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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”?

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

Ch 1.1

A2

- Temperature:
- Food spoils more quickly at room temperature than in a refrigerator
- bacteria grow faster at RT than at lower temperatures

- Food spoils more quickly at room temperature than in a refrigerator

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

How do factors affect reaction rates?

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

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

Ch 1.1

A2

What does “rate of reaction mean?

A → B

Concentration of [A] decreases with time as the concentration of [B] increases.

Chemical reaction rates

Ch 1.1

A2

Predict the general shape of the graph you would expect for

A → Product

What do reaction rate graphs look like?

Chemical reaction rates

Ch 1.1

A2

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

Ch 1.1

A2

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

Ch 1.1

A2

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

Ch 1.1

A2

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

Ch 1.1

A2

- 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 changealways 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?

Reaction Rates

Ch 1.1

A2

Consider the chemical reaction:

AB

How do you find reaction rates?

t = 40. min

2.0 mol A

8.0 mol B

t = 20. min

5.0 mol A

5.0 mol B

Time = 0.

10. mol A

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?

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.

Ch 1.1

A2

Time = 0

10 mol A

t = 20 min

5.0 mol A

5.0 mol B

- 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

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

Ch 1.1

A2

Given: [HCl]54 min = 1.58 M

[HCl]215 min = 1.02 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?

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…

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

Ch 1.1

A2

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

Ch 1.1

A2

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

How do you find reaction rates?

x2

x2

x2

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 0.10.2 1.6 x 10-2

2 0.10.4 3.2 x 10-2

3 0.2 0.2 6.4 x 10-2

x2

x4

x1

How do you find reaction rates?

x1

x2

x2

[A] stays the same

[B] doubles

the rate doubles

[B] rate

[A] doubles

[B] stays the same

[A]2 rate

the rate is x4

Ch 1.1

A2

How do you find reaction rates?

What is the rate equation for this reaction?

Rate = k[A]2 [B]

[A] stays the same

[B] doubles

the rate doubles

[B] rate

The reaction is second order in respect of A and first order in respect of B. The overall reaction order is 3.

[A] doubles

[B] stays the same

[A]2 rate

the rate is x4

Second Order Reactions

Ch 1.1

A2

x1

x1

x1

NE

x1

X0.5

How do you find reaction rates?

[Z] triples

[X] &[Y] stay the same

[Z] rate

x1

x3

x3

[X] halves

[Y] & [Z] stay the same

the rate goes up by 16 (ie 42 )

the rate is

the same

the rate trebles

[X]0 rate

x4

x1

x1

[Y] quadruples

[X] & [Z] stay the same

[Y]2 rate

Ch 1.1

A2

[Z] triples

[X] &[Y] stay the same

[Z] rate

[X] halves

[Y] & [Z] stay the same

the rate is

the same

[X]0 rate

[Y] quadruples

[X] & [Z] stay the same

[Y]2 rate

How do you find reaction rates?

What is the rate equation for this reaction?

Rate = k[Y]2 [Z]

the rate goes up by 16 (ie 42 )

the rate trebles

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

A2

- 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?

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