Chapter 15: Kinetics The speed with which the reactants disappear and the products form is called the rate of the reaction A study of the rate of reaction can give detailed information about how reactants change into products The series of individual steps that add up to the overall observed reaction is called the reaction mechanism

The progress of the reaction A  B The progress of the reaction A  B. The number of A molecules (in red) decreases with time while the number of B molecules (in blue) increases. The steeper the concentration versus time curve, the faster the reaction rate. The film strip represents the relative number of A and B molecules at each time.

There are five principle factors that influence reaction rates: Chemical nature of the reactants Ability of the reactants to come in contact with each other Concentration of the reactants Temperature Availability of of rate-accelerating agents called catalysts

Chemical nature of the reactants Bonds break and form during reactions The most fundamental difference in reaction rates lie in the reactants themselves Some reactions are fast by nature and others slow Ability of the reactants to meet Most reactions require that particles (atoms, molecules, or ions) collide before the reaction can occur This depends on the phase of the reactants

In a homogeneous reaction the reactants are in the same phase For example both reactants in the gas (vapor) phase In a heterogeneous reaction the reactants are in different phases For example one reactant in the liquid and the second in the solid phase In heterogeneous reactions the reactants meet only at the intersection between the phases Thus the area of contact between the phases determines the rate of the reaction

Effect of crushing a solid Effect of crushing a solid. When a single solid is subdivided into much smaller pieces, the total surface area on all of the pieces becomes very large.

Concentration of the reactants Both homogeneous and heterogeneous reaction rates are affected by reactant concentration For example, red hot steel wool bursts into flames in the presence of pure oxygen Temperature of the system The rates for almost all chemical reactions increase as the temperature is increased Cold-blooded creatures, such as insects and reptiles, become sluggish at lower temperatures as their metabolism slows down

A rate is always expressed as a ratio Presence of a catalyst A catalysts is a substance that increases the rate of a chemical reaction without being consumed Enzymes are biological catalysts that direct our body chemistry A rate is always expressed as a ratio One way to describe a reaction rate is to select one component of the reaction and describe the change in concentration per unit of time

Molarity (mol/L) is normally the concentration unit and the second (s) is the most often used unit of time Typically, the reaction rate has the units

A reaction rate is generally not constant throughout the reaction Since most reactions depend on the concentration of reactants, the rate changes as they are used up The rate at any particular moment is called the instantaneous rate It can be calculated from a concentration versus time plot

A plot of the concentration of HI versus time for the reaction: 2HI(g)  H2(g) + I2(g). The slope is negative because we are measuring the disappearance of HI. When used to express the rate it is used as a positive number.

The rate of a homogeneous reaction at any instant is proportional to the product of the molar concentrations of the reactants raised to a power determined from experiment

Consider the following reaction: From experiment, the rate law (determined from initial rates) is At 0oC, k equals 5.0 x 105 L5 mol-5 s-1 Thus, at 0oC

The exponents in the rate law are generally unrelated to the chemical equation’s coefficients Never simply assume the exponents and coefficients are the same The exponents must be determined from the results of experiments The exponent in a rate law is called the order of reaction with respect to the corresponding reactant

Exponents in a rate law can be fractional, negative, and even zero For the rate law: We can say The reaction is first order with respect to H2SeO3 The reaction is third order with respect to I- The reaction is second order with respect to H+ The reaction order is sixth order overall Exponents in a rate law can be fractional, negative, and even zero

Looking for patterns in experimental data provide way to determine the exponents in a rate law One of the easiest ways to reveal patterns in data is to form ratios of results using different sets of conditions This technique is generally applicable Again consider the hypothetical reaction

Suppose the experimental concentration-rate data for five experiments is:

For experiments 1, 2, and 3 [B] is held constant, so any change in rate must be due to changes in [A] The rate law says that at constant [B] the rate is proportional to [A]m Thus m=1

This means that the reactions is first order with respect to reactant A For experiments 3, 4, and 5 [A] is held constant, so any change must be due to changes in [B] The rate law says that at constant [A] the rate is proportional to [B]n Using the results from experiment 3 and 4:

The reaction is second order in B and rate=k[A][B]2 Thus n=2

The rate constant (k) can be determined using data from any experiment Using experiment 1: Using data from a different experiment might give a slightly different value