Describing Reaction RatesKinetics

How fast does the reaction go?

Reaction Rate :the change in the amount of reactant or product per unit time.

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slow fast very fast

• The word rate can be used as a synonym of speed• Rates measure the speed of any change that

occurs within a time interval • The interval of time may range from fractions of a

second to centuries

Describing Reaction Rates•The figure below illustrates the progress of a typical reaction. Over time, the amount of reactant decreases and the amount of product increases.

Graphing rates of reaction

Rates of chem rxns are related to the properties of atoms, ions, and molecules through a model called

Rates of chem rxns are related to the properties of atoms, ions, and molecules through a model called

Part 1: Collision Theory• Reactions don’t happen

unless the substances are in contact.

Collision Theory and Reaction Rates

According to collision theory, atoms, ions, and molecules can react to form products when they collide if the particles have enough kinetic energy.

Particles that do not have enough energy to react bounce apart unchanged when they collide.

Collision Theory

Describing Reaction Rates

The minimum energy that colliding particles must have in order to react is called the activation energy. An activated complex is an unstable arrangement of atoms that forms for a moment at the peak of the activation-energy barrier. Very brief existence; sometimes called the transition state.

The activation-energy barrier must be crossed before reactants are converted to products.

Collision Theory

Enough energy: effective collision – product formed

Not enough energy: ineffective collision – no product formed

Particle orientation is also important

The minimum energy colliding particles must have in order to react is called the… a. activated complexb. reaction ratec. reaction energyd. activation energy

The study of a reaction’s progress over a period of time is called

A) thermochemistryB) kineticsC) stoichiometryD) catalyst

What happens if atoms collide without enough energy?

A) they stick togetherB) they bounce off each otherC) they form productsD) they react

Which of the following is the theory that says that atoms, molecules and ions must bump into each other for a chemical reaction to occur?A) reaction theoryB) mechanismC) collision theoryD) accident theory

The minimum energy colliding particles must have in order to react is called the… a. activated complexb. reaction ratec. reaction energyd. activation energy

The study of a reaction’s progress over a period of time is called

A) thermochemistryB) kineticsC) stoichiometryD) catalyst

What happens if atoms collide without enough energy?

A) they stick togetherB) they bounce off each otherC) they form productsD) they react

Which of the following is the theory that says that atoms, molecules and ions must bump into each other for a chemical reaction to occur?A) reaction theoryB) mechanismC) collision theoryD) accident theory

Part 2: Rate LawsIt is not enough to say that a reaction is fast or

slow. Industries must know HOW fast or slow and WHY the reaction is fast or slow.

H2SO4 Sulfuric acid usesfertilizers gelatinPlastics perfumesDyes car batteries Paper glueExplosives rayonDetergent paints Steel printing ink Disinfectantschemotherapy drugs

Rate laws express the rate of a reaction interms of the concentration [ ] of the reactants.For example Rate = k[A]x[B]y

where [A] and [B] are concentration in molarity and x and y are the powers of the concentrations of the reactants. 

The specific rate constant (k) for a reaction relatesthe concentrations of reactants to the rate of thereaction.

k is large if products form quickly

k is small if products form slowly

Part 4: What affects reaction rate?

1. Nature of the reactants 2. Temperature 3. Concentration or pressure of a reactant4. Surface area of a solid reactant 5. Presence/absence of a catalyst.

The Nature of the Reactants:Individual properties of substances also affect

reaction rates. Some of the properties in this category are state of matter, molecular size, bond type and bond strength.

In general, gases react more rapidly than liquids than solids.

Temperature and particle collisions

More collisions More molecules with enough kinetic energy to slip

over the activation-energy barrier.↑ temperature causes products to form faster.

Temperature: Particles turn heat energy into kinetic energy.When they get hotter they move faster

Generally, ↑temperature ↑reaction rate. ↓temperature ↓reaction rate.

The effect of temperature on rate

The higher the temperature, the faster the rate of a reaction.

In many reactions,a rise in temperatureof 10 °C causes therate of reaction toapproximately double.

Temperature and Reaction Rate

Concentration and particle collisions

↑collision frequency ↑ reaction rate.

Concentration

More particles means more collisions.

Surface area and particle collisions

Grind up Chemicals – Smaller Particle Size

Large particles

Small particles

The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

• They don’t speed up the actual molecules but the reaction happens a lot faster. How?

Catalysts

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They change the pathway of the reaction

More About How Catalysts Work?

CATALYST – probably a metal like platinum

REACTANTSPRODUCTS

The bonds in the reactants are weakened because the metal atoms attract electrons away from the "bonded" atoms. The bonds of the reactant break with much less energy input. 

Catalysts lower the activation energy

Is a catalyst a reactant, since it isinvolved in the reaction? No, because it is not used up. Just like the bridge it will be there after it is used.

How Do Catalysts Work?

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At normal body temperature (37C), reactions in the body would be too slow without catalysts.

• The catalysts that increase the rates of biological reactions are called enzymes.

Biological catalysts - enzymes

An inhibitor is a substance that interferes with the action of a catalyst.

Why is it cost effective to use expensive metals such as platinum and palladium in chemical reactions?

It is a one time investment since they are not used up in the reaction.

Which of the following factors could be increased in order to decrease a reaction rate?

A.Catalyst concentration

B.Concentration

C.Temperature

D.Particle size

A catalyst works by ____.a.lowering the activation energy barrier

b. shifting the equilibrium position toward the products

c. changing the temperature of the reactants

d. changing the particle size of the reactants

Which will have a faster reaction rate?

A. One reactant is a solid; the other dissolved.

B. Both reactants are dissolved

Which of the following factors could be increased in order to decrease a reaction rate?

A.Catalyst concentration

B.Concentration

C.Temperature

D.Particle size

A catalyst works by ____.a.lowering the activation energy barrier

b. shifting the equilibrium position toward the products

c. changing the temperature of the reactants

d. changing the particle size of the reactants

Which will have a faster reaction rate?

A. One reactant is a solid; the other dissolved.

B. Both reactants are dissolved