S) is and enthalpy (H) is .

Thus, the reaction is .

Ex2:CO2(g) + N2(g)+ H2O(g)+ heat  C3H5(ONO2)3(l)

pieces and heat

Entropy (S) is and enthalpy (H) is .

Thus, the reaction is .

Ex3: H2O (s)H2O (l)

does it happen?

Entropy (S) is and enthalpy (H) is .

Thus, we determine the spontaneity based on the information given. Gibbs

Free energy formula is used to determine the spontaneity.

Free energy of a system: G = Gibbs Free Energy (combined enthalpy/entropy function)

G = H - (TS)(use absolute temperature)

if G is negative, the reaction is

Possible combinations of entropy and enthalpy:

Spontaneous / Non-Spontaneous / Maybe / Maybe
Enthalpy
Entropy

What determines a maybe? ______

Is this reaction spontaneous?

H2O+ CCO+H2

H = + 131.3 kJ/mole

S =+ 0.134 kJ/mole.K at 25 Co

G =

G = The reaction is ______.

How about at 900 Co?

G =

G = The reaction is ______.

18.5 The Progress of Chemical Reactions – Rate Laws

For:A + B  C

rate = k [ A ]x [ B ]y(determined experimentally)

If a reaction is one step, then the coefficients ______the exponents.

If a reaction is multi step, then the coefficients ______the exponents.

Ex.2 H2 +2 NON2 + 2 H2O

If the reaction is one step, the reaction rate formula is …

R = k [ H2 ]2 [ NO ]2

However, experiments show:

2 times [ H2 ]= rate increases 2 times

3 times [ H2 ]= rate increases 3 times

So, rate is proportional to

2 times [ NO ] = rate increases 4 times

3 times [ NO ] = rate increases 9 times

So, rate is proportional to

So, the rate is proportional to

Thus, the reaction rate formula is …R =

Ex.X+2YXY2 (a single step reaction)

Write the rate law:

If you double X, the rate .

If you double Y, the rate .

If Y is reduced to 1/3, the rate is .

If X is cut in half and Y is doubled, the rate .

Reaction Mechanisms and the Rate of Reaction:

Different steps take place at .

The slowest step is the step.

Increasing the [ R ] in the step increases the rate of reaction.

Ex:A+BInt1fast step

A+Int1Int2slow step

C+Int2Dfast step

So, increasing or doesn’t speed up the reaction, but increasing does.