Regents Chemistry

KINETICS

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-ACTIVATION ENERGY!!!!!

1- Correct orientation2- Sufficient activation energy!!!

1- Wrong orientation or not enough activation energy.

2- ACTIVATION ENERGY: the minimum amount of energy needed to form the activaion complex and start a reaction. : Kinetic energy is converted to potential energy

Why 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!

1- The greater concentration of reactants, the more collisions

2- Increase concentration = increase collisions = increase rate of reaction!!! Chemical reaction is faster!!!

TEMPERATURE

1- Increase in temperature = increase in rate of reaction

2- WHY?

ANS: increase in kinetic energy and motion of particles- more collisions of particles

Surface Area

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

Ex: one centimeter of a cube of Mg reacting with HCl reacts more slowly then increasing the surface area by breaking Mg down into smaller pieces

INCREASING SURFACE AREA

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 an alternative pathway

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

Regents Chemistry

Potential Energy Diagrams

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

chemical potential energy (when bonds are formed)

CHEMICAL ENERGY: energy associated with the breaking and forming of chemical bonds

This is a form of Potential energy!

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..

-ENERGY IS ON REACTANT SIDE!

-ENERGY RELEASED: ON PRODUCT SIDE!!!!

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

change that occurs during a chemical reaction

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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

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