I. Kinetics

Reaction rateVelocity (speed) at which a reaction occurs.

Reaction MechanismSteps involved in producing the overall reaction

-study of

- Amt / time

CH3CH2OH(l) + 3 O2(g) 2 CO2(g) + 3 H2O(g) + 1236 kJC12H22O11(s) + 8 KClO3(s) 12 CO2(g) + 11 H2O (g) + 8 KCl (s)

C12H22O11(s) 12 C (s) + 11 H2O(g) + 429 kJ

Collision Theory and Rate of Reaction

Collision Theory

- Particles must collide in order for a reaction to occur and they must have “effective” collisions

1. Sufficient Energy – hard enough2. Proper orientation ( direct hits )

A-B and C-D bonds breakA-D and B -C bonds form

The collision must also have the correct orientation

AB + CD --> AD + CB

A

B

D

CThe bond between A and D cannot form.There is no reaction between molecules

Effective collision

Ineffective collision = no reaction!Ineffective collision = no reaction!

A

B

D

C

1. Nature of Reactants- Covalent substances react slower than ionic substances

- Fastest reactions happen with aqueous solutions

In AB + CD--> AD + CB, first AB and CD mustbreak apart

A BBBBB AB--> A+ + B- Now they can react with CD

In ionic solutions, ionic substances will dissociate, bonds are already broken

A BBBBB C DDBDD

They have already completed the first step of the reaction before they are mixed

Therefore, ionic substances in solution react Therefore, ionic substances in solution react faster than in solid formfaster than in solid form

2. Concentration of Reactants[NaCl]“Concentration of NaCl”

Usually molarity (M)

[Cl-]“Concentration of chlorine ions”

Higher concentration means more molecules are closer together

A

AA

AA

AA

A

B

B BB

B B

B

B

B

As molecules move, more collisions will occur if they are closer together

A

A

AB

B B

More collisions = faster reaction

As concentrations increase, reaction rate increasesAs concentrations increase, reaction rate increases

Pressure

Only gases show a noticeable change in reaction rate as pressure is increased

As pressure increases, we force the molecules closer together

AA

A

B

B

BA

A

AB

B B

Closer together means more collisions and a faster reaction rate

As pressure on As pressure on gasesgases increases, reaction rate increases increases, reaction rate increases

3. TemperatureAverage kinetic energy

As we increase temperature, we speed up the molecules

Cold Hot

Faster molecules means more collisions per second = Faster reaction

As temperature increases, reaction rate increasesAs temperature increases, reaction rate increases

Usually a 10oC temperature change will double the reaction rate

4. Surface Area

More surface area = more places for collisions to occur

In this case, the liquid can only react at these places

When we break the object, now we have more places for the liquid molecules to collide

MoreMore surface area = Faster reaction rate surface area = Faster reaction rate

Powder reacts faster than cube

Dragons Breath

Describe what happens when the pile of lycopodium powder is lit with the lighter.

Describe what happens when the lycopodium powder is ignited after being blown over a flame.

Grain Silo Explosion

Describe what happens when the lycopodium powder is ignited inside the coffee can.

Why do you think what you observed happened?

A

B

5. CatalystSubstance which lowers Ea (activation energy), but is not changed

Catalyzed reactions occur faster

Catalysts help molecules collide properly properly

Allows for more effective collisions,

Not more collisions, better collisions

Different catalysts for different reactions

A

B

D

C

D

C

D

C

D

C

D

C

D

C

D

C

D

C

D

C

A

B

D

C

D

C

D

C

D

C

D

CB CB C

B C

Catalyst = Faster reaction rateCatalyst = Faster reaction rate

Catalytic acetone combustion

O2(g) + C3H6O(g) H2O(g) + CO2(g) + heat

The above reaction can be described as a combustion reaction as it gives off water and carbon dioxide as products. What is another way to describe the above reaction.

Balance the above reaction. Hint start with the acetone.

In the reaction above, the copper from the penny is not shown. Why do you suppose it is not written in the equation?

PE(kcals)

Reaction coordinate

PE of reactants

PE of products

H

PE of products

Ea

PE of activated complex

Reaction Coordinate- Represents progress of reaction

Activation Energy

-energy needed to make the activated complex- Needed to start reaction

PE(kcals)

Reaction coordinate

PE of reactants

PE of products

H

PE of products

Ea

PE of activated complex

Heat of Reaction (ΔH)

- Energy released or absorbed during a chemical reaction- Measured in joules

Activated Complex -Occurs when bonds are in process of breaking and

forming

- Occurs when reactants collide in proper orientatation

- Very unstable, only lasts moments

∆H = Hproducts - Hreactants

H = Negative H = Negative Products have less energy than reactants

Energy is given off

- Product is more stable than the reactant- Enthalpy decreases

Chlorination of acetylene pg 47

There are several reactions that take place in this demonstration. First….

CaC2(s) + H2O(l) C2H2(g) + Ca(OH)2(aq)Balance the above and identify the compound that has both ionic and covalent bonds in it.

NaClO(aq) + HCl(aq) Cl2(g) + NaOH (aq)Balance the above and name NaClO _________________

Second….

Third…

C2H2(g) + Cl2(g) HCl (aq) + C (s) + heat

Would ∆H be positive or negative for this equation?

Draw the potential energy diagram for the above reaction. Put the reactants and products on the correct line and label the activation energy.

Fourth…

C2H2(g) + O2(g) 4 CO2 + 2H2O(g) + heat

Draw the electron dot diagram for oxygen, carbon dioxide and water.

H = PositiveH = PositiveProducts have more energy than reactants

Energy is gained

- Product is less stable than the reactant

- Enthalpy increases

Example- What is the H of the following

a. N2 + 2 O2 + 66.4 kJ --> 2 NO2

b. H2(g) + O2(g) 2 H2O (g) + 486.3 kJ

H = +66.4 kJ

H = -486.3 kJ

Table I Heats of Reactions

Shows the H for various reactions - H is based on the MOLES of reactants and products used.

Example A. How much energy is given off when carbon and oxygen gas make one mole of carbon dioxide?

B. When 4.0 moles of HI is made from pure hydrogen and iodine, is energy released or absorbed? How much?

393.5 kJ

Since H is +53.0 kJ, energy is absorbed.

The written reaction is for 2 moles of HI, so for 4 moles, we absorb 106.0 kJ

H2 + I2 2 HI

Heating with sulfuric acid pg 155

When diluting an acid with water the little ditty “Do as you oughta, add acid to water” is often utilized. After watching the temperature change as the acid is added to the water why do you think that ditty is important to remember?

Is the ∆H for this reaction positive or negative?

Draw a simple energy diagram showing the reaction that happened.

PE(kcals)

Reaction coordinate

PE of reactants

PE of products

H

PE of products

Ea

Ea with a catalyst

Catalyst -Compound with lowers Ea

Is not permanently changed in the reaction

Can be reusedDoes not change the H

3. Reversible ReactionSome reactions can run back to products

A + B --> AB A + B <-- AB

PE(kcals)

Reaction coordinate

A + B

AB

FWD

Eaf

RVS

Ear

Reactions are oppositeOne is exothermic, one is endothermic

H is the same, but has opposite signs

H

Eaf and Ear differ by H

Equilibrium

- Forward and reverse reactions happen at the same time1.) Takes place in a closed system

2.) Is dynamic - Overall appearance does not change

3.) Does not mean same amounts of products and reactants

- Means that amts of products and reactants remain constant

Types of Equilibrium

-One compound in equilibrium in two phases

Ex. H2O(l) <= => H2O(g)

Water molecules

Vapor molecules

Again, the forward and reverse reaction still occur, but the total number of vapor and liquid molecules remains unchanged

Phase Equilibrium

Sealed jar

- Amount turning to liquid = amt turning to solid- Amount turning to liquid = amt turning to gas

-Equilibrium is reached when number of particles being dissolved = number of particles being crystalized

Solid sugar

Dissolve sugar

Ex. C6H12O6(s)<==> C6H12O6(aq)

- Forward and Reverse reaction are equal

But since the forward and reverse reaction are equal, the total number of solid and dissolved molecules remains the same

- Solutions in this condition are considered saturated

Solution EquilibriumSolids in Liquids

Electrolytes- With electrolytes in the same situation, the electrolyte dissociates as it goes into solution

NaCl(s) ---> Na+(aq) + Cl-

(aq)

NaCl NaCl NaCl

Na+Cl-

Cl-

Cl-

Na+ Na+

NaCl(s)<---- Na+(aq) + Cl-

(aq)

Cl-Na+

-And reassociates as it fall out of solution

NaCl

NaCl(s) <==> Na+(aq) + Cl-

(aq)

Nonelectrolytes (They don’t dissociate in water!)

Ex. C6H12O6(s)<==> C6H12O6(aq)

Gases in Liquids- Equilibrium is reached when number of particles being dissolved = number becoming vapor- Only occurs in a closed system ( bottle of soda)

CO2 (g) = CO2 (aq)

- Soluabilty of gases are influenced by 1.) temperature

2.) pressure

Equilibrium only occurs when the rate of the forward reaction equals the rate of the reverse reaction

At equilibrium the amount of reactants do not equal the amount of products

At equilibrium, the concentration of reactants and products do not change

The reverse reaction only begins when enough product has been created

Chemical Equilibrium- A dynamic equilibrium is reached when both the forward and reverse reactions are equal

N2 + 3 H2 2 NH3

Three indications that a reaction is at equilibrium:

No change in:

1.) color

2.) temperature

3.) pressure

LeChatelier’s Principle

If a stressstress is applied to a system at equilibrium, the equilibrium is shiftedshifted in a way that tends to reduce the effects of the stress.

A + B C + D

If we apply a stress, the equilibrium will shift to the left to create more products, or to the right to create more reactants

What you kids give me!Change in temperature, pressure, concentration of chemicals and other factors

Types of Stress**** A catalyst is NOT a stress and has

no effect on equilibrium

A. Concentration

- By adding / removing product or reactant

Reaction shifts to

1.) Replace the loss

2.) Consume the addition - Increase [reactants], equilibrium is pushed to

products

A + B C + D

-Increase the [products] , equilibrium is pushed to reactants

A + B C + D

A C + D A + B

CA + B C + D

If a reactant or product is removed, the reaction will shift to replace the missing substance

In an open container, gas D floats away. Equilibrium is constantly pushed to products

A(s) + B(s) C(s) + D(g)

D(g)

D(g)

D(g)

D(g)

A(s) + B(s) C(s) + D(g)

D(g)

D(g)

D(g)

D(g)

A(aq) + B(aq ) C(s) + D(g)

D(g)

D(g)

D(g)

D(g)

Example - For the equation 3 H2 + N2 2 NH3, List three ways to increase the concentration of the products and three ways to increase the concentration of the reactants.

More products

Add more N2

Add more H2

Remove some NH3

More reactants

Remove some N2

Remove some H2

Add more NH3

b. Temperature

Consider heat as either a product or a reactant

3 H2 + N2 2 NH3 + 91.8 kJ

In this case, heat is a product. If we increase the temperature, it would be the same as adding a product

If heat is on the reactant side then adding temperature is adding a reactant. The equilibrium will shift to the products

A + B + 43 kJ D + C

H2 H2

H2H2 N2 N2

NH3NH3

NH3

3 H2 + N2 2 NH3 + 91.8 kJ

H2 N2

H2 H2

For endothermic reactions, as temperature increases

[products] increases

For exothermic reactions, as temperature increases

[reactants] increases

a. The container was heated?

Equilibrium shifts to reactants

b. The container was cooled?

Equilibrium shifts to products

Example – In the reaction XY + B XB + Y + 2 kJ , what will happen when

• Balance the above equation.

• Find the ionic charge of the reactant side and product side.

• Predict what you will observe when:– Water is added to the cobalt(II)chloride solution.

• Use arrows to show shift and changes of concentration.

LeChatliers principle demo pg 236

• Predict what you will observe when:– HCl is added to the cobalt(II)water complex

solution.• Use arrows to show shift and changes of concentration.

• Using table F what will Silver Chloride look like in solution.

• Using table F what will Silver Nitrate look like in solution.

• What happened when AgNO3 was added?

– Explain why the observed results happened.• Use arrows to show shift and changes of concentration.

• Predict what you will observe when:– Heat is added to the cobalt(II)water complex

solution.• Use arrows to show shift and changes of

concentration.

• Predict what you will observe when:– Ice is applied to the cobalt(II)chloride solution.

• Use arrows to show shift and changes of concentration.

C. Pressure - Only effects reactions with gases

- Increasing pressure is like increasing concentration of gas

- Shifts toward side without gas

CO2(g) CO2(aq)

What if gases are found on BOTH sides of the reaction?

N2(g) + 3 H2(g) = 2 NH3(g)

- Equilibrium will shift to side with least number of moles of gas ( takes up less space)- If same number of moles on each side then

no effect on equilibrium

In carbonated soft drinks the cap on the soda keeps a high pressure on solution so more gas is dissolved. When soda is opened pressure is released and according to LC Principle carbon dioxide gas leaves solution to relieve stress.

A. Enthalpy

All matter has energy stored in it. Either as a chemical bond, a phase, or kinetic energy

Enthalpy Total energy of a substance

Very difficult to measure, but we can determine the change in enthalpy

Called HH aka - the heat of reactionaka - the heat of reaction

Nature favors lower energy - - HH

-Measurement of disorder

-Measurement of randomness

-Represented by S

S = Change in randomness

+ S = Becoming more random, or more disordered

- S = Becoming less random, or less disordered

Solid Liquid Gasorganized

Low entropy

Less organizedHigher entropy

Very disordered

Highest entropy

Nature likes disorder

Example - When CO2(s) CO2(g), is entropy increasing or decreasing?

Increasing - molecules becoming more random

Which has an increase in entropy (+S)?

b. C + O2 --> CO2

c. NaCl --> Na+ + Cl-

d. condensation

All reactions in nature tend to run to

Lowest energy and highest disorder

Decrease enthalpy and increase entropy

- H and + S

a. H2O(l) --> H2O(s)