Regents Chemistry

Kinetics and Equilibrium

What is Kinetics?

Kinetics is the branch of chemistry that deals with rates of chemical reactions

Different factors affect how quickly chemical reactions occur

In order for a reaction to occur, reactant particles must collide This is called COLLISION THEORY

Reactions occur between collisions of particles that are orientated correctly and have sufficient amounts of energy!

What is Kinetics Important?

Kinetics allows chemists to predict how fast a reaction will occur

Important in the synthesis of all kinds of compounds

In manufacturing, it is essential to making products, timing rates between chemical

reactions to get a desired product

What Factors Affect Rate of Reaction?

The rate of a chemical reaction depends on a number of factors that affect the number of effective collisions between particles Nature of Reactants Concentration Surface Area Pressure Temperature Presence of a Catalyst

Nature of Reactants

Reactions involve the breaking of existing bonds and the formation of new bonds

Generally, covalently bonded substances are slower to react than ionic substances as they have move bonds to be broken Breaking more bonds requires that the

particles must have more energy when they collide

Concentration

Most chemical reactions will proceed at a faster rate if the concentration of one or more of the reactants is increased

Ex: Combustion of Paper Normal air is 20% oxygen, if we use pure

oxygen it burns much faster! Why? KMT says that more collisions

between oxygen and paper particles = faster rate of reaction!

Surface Area

When more surface area of a substance is exposed, there are more chances for reactant particles to collide

Ex: Lycopodium solid vs. powder phases

Pressure

Pressure has little or no effect on rates of reactions between liquids and solids…

It does have an effect on gases!An increase in pressure has the

effect of increasing the concentration of gaseous particles Therefore, it increases the rate of a

reaction that involves only gases

Presence of a Catalyst

Catalysts are substances that increase the rate of a reaction by providing a different and easier pathway for a chemical reaction

Catalysts take part in a reaction, but they are unchanged when the reaction is complete So they are present in the reactants and

products and only assist in the rate of the reaction

Temperature

By definition, temperature implies that the greater the temperature, the faster the molecules will move (higher kinetic energy)

When particles are moving faster, more collisions occur and increasing the likelihood of a reaction Higher Temp = particles with more kinetic

energy = more effective collisions

worksheetVIDEO

Regents Chemistry

Potential Energy Diagrams

What’s Potential Energy?Chemical bonds are large sources of

chemical potential energyPotential energy has ability to do stuff!Similar to gravitational potential energy

Gravitational PE can be increased by raising an object higher from the Earth’s surfaceAnd

Reduced by lowering closer to the ground

Chemical Bonds Have PE!

Chemical bonds have large supplies of potential energy

Bonds are broken in chemical reactions and new bonds form in products

Energy can be released or gained in these chemical reactions, so…

Potential energy can be increased or decreased depending on the reaction..

What’s a PE Diagram?..First lookA PE diagram illustrates the potential energy

change that occurs during a chemical reaction

---

Pot

enti

al e

nerg

y

Reaction Coordinate

Products

Reactants

Activated ComplexReaction Coordinate = progress of reaction

Potential Energy Diagrams

In order for a reaction to occur, the reactants must have sufficient energy to collide effectively

As reactant particles approach each other, kinetic energy is converted into potential energy

The molecules must also have proper orientation to come together…this leads to..

Activated Complex

When molecules collide with the proper orientation, an intermediate product is formed

Activated Complex – is a temporary, intermediate product that may either break apart and reform the reactants or rearrange the atoms and form new products

Example of Collisions

HI molecules collide in a reaction and form H2 and I2 products

A + B C + D + Heat

Heat is a product because the PE is lower for the products than in the reactants

DifferenceIn PE

Reactants and Products on a PE Diagram

H =Heat of reaction

Activationenergy

Activation Energy and Heat of Reaction

The amount of energy needed to form the activated complex from the reactants is called the activation energy

The diagram is typically read from left to right and vice versa So..we specify by saying forward or reverse

HEAT OF REACTION – is the heat required to form products or reactants in this specific reaction Can be negative or positive depending on viewing the

reaction as forward or reverse

Regents Chemistry

Agenda Finish Potential Energy Diagrams

LectureYOU NEED HANDOUT FROM

WEDNESDAY

Worksheet HW: Finish remaining problems

The Effect of a Catalyst

Catalysts are added to a reaction to lower the activation energy, which in turn speeds up the reaction

The catalyst speed up the reaction by providing a new pathway

In turn, the activation energy of the reverse reaction is also lowered…but

The H remains unaffected..see why

Effect of a Catalyst

What does 5 represent?

Two Outcomes of PE Diagrams1. PE of products is lower than the

reactants, so energy is lost and released to the environment – Exothermic! (H = -)

PE of the products is greater than the reactants, so energy is absorbed to make the products – Endothermic (H = +)

Two Outcomes of PE Diagrams

Practice Problem

Regents Chemistry

Physical and Chemical Equilibrium

What’s Equilibrium?

The potential energy diagrams typically show a forward reaction – Left to Right Reactants Activated Complex Products

But the reverse can also happen… Products Activated Complex Reactants

And they can both happen at the same time!

When they occur at the same rate, the system is said to be in equilibrium

Describing Equilibrium

A double arrow is used in the reaction in place of a single arrow

Equilibrium is a state of balance between the rates of two opposite processes that are taking place at the same rate

Exists only in a closed system!

vs.

Equilibrium…

Is important because many chemical reactions and physical processes are reversible

We will look at two kinds: Physical and Chemical Equilibriums

Is an equilibrium between rates, not amounts of reactants and products

For example…

Equilibrium in a Closed Container

Open Container

H2O(l) H2O(g)

Closed Container

H2O(l) H2O(g)

Evaporation continuesin the closed containerbut is balanced bycondensing vapor.This is an equal processat equilibrium!

End

Physical Equilibrium (PE)

The evaporation – condensation of water in a closed system is an example of PE

Two examples of PE are Phase Equilibrium Solution Equilibrium

Phase Equilibrium…

Can exist between the solid and liquid phases of a substance.

This is called the melting point of a solid phase or the freezing point of the liquid phase in a closed container

Example: Water at 0 Celsius (closed container) Some of the ice is melting and some of the water

is freezing H2O(s) H2O(l)

Remember, rates are the same..not amounts!

Solution Equilibrium

You’ve seen this before..saturated solutions!

Solids in liquids exist in equilibrium in a saturated solution

When we add more solute to a saturated solution, the solute may dissolve but some will also recrystallize out of solutionKCl(s) KCl(aq)

Solution Equilibrium cont…

Equilibrium may also be attained in a closed system between a gas dissolved in a liquid and the undissolved gas

For example In a closed Pepsi can, there is an equilibrium between the gaseous and dissolved state of CO2

CO2(g) CO2(aq)

Temperature and Physical Equilibrium

Equilibrium is affected by temperature If the temperature is raised, a solid generally

becomes more soluble in a liquidFor a short time the rate of dissolving

exceeds the rate of crystallizationHowever, as more solid is placed into

solution, the rate of recrystallization increases until a new equilibrium is reached

Temperature and Physical Equilibrium….

Opposite is true for gases in liquidsAs the temperature increases, the rate of

the gas escaping from the liquid increases while the rate at which the gas particles dissolves decreases

This decreases the solubility of the gas in the liquid

As the temperature rises, the solubility of all gases decreases in a liquid decreases

Chemical Equilibrium

When reactants are first mixed and no products are present, only the forward reaction can occur

For example: water vapor and methaneCH4(g) + H2O(g) 3H2(g) + CO(g)

• As time progresses, the concentrations of the reactants decreases, causing the forward reaction to slow

• While the concentrations of the products increases, causing the rate of the reverse reaction to increases

This process continues until equilibrium is reached This can be represented in a graph summary – p. 115

Remember..the system must be closed!No product or reactant can leave the

system If a precipitate or a gas is formed in a system

that is not closed, equilibrium will not be reached The effect of any change on equilibrium is explained

by Le Chatelier’s principle

CH4(g) + H2O(g) 3H2(g) + CO(g)

End

Regents Chemistry

•Le Chatelier’s Principle

Le Chatelier’s Principle

Any change in temperature, concentration or pressure on an equilibrium system is called a stress

Le Chatelier’s principle explains how a system at equilibrium responds to relieve any stress on the system

We will look at these separately…

Concentration Changes

CH4(g) + H2O(g) 3H2(g) + CO(g)

If the stress is the addition of more methane (CH4), the rate of the forward reaction will increase and more products will form

As more product forms, the reverse reaction will also increase until reactions are equal againOverall, if the concentration of one substance is increased, the reaction that reduces the amount of the addedsubstance is favored

CH4(g) + H2O(g) 3H2(g) + CO(g)

Concentration Changes cont..

If the concentration of methane is reduced, the rate of the forward reaction decreases

When the concentration of a substance decreases, the reaction that produces that substance is favored

Initially the reverse reaction will take place faster than the forward reaction, and the system is said to be shifting to the left, or toward the reactant side

Concentration Changes - Ex #2

Stress: More NH3

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat

Stress: Less NH3

+

+

+++-

- - - -

Temperature Changes

Increasing or decreasing the temperature changes the amount of heat going into / leaving the system

We can consider heat as a reactant or product, Increasing the amount of heat drives the

reaction in the opposite direction of the location of the heat

Decreasing the heat drives the reaction towards the location of the heat

Temperature - Example

Stress: More Heat

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat+ + +- -

Stress: Less Heat

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat- - -+ +

Pressure Changes

Pressure does not affect the rate of reactions of solids and liquids…however

Gases are affected!We will look at how pressure affects

CO2 gas in aqueous solution (contains 1 gaseous molecule) Nitrogen and hydrogen gases in the production of ammonia gas (contains

more than 1 gaseous molecule)

CO2(g) CO2(aq)

Pressure increases System shifts to the right forming more

CO2(aq)

Pressure decreases System shifts to the left forming more CO2(g)

We see this when we open a can of pop Pressure decreases, so CO2(g) increases and

it comes out of solutionAn increase in pressure favors the side of the reaction that contains the gas

1 gas molecule

Systems with More than 1 Gaseous Substance

An increase in pressure will increase the increase the concentration of gaseous molecules on both reactant and product sides of the reaction, but the effects will be unequal

An increase in pressure will favor the reaction toward the side with fewer gas molecules

Reactants have 4 gas moleculesProducts have 2 gas moleculesAn increase in pressure will favor the

reaction towards the productsA decrease in pressure will have the

opposite effectSo a decrease favors more N2 and H2 and

less NH3

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

H2(g) + Cl2(g) 2HCl(g)

Both sides have the same # of gas molecules

In this case, pressure changes have no effect on the system

Also, don’t forget catalysts!

Equal gasmolecules

End

Regents Chemistry

Entropy and Enthalpy

Entropy and Enthalpy

Many factors cause and chemical and physical changes to occur.. For example: Pressure, temperature,

concentration changesWe see how these changes occur,

but why exactly do they occur?