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Lecture 9:Chemical Kinetics

Course lecturer :

Jasmin Šutković

22 th April 2014

Contents International University of Sarajevo

1. Factors that affect the chemical kinetics 2. Reaction rates and rate laws3. The Collision Model of Chemical Kinetics4. Reaction rate order and type of order reactions 5. Activation energy6. Catalysts and Catalysis

1. Factors That Affect Reaction Rates

Chemical kinetics

– Study of reaction rates, or the changes in the concentrations of reactants and products with time

– By studying kinetics, insights are gained into how to control reaction conditions to achieve a desired outcome

Chemical kinetics of a reaction depend on various factors

1. Reactant concentrations

2. Temperature

3. Physical states and surface areas of reactants

4. Solvent and catalyst properties

Concentration Effects

Two substances cannot react with each other unless their constituent particles come into contact; if there is no contact, the rate of reaction will be zero.

The more reactant particles that collide per unit time, the more often a reaction between them can occur.

The rate of reaction usually increases as the concentration of the reactants increases.

Temperature Effects

Increasing the temperature –increase of kinetic energy of its constituent particles.

the average kinetic energy increases- the particles move faster, so they collide more frequently per unit time and possess greater energy when they collide, causing increases in the rate of the reaction.

Rate of all reactions increases with increasing temperature and decreases with decreasing temperature.

Homogenous VS Heterogeneous solutions

Homogeneous Mixtures

are composed of two or more components that are equally (uniformly) distributed throughout the system, examples: Salt dissolved in water, Soapy water,  Hydrochloric acid & water,  Alcohol & water, Sugar dissolved in water, 

Heterogeneous Mixtures

are composed of two or more components that are unequally (not uniformly) distributed though out the system, examples: Sand & water (liquid & solid),  Oil & water (immiscible liquids),  Table salt crystals & sugar crystals (solids & solids), 

REMINDER !!

Phase and Surface Area Effects

If reactants are uniformly dispersed in a single homogeneous solution, the number of collisions per unit time depends on concentration and temperature.

If the reaction is heterogeneous, the reactants are in two different phases, and collisions between the reactants canoccur only at interfaces between phases;therefore, the number of collisions betweenthe reactants per unit time is reduced, as is the reaction rate.

The rate of a heterogeneous reaction depends on the surface area of the more condensed phase.

Solvent Effects on rate of reaction

The nature of the solvent can affect the reaction rates of solute particles.

Solvent viscosity is also important in determining reaction rates.

1. In highly viscous solvents, dissolved particles diffuse much more slowly than in less viscous solvents and collide less frequently per unit time.

2. Rates of most reactions decrease rapidly with increasing solvent viscosity.

Viscosity ….REMINDER!!!

The viscosity of a fluid is a measure of its resistance on gradual deformation by shear stress or tensile stress. 

Catalyst Effects

Catalyst is a substance that participates in a chemical reaction and increases the rate of the reaction without undergoing a net chemical change itself.

2. Reaction Rates

The rate of a reaction is the speed at which a reaction happens.

If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate.

Some reactions take hundreds, maybe even thousands, of years while others can happen in less than one second. 

Reaction Rates cont.…

There is another big idea for rates of reaction called collision theory.

The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other.

So..more collisions results in more combinations!!

If there are a higher number of collisions the reactions will go faster and the rate of that reaction will be higher.

Concentration

If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction

Sometimes when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker.

Temperature

Raising the temperature of a system, the molecules bounce around a lot more (more faster ) because they have more energy.

If they are faster than its is more likely that they collide. When you lower the temperature, the molecules are slower and collide less.

That temperature drop lowers the rate of the reaction.

PRESSURE

Pressure affects the rate of reaction, especially when you look at gases.

When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions.

When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction. 

Interaction of all factors

Activation Energy

A minimum energy (activation energy, Ea) is required for a collision between molecules to result in a chemical reaction.

Reacting molecules must have enough energy to overcome electrostatic repulsion and a minimum amount of energy to break chemical bonds so that new ones may be formed.

Catalysis – Catalysts

Substances that increase the rate of a chemical reaction without being consumed in the process

Catalyzed pathway has a lower Ea, but the net change in energy that results from the reaction (the difference between the energy of the reactants and the energy of the products) is not affected by the presence of a catalyst

Catalysis

A catalyst decreases the height of the energy barrier, and its presence increases the rates of both the forward and the reverse reactions by the same amount

There are three major classes of catalysts

1. Heterogeneous catalysts 2. Homogeneous catalysts 3. Enzymes

Enzymes

• Enzymes are catalysts that occur naturally in living organisms and are almost all protein molecules

• Enzymes can increase reaction rates by enormous factors and tend to be very specific, typically producing only a single product in quantitative yield.

Enzymes

• Enzymes are expensive, and often cease functioning at temperatures higher than 37ºC, and have limited stability in solution.

• Enzyme inhibitors cause a decrease in the rate of an enzyme-catalyzed reaction by binding to a specific portion of an enzyme and thus slowing or preventing a reaction from occurring.

Readings …

Pages : 921-994 – but follow this ppt slides - Book chapter 14 – skip those parts that are not mentioned in this lecture (subtitles and titles)