Chapter 1 Rate of Reaction

Reaction Rates What makes “superglue” bond instantly while Elmer’s glue 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”? Ch 1.1 A2 The study of the speed or rate at which chemical reactions occur Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs). What are chemical kinetics and what affects them?

What are “kinetics”? Temperature Pressure Concentration Particle size Ch 1.1 A2 Temperature Pressure Concentration Particle size Catalyst How fast a chemical reaction goes is affected by many factors. List four of these. What are chemical kinetics and what affects them? Discuss in your groups how each factor affects the reaction rate

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

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

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

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

Ways to measure the rate of reaction Ch 1.1 A2 Gas Volume – you can measure the rate of reaction by measuring how much gas is released What do reaction rate graphs look like? Colour change – use a colorimeter to measure the change in a colour change reaction. Electrical conductivity – as the number of ions change during the reaction, the conductivity changes.

Practical Design a practical to show that concentration of HCl affects the rate of reaction with calcium carbonate. HCl (aq) + CaCO3(s)  CaCl2(aq) + H20(l) + CO2(g)

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

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

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

Chemical reaction rates The steeper the curve the faster the rate of reaction Why is the initial slope of the graph the steepest? What does “rate of reaction mean? The rate of reaction is highest here because there is more likelihood of collision

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

Chemical reaction rates Why does [B] fall faster than [A]? What do reaction rate graphs look like? For each molecule or atom of A, two B particles of B are used up.

Chemical reaction rates 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 What does “rate of reaction mean?

Finding the rate Ch 1.1 A2 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 = Dd Dt How do you find reaction rates? Remember, the term change always refers to final value minus initial value.

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

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

Finding the rate Reaction Rates Consider the chemical reaction: A B t = 20. min 5.0 mol A 5.0 mol B t = 40. min 2.0 mol A 8.0 mol B How do you find reaction rates? Time = 0. 10. mol A

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

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

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

Finding the rate Ch 1.1 A2 C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times, t. How do you find reaction rates?

Finding the rate Ch 1.1 A2 The average rate of the reaction over each interval is the change in concentration divided by the change in time: C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) How do you find reaction rates?

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

Finding the rate Ch 1.1 What do you notice about the average rate? How do you find reaction rates? The average rate decreases as the reaction proceeds. As the reaction goes forward, there are fewer collisions between reactant molecules. Why?

Example Ch 1.1 A2 : Given the following data, what is the average rate of the following reaction over the time interval from 54.0 min to 215.0 min? CH3OH (aq) + HCl (aq) → CH3Cl (aq) + H2O (l) How do you find reaction rates? Time (min) [HCl] (M) 0.0 1.85 54.0 1.58 107.0 1.36 215.0 1.02

Finding the rate Ch 1.1 Given: [HCl]54 min = 1.58 M Find: avg. rate of disappearance of HCl How do you find reaction rates? Avg. rate = - D [HCl] D t = - (1.02 M - 1.58 M) 215 min - 54 min = 0.0035M / min

Finding the rate Ch 1.1 A2 C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) 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. How do you find reaction rates?

Finding the rate Ch 1.1 A2 The rate of reaction (and reaction order) can only be worked out experimentally. You can calculate the rate (gradient) at different points, and produce a rate-concentration graph. How do you find reaction rates?

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

Rate Order The order of a reaction will tell you how the change in concentration affects the rate of reaction. If you double the concentration of a reactant, and the rate stays the same, the reaction is zero order with respect to the reactant If you double the concentration of a reactant, and the rate doubles, the reaction is first order with respect to the reactant If you double the concentration of a reactant, and the rate quadruples, the reaction is second order with respect to the reactant

What this tells us… A + B  C The order of reaction for a reactant tells us how it will affect the rate of reaction. A + B  C By completing rate experiments, you may find that doubling the concentration of A, the rate remains constant… BUT doubling the concentration of B, the rate also doubles. We can say it is zero order with respect to [A] and first order with respect to [B].

Finding the rate 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 How do you find reaction rates?

First Order Reactions First Order Reaction Ch 1.1 Expt [A] (M) Rate (M/s) 1 0.50 1.00 2 1.00 2.00 3 2.00 4.00 As [A] doubles, the rate doubles [A]  rate x2 How do you find reaction rates? x2 First Order Reaction Overall reaction order = 1 Rate = k[A]

Second Order Reactions Ch 1.1 A2 Expt Initial [A] (M) Initial [B] (M ) Rate (mol dm-3 s-1) 1 0.1 0.2 1.6 x 10-2 2 0.1 0.4 3.2 x 10-2 3 0.2 0.2 6.4 x 10-2 x2 x1 x2 x4 x1 How do you find reaction rates? [A] stays the same [B] doubles the rate doubles [B]  rate [A] doubles [B] stays the same the rate is x4 [A]2  rate

Second Order Reactions Ch 1.1 A2 [A] doubles [B] stays the same [A] stays the same [B] doubles the rate doubles [B]  rate the rate is x4 [A]2  rate How do you find reaction rates? 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.

Second Order Reactions Ch 1.1 A2 Initial [X]/M Initial [Y]/M Initial [Z] / M Initial rate/ mol dm-3 s-1 0.10 2.40 x 10-3 0.30 7.20 x 10-3 0.05 0.40 3.84 x 10-2 X0.5 x1 x3 x4 x1 x1 x1 x1 NE x1 How do you find reaction rates? [Z] triples [X] &[Y] stay the same the rate trebles [Z]  rate [X] halves [Y] & [Z] stay the same the rate is the same [X]0  rate [Y] quadruples [X] & [Z] stay the same the rate goes up by 16 (ie 42 ) [Y]2  rate

Second Order Reactions Ch 1.1 A2 [X] halves [Y] & [Z] 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 the rate goes up by 16 (ie 42 ) [Y]2  rate 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.

Summary Ch 1.1 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 How do you find reaction rates?