Chemical Kinetics

What is Kinetics? The study of the rate at which a chemical process occurs. In chemical equations, we see the starting reactants and the ending products, but what about the middle? Important because  kinetics is involved in how quickly medicine reacts in the body.  Industrial applications on how to make a reaction more efficient, and quicker.

Factors That Affect Reaction Rates Physical State and Surface Area  If reactants are in gaseous forms, there is a greater ability to react because of a greater kinetic energy. Medicine in the form of a powder enters the bloodstream more quickly than a solid tablet. Concentration of Reactants  As the concentration of reactants increases, so does the likelihood that molecules will collide and cause an increase reaction speed.

Factors That Affect Reaction Rates Temperature  At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. Catalysts  Speeds up a reaction by changing the mechanism. It decreasing the activation energy required for the reaction to take place.

Reaction Rates The speed at which the reaction occurs is solved for by dividing the change in concentrations over the time the reaction takes place. Reaction Ex: A  B rate = -  [A] tt rate = [B][B] tt

Reaction Rates In this reaction, the concentration of butyl chloride, C 4 H 9 Cl, was measured at various times during the experiment. It makes sense that the concentration [C 4 H 9 Cl] is decreasing. The reactant is being used up. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) [C 4 H 9 Cl] M

Average Rates The average rate of the reaction is determined by subtracting the change in concentration divided by the change in time: Average Rate, M/s ( – ) (50.0 – 0) x M/s Negative value b/c it is decreasing in amount.

Reaction Rates Note that the concentration and the average rate are both decreasing as the reaction proceeds. This is because as the reaction goes forward, there are fewer collisions between reactant molecules, thus a slower rate. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Reaction Rates A plot of concentration vs. time for this reaction yields a curve like this. The slope of a line tangent to the curve at any point is the instantaneous rate at that time. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Reaction Rates The reaction slows down with time because the concentration of the reactants decreases. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) average rate = -  [Br 2 ] tt = - [Br 2 ] final – [Br 2 ] initial t final - t initial slope of tangent slope of tangent slope of tangent instantaneous rate = rate for specific instance in time 13.1

Practice Problem In an experiment, the concentration of substance D was measured at different times. Determine the average rate of the reaction between 3.0 and 7.0 seconds. Time (S)[D] M Rate = ( ) (7.0 – 3.0) Rate = M/s Is “D” a reactant or a product?

Reaction Rates and Stoichiometry In this reaction, the ratio of C 4 H 9 Cl to C 4 H 9 OH is 1:1. For every mole of C 4 H 9 Cl lost, there was a mole of C 4 H 9 OH created. Thus, the rate of disappearance of C 4 H 9 Cl is the same as the rate of appearance of C 4 H 9 OH. They are directly related. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) Rate = -  [C 4 H 9 Cl]  t =  [C 4 H 9 OH]  t

Reaction Rates and Stoichiometry What if the ratio is not 1:1? For every 1 mole of H 2 that react, 2 moles of HI are produced. In this reaction, the rate that Hydroiodic acid is produced is twice the rate Hydrogen gas is being depleted. In order to get one correct reaction rate for all parts of the equation, we must account for reaction stoichiometry. H 2 (g) + I 2 (g)  2 HI (g)

Reaction Rates and Stoichiometry So that the reaction rates for all chemical compounds will be the same, the rate for Hydroiodic acid needs to be divided by 2. (Remember it produced twice as much, so dividing it by two will make the reaction rate the same for all compounds in the reaction) H 2 (g) + I 2 (g)  2 HI (g)

Reaction Rates and Stoichiometry So how would we write the rates for the reaction of nitrogen and hydrogen, in the production of ammonia (NH 3 )? Rate = -  [N 2 ] = -1  [H 2 ] = 1  [NH 3 ]  t 3  t 2  t N 2 (g) + 3 H 2 (g)  2 NH 3 (g) Reactants are negative, Products are positive.

Generalized Rate Expression Use this generalized equation to write rates for a reaction aA + bBcC + dD Reactants decrease (Negative rates) Products increase (Positive rates)

Practice Problem: Write the rate expression for all molecules in the following equation: 2 N 2 O  2 N 2 + O 2 Rate = -1  [N 2 O] = 1  [N 2 ] =  [O 2 ] 2  t 2  t  t

Practice problem with calculations The disappearance of dinitrogen monoxide occurs at a rate of x Determine the rate of nitrogen and oxygen gas appearance. - 1 [-3.25 x 10 6 ] = 1  [N 2 ] N 2 = 3.25 x  t -1 [-3.25 x 10 6 ] =  [O 2 ] O 2 = 1.63 x  t Rate = -1  [N 2 O] = 1  [N 2 ] =  [O 2 ] 2  t 2  t  t

Rate Expression and Calculations Practice