KINETICS

Kinetics – What makes “superglue” bond instantly while Prit- stick does not? – What factors determine how quickly food spoils? – Why do “glow sticks” last longer when stored in the freezer? – How do catalytic converters remove various pollutants from car exhaust?

What are “kinetics”? The study of the speed or rate at which chemical reactions occur Ch 1.1 A2 What are chemical kinetics and what affects them? Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).

What are “kinetics”? Ch 1.1 A2 What are chemical kinetics and what affects them? How fast a chemical reaction goes is affected by many factors. List four of these.

Chemical reaction rates Ch 1.1 A2 How do factors affect reaction rate? Temperature: Food spoils more quickly at room temperature than in a refrigerator Temperature: Food spoils more quickly at room temperature than in a refrigerator simulation At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy.

Factors affecting reaction rates Ch 1.1 A2 How do factors affect reaction rates? Concentration: As the concentration of reactants increases, so does the likelihood that reactant molecules will collide [the collision frequency increases], the rate of reaction generally increase. Concentration: As the concentration of reactants increases, so does the likelihood that reactant molecules will collide [the collision frequency increases], the rate of reaction generally increase. There are fewer red particles inc the same volume so there is less chance of a collision There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Factors affecting reaction rates Catalyst a substance that increases the rate of a reaction without being consumed in the reaction Enzymes – biological catalysts – proteins that increase the rate of biochemical reactions Catalyst a substance that increases the rate of a reaction without being consumed in the reaction Enzymes – biological catalysts – proteins that increase the rate of biochemical reactions Ch 1.1 A2 How do factors affect reaction rates? Surface area as surface area increases the rate of reaction generally increases Surface area as surface area increases the rate of reaction generally increases

Factors affecting reaction rates Ch 1.1 A2 How do factors affect reaction rates? Surface area As surface area increases the rate of reaction generally increases as collision frequency increases Surface area As surface area increases the rate of reaction generally increases as collision frequency increases The particles on the surface can react When cut into smaller pieces the particles on the inside can react

Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? A → B [A] decreases with time as the [B] increases. A → B [A] decreases with time as the [B] increases.

Chemical reaction rates Ch 1.1 A2 What do reaction rate graphs look like? Predict the general shape of the graph you would expect for A → B

Chemical reaction rates Ch 1.1 A2 What do reaction rate graphs look like? Predict the shape of the graph for the reaction A + 2B →C Describe what is happening in this graph in terms of reaction rate and concentrations

Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? The steeper the curve the faster the rate of reaction Why is the initial slope of the graph the steepest?

Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? Why does the slope of the reaction decrease with time? The rate of reaction slows down over time because there are fewer reactant particles left i.e. the concentration of the reactant decreases.

Chemical reaction rates Ch 1.1 A2 What do reaction rate graphs look like? Why does [B] fall faster than [A]?

Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? Summary The reaction rate for a chemical reaction can be expressed as either: the increase in concentration (or number of moles) of a product as a function of time. the decrease in concentration (or number of moles) of a reactant as a function of time

Finding the rate Ch 1.1 A2 How do you find reaction rates? The speed of an object or event is the change that occurs in a given time interval. Speed of a car = change in distance time interval =  d  t Remember, the term change always refers to final value minus initial value.

Finding the rate Ch 1.1 A2 How do you find reaction rates? Similarly, the rate (or speed) of a reaction can be determined: Rate = change in concentration (or moles) of product time interval Rate =  (conc. or moles)  t

Finding the rate Ch 1.1 A2 How do you find reaction rates?

Reaction Rates Consider the chemical reaction: A B Time = mol A t = 20. min 5.0 mol A 5.0 mol B t = 40. min 2.0 mol A 8.0 mol B Finding the rate Ch 1.1 A2 How do you find reaction rates?

Finding the rate Ch 1.1 A2 How do you find reaction rates? If the number of moles of A and B are measured and plotted, a graph such as this one can be obtained This data can be used to find the reaction rate.

Finding the rate Ch 1.1 A2 How do you find reaction rates? In this reaction: Average rate of appearance of B = change in n. moles of B change in time =  (mol B)  t In this reaction: Average rate of appearance of B = change in n. moles of B change in time =  (mol B)  t We can calculate the average rate for any time interval involved in the reaction.

Finding the rate Ch 1.1 A2 How do you find reaction rates? the rate of appearance of B over the first 20 minutes of reaction: Average rate of appearance of B =  (mol B)  t = 5.0 mol B – 0.0 mol B 20. min – 0. min = 0.25 mol/min the rate of appearance of B over the first 20 minutes of reaction: Average rate of appearance of B =  (mol B)  t = 5.0 mol B – 0.0 mol B 20. min – 0. min = 0.25 mol/min Time = 0 10 mol A t = 20 min 5.0 mol A 5.0 mol B

Finding the rate Ch 1.1 A2 How do you find reaction rates? The average rate of appearance of B during the second 20 minutes of the reaction: Avg. rate = 8.0 mol B – 5.0 mol B 40. min – 20. min =0.15 mol/min The average rate of appearance of B during the second 20 minutes of the reaction: Avg. rate = 8.0 mol B – 5.0 mol B 40. min – 20. min =0.15 mol/min t = 20 min 5.0 mol A 5.0 mol B t = 40. min 2.0 mol A 8.0 mol B

The rate of a reaction can also be expressed as the disappearance of A as a function of time. For this particular reaction, when 1 mole of B is formed, 1 mole of A must disappear. A → B Hence  B/  t = -  A/  t The rate of a reaction can also be expressed as the disappearance of A as a function of time. For this particular reaction, when 1 mole of B is formed, 1 mole of A must disappear. A → B Hence  B/  t = -  A/  t Finding the rate Ch 1.1 A2 How do you find reaction rates?

For reactions with 1:1 stoichiometry: Avg. rate =  (moles product)  t =-  (moles reactant)  t For reactions with 1:1 stoichiometry: Avg. rate =  (moles product)  t =-  (moles reactant)  t Finding the rate Ch 1.1 A2 How do you find reaction rates?

For most reactions, the reaction rate is expressed as a change in concentration of a particular reactant or product Average Rate =  [Product] = -  [Reactant]  t With concentration in mol dm -3 For most reactions, the reaction rate is expressed as a change in concentration of a particular reactant or product Average Rate =  [Product] = -  [Reactant]  t With concentration in mol dm -3 Finding the rate Ch 1.1 A2 How do you find reaction rates?

Finding the rate Ch 1.1 A2 How do you find reaction rates? In this reaction, the concentration of butyl chloride, C 4 H 9 Cl, was measured at various times, t. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Finding the rate Ch 1.1 A2 How do you find reaction rates? The average rate of the reaction over each interval is the change in concentration divided by the change in time: C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Finding the rate Ch 1.1 A2 How do you find reaction rates?

Finding the rate Ch 1.1 A2 How do you find reaction rates? The average rate decreases as the reaction proceeds. Why? As the reaction goes forward, there are fewer collisions between reactant molecules because [C 4 H 9 Cl] decreases.

Example Ch 1.1 A2 How do you find reaction rates? : Given the following data, what is the average rate of the following reaction over the time interval from 54.0 min to min? CH 3 OH (aq) + HCl (aq) → CH 3 Cl (aq) + H 2 O (l) Time (min)[HCl] (M)

Finding the rate Ch 1.1 A2 How do you find reaction rates? Given: [HCl] 54 min = 1.58 M [HCl] 215 min = 1.02 M Find: avg. rate of disappearance of HCl Given: [HCl] 54 min = 1.58 M [HCl] 215 min = 1.02 M Find: avg. rate of disappearance of HCl Avg. rate = -  [HCl]  t = - (1.02 M M) 215 min - 54 min = M / min Avg. rate = -  [HCl]  t = - (1.02 M M) 215 min - 54 min = M / min

Finding the rate Ch 1.1 A2 How do you find 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. 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)

Finding the rate Ch 1.1 A2 How do you find reaction rates? Rate laws must be determined experimentally. Measure the instantaneous reaction rate at the start of the reaction (i.e. at t = 0) for various concentrations of reactants. You CANNOT determine the rate law by looking at the coefficients in the balanced chemical equation!

Finding the rate Ch 1.1 A2 How do you find reaction rates?

Finding the rate Ch 1.1 A2 How do you find reaction rates? For a generalized chemical reaction: w A + x B y C + z D the general form of the rate law is: Rate = k[A] m [B] n where k = rate constant m, n = reaction order

First Order Reactions Ch 1.1 A2 How do you find reaction rates? Expt[A] (M)Rate (M/s) x2 As [A] doubles, the rate doubles [A]  rate As [A] doubles, the rate doubles [A]  rate First Order Reaction – Overall reaction order = 1 – Rate = k[A] First Order Reaction – Overall reaction order = 1 – Rate = k[A]

Second Order Reactions Ch 1.1 A2 How do you find reaction rates? Expt Initial [A] (M) Initial [B] (M ) Rate (mol dm -3 s -1 ) x x x x1 x2 [A] stays the same [B] doubles [A] stays the same [B] doubles x2 x1 x4 the rate doubles [B]  rate [A] doubles [B] stays the same [A] doubles [B] stays the same the rate is x4 [A] 2  rate

Second Order Reactions Ch 1.1 A2 How do you find reaction rates? [A] doubles [B] stays the same [A] doubles [B] stays the same [A] stays the same [B] doubles [A] stays the same [B] doubles the rate doubles [B]  rate the rate is x4 [A] 2  rate What is the rate equation for this reaction? Rate = k[A] 2 [B] The reaction is second order in respect of A and first order in respect of B. The overall reaction order is 3.

Initial [X]/MInitial [Y]/MInitial [Z] / MInitial rate/ mol dm -3 s x x x x Second Order Reactions Ch 1.1 A2 How do you find reaction rates? x1 x3 [Z] triples [X] &[Y] stay the same [Z] triples [X] &[Y] stay the same X0.5 x1 the rate trebles [Z]  rate [X] halves [Y] & [Z] stay the same [X] halves [Y] & [Z] stay the same the rate is the same [X] 0  rate NE x1 [Y] quadruples [X] & [Z] stay the same [Y] quadruples [X] & [Z] stay the same the rate goes up by 16 (ie 4 2 ) [Y] 2  rate x1 x4 x1

Second Order Reactions Ch 1.1 A2 How do you find reaction rates? What is the rate equation for this reaction? Rate = k[Y] 2 [Z] The reaction is second order in respect of Y and first order in respect of Z. The overall reaction order is 3. [X] halves [Y] & [Z] stay the same [X] halves [Y] & [Z] stay the same [Z] triples [X] &[Y] stay the same [Z] triples [X] &[Y] stay the same the rate trebles [Z]  rate the rate is the same [X] 0  rate [Y] quadruples [X] & [Z] stay the same [Y] quadruples [X] & [Z] stay the same the rate goes up by 16 (ie 4 2 ) [Y] 2  rate

Summary Ch 1.1 A2 How do you find reaction rates? Rate Constant (k) a proportionality constant that relates the concentration of reactants to the reaction rate Reaction Order the power to which the concentration of a reactant is raised in a rate law Overall reaction order The sum of all individual reaction orders