Kinetics - Reaction Rates

Reaction Kinetics Reaction Rate is a measure of amount of reactants used (or amount of products made) per unit of time. Units for reaction rate can be any measure of amount (g, mol), per unit of time (s, min, hr). ex: g/s For a reaction to occur, 3 things must occur. This is called COLLISION THEORY Molecules must be in contact (collide) There must be enough energy to separate the original bonds They must have the correct orientation collision theory

Reaction Kinetics What affects reaction rate? Temperature Higher temp means that more molecules have the activation energy and are therefore more likely to react when they collide.

Reaction Kinetics What affects reaction rate? Concentration Higher concentration of reactants causes more collisions

(1) Gases, Liquids, Solutions (High P & Conc.) Which will react faster? Reaction Kinetics Surface Area More surface area leads to more collisions Gases react faster than solids. Powdered solids react faster than chunks. (1) Gases, Liquids, Solutions (High P & Conc.) (2) Solids

Reaction Kinetics Catalysts Lower the activation energy needed for reaction, therefore the reaction happens faster. Elephants Toothpaste

Reaction Kinetics • Calculated by change in reactants (or products) over change in time.

Reaction Kinetics • Calculated by change in reactants (or products) over change in time. Units for reactants are usually gram or moles Units for time will depend on the reaction. Seconds for fast reactions up to years for very slow reactions.

Reaction Kinetics • Calculated by change in reactants (or products) over change in time. Units for reactants are usually gram or moles Units for time will depend on the reaction. Seconds for fast reactions up to years for very slow reactions.

Reaction Kinetics the Balanced Equation is a mole ratio of the reaction. This can be used to determine the rate, in moles, of other substances in a reaction. The rate of N2 is 1.2 mol/min. What us the rate of H2 and NH3? N2 + 3H2  2NH3

Reaction Kinetics • In many reactions, the rate of reaction changes as the reaction progresses. • Initially the rate of reaction is relatively large, while at very long times the rate of reaction decreases to zero (at which point the reaction is complete). • In order to characterize the kinetic behavior of a reaction, it is desirable to determine how the rate of reaction varies as the reaction progresses.

Rate Changes and Rate Laws • Since most reactions are dependent on concentration, chemists use rate laws to describe the relationship between rate and concentration • Although many rate orders exist, most chemical reactions obey one of three differential rate laws; 0, 1st , or 2nd order.

Zero Order Rate • For a zero-order reaction, the rate of reaction is a constant. It does NOT change with concentration. The Rate Law: r = k[A]0 or essentially r = k

First Order Reactions • The rate is directly proportional to the concentration of the reactant. For example, if the concentration is cut in half, the rate is cut in half. The Rate Law: r = k [A]1

Second Order Reactions The rate of reaction is directly proportional to the square of the concentration of the reactant. For example, if the concentration triples, then the rate is multiple by 32 The Rate Law: r = k [A]2

How to determine the reaction order for the reaction? This can only be done by analyzing experimental data, by comparing ratio of concentration to reaction rate.