Describing Reaction Rates Kinetics How fast does the reaction go? Reaction Rate : the change in the amount of reactant or product per unit time.

rustingbakingexplosion 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

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 complex b. reaction rate c. reaction energy d. activation energy

The study of a reaction’s progress over a period of time is called A) thermochemistry B) kinetics C) stoichiometry D) catalyst

What happens if atoms collide without enough energy? A) they stick together B) they bounce off each other C) they form products D) 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 theory B) mechanism C) collision theory D) accident theory

The minimum energy colliding particles must have in order to react is called the… a. activated complex b. reaction rate c. reaction energy d. activation energy

The study of a reaction’s progress over a period of time is called A) thermochemistry B) kinetics C) stoichiometry D) catalyst

What happens if atoms collide without enough energy? A) they stick together B) they bounce off each other C) they form products D) 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 theory B) mechanism C) collision theory D) accident theory

Part 2: Rate Laws It 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. H 2 SO 4 Sulfuric acid uses fertilizers gelatin Plasticsperfumes Dyescar batteries Paperglue Explosivesrayon Detergent paints Steel printing ink Disinfectants chemotherapy drugs

Rate laws express the rate of a reaction in terms 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 relates the concentrations of reactants to the rate of the reaction. k is large if products form quickly k is small if products form slowly

Determining Rate Laws What is the rate law for the reaction : 2 H NO  N H 2 O. What is the value of the rate constant? [H 2 ] o [N 2 ]rate (mM/minute) H 2 concentration doubles: rate doubles N 2 concentration doubles: rate quadruples

Order of Reaction If doubling the concentration (other reactant constant) doubles the rate, the reaction is first order with respect to that reactant. [A] 1 If doubling the concentration (other reactant constant) quadruples the rate, the reaction is second order with respect to that reactant. [B ] 2 If doubling the concentration has no effect on the rate, the reaction is zero order with respect to that reactant [C] 0 (not included in rate law)

Determining Rate Laws 2 H NO  N H 2 O. Rate Law: Rate = k[H 2 ] 1 [NO ] 2 What is the value of the rate constant? Substitute data. (0.1) mol/minute = k (0.1 mol/L) 1 (0.1 mol/L) 2 k= 100 L 3 mol 2 min H 2 concentration doubles: rate doubles NO concentration doubles: rate quadruples [H 2 ] 1 [NO ] 2

Part 3: Reaction Mechanisms The chance of more than two particles colliding simultaneously with correct geometry and minimum energy required is very small. If there are more than 2 reactants, the reaction must occur by a number of simpler steps

Reaction Mechanisms A reaction mechanism is a series of simple steps that ultimately lead from the initial reactants to the final products of a reaction. The mechanism must account for the experimentally determined rate law. The mechanism must be consistent with the stoichiometry of the overall or net reaction.

Reaction Mechanisms Elementary Steps - The individual steps in a mechanism, they each describe only one event in the mechanism. Intermediate products: Substances that are produced in one step but consumed in a later step. All steps in a reaction mechanism do not have the same rate. The step with the slowest rate limits the overall reaction, called the rate-determining, or rate-limiting step.

Reaction Mechanisms The reaction of NO 2(g) + CO (g)  NO (g) + CO 2(g) occurs by a multistep mechanism. Its rate law is Rate = k[NO 2 ] 2 Why doesn’t its rate depend on the concentration of CO, and why [NO 2 ] squared?

Reaction Mechanisms Step 1 NO 2(g) + NO 2(g)  NO 3(g) + NO (g) slow Step 2 NO 3(g) + CO (g)  NO 2(g) + CO 2(g) fast Overall NO 2(g) + CO (g)  NO (g) + CO 2(g) Step 1 is slow: rate determining step Does not include CO, so CO not in rate law Does have 2 molecules of NO 2, so [NO 2 ] 2 What is NO 3 in Step 1 and 2? Intermediate

Reaction Mechanisms Step 1 NO 2(g) + NO 2(g)  NO 3(g) + NO (g) slow The exponent is 2 because there are two molecules of NO 2 of in the determining step The total number of molecules or atoms whose concentration determine the rate of reaction is known as order of reaction. This is a second order reaction.

Part 4: What affects reaction rate? 1.Nature of the reactants 2.Temperature 3.Concentration or pressure of a reactant 4.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 temperature of 10 °C causes the rate of reaction to approximately 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 lake Boone Your house They change the pathway of the reaction

More About How Catalysts Work? CATALYST – probably a metal like platinum REACTANTS PRODUCTS 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 is involved 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? Lake Boone Your house

At normal body temperature (37  C), 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 reactant s

Which will have a faster reaction rate? A. One reactant is a solid; the other dissolved. B. Both reactants are dissolved