reaction rate notes complet

8/7/2019 Reaction Rate Notes Complet

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Rates of Reaction

Introduction to Rate



Rate of Reaction: the change in “something” of the reactants or

products over time or per unit timemass or moles over time**

concentration over time

color over time

the rate of a reaction can be fast, slow or zero



Chemical Reactions

are the result of collisions between atoms, ions or molecules

involve bond breaking (releases energy)

NaCl

involve bond making (requires energy)



Collision Theory

explains why reactions have different rates

1. the reacting particles must collide

2. the particles must have enough energyfor the breaking & making of bonds

3. the colliding particles must collide at thright orientation or geometry

Explains why so

me reaction

s don’t o

ccur

at room temp

Not eno

ugh ener

gy to break & form

bonds



Good

Orienta

tion

R EAC

TION!!

Wrong O

rientation

NO R E

ACTIO

N!!

Wrong OrientationNO REACTION!!

W r o n g O r i e n t a t i o n

N O R E A C T I O N ! !



Activation Energy

the min amount of energy that colliding particlesmust have in order to react

“barrier” or “hurdle” reactants mustovercome



Activation Energy

Progress of Reaction

Energ y

Energy

Reactants

Energy/Enthalpy

Products

Energy of ActivationComplex

Activation

Energy (EA)



The diagram shows the following reactionA + B →AB

According to this diagram, what is the

activation energy of the reaction?

Progress of reaction

Enthalpy

(kJ/mol)

AB

A + B

-135

035

EA 35 kJ/mol – 0 kJ/mol

EA = 35 kJ/mol



Enthalpy (kJ)

Progress of the reaction

C + D

A + B

+800

+600

+400

+200

0

-200

-400

-600

-800

-1000

-1200

The graph below shows the potentialenergy changes for: A + B → C + D

What is the activation energy of the

reaction? The enthalpy?

EA

550 – 100 kJ/mol

EA = 450 kJ/mol

-1000 – 100 kJ/mol

∆H = -1100kJ/mol

∆H



Another Way to Graph EA

Nu

mb

erofPar ti

cles

Energy

EA

Area underthe curverepresents

the # of moleculesthat havereached EA



Factors Affecting Rates of Reactions



Factors Affecting Rates:

Various factors will either increase ordecrease a reaction’s rate

We will study: Temperature

Concentration

Particle Size / Surface Area Catalyst



Temperature:

Increasing the temp, increases the rate

Decreasing temp, decreases the rate

Why?Molecules absorb energy, so…

the reactants reach activation energy &

the number of collisions increase, so reactants step

over the activation energy barrierMore molecules with enough energy to reach E

A



Nu

mb

erofPar ti

cles

Energy

EA

Room TempAt 50oC

Particlesabsorb

energy, somore

particles(area undercurve) are

able to reachEA



Concentration:

Increasing concentration, increases the rate

Decreasing concentration, decreases the rate

Why?

Increasing conc increases the number of particles: therefore more frequent collisions

allowing reactants to reach activation energy

Increase Conc



Particle Size & Surface Area:

Larger surface area (smaller particles) have increased rates

Smaller surface area (larger particles)

have decreased rates Why?

Large SA gives a larger area for collisions,

allowing reactants to reach EA

Increase SA



Catalysts:

Catalysts increase the rate

Why?Catalysts provide reactants with another reaction

path that has a lower activation energy

Tunnel = Catalyst



ActivationEnergy (EA)


Energ y

Activation Energy (EA)

with CATALYST

EA is LOWER

with CATALYST



Nu

mb

erofPar ti

cles

Energy

EA

CATALYST

EA is LOWER

with CATALYSTEA



Inhibitors

Inhibitors decrease the rate

Why?

Inhibitors provide reactants withanother reaction path that has ahigher activation energy



EA with

INHIBITOR


Energ y

Activation

Energy (EA)

EA is HIGHER

with INHIBITORS



Nu

mb

erofPar ti

cles

Energy

EA

INHIBITOR

EA

EA is HIGHER

with INHIBITOR



Reactants

Products

REACTION PROCEEDS

1--

2--

ENERGY

Curves 1 and 2 on the graph below represent energypathways for the same chemical reaction. The reaction

rate is faster for pathway number 2.

Which factor is responsible for curve 2?

A) Increase in Temp

B) Increase in SA

C) Decrease in Conc

D) Presence of Catalyst



When you light a fire you use paper and kindling woodrather than trying to light a log with a match.

Use the collision theory to explain why.

Kindling has a greater surface area than a log

Increase SA = more area for collisions

More collisions = faster rateb/c reach EA sooner



The four graphs below represent the number of moleculesof reactants as a function of their kinetic energy.

Which graph represents the fastest reaction?

Kinetic energy (kJ)

Number of molecules

E E

Kinetic energy (kJ)

Number of molecules

E

Kinetic energy (kJ)

Number of molecules

E

Kinetic energy (kJ)

Number of molecules

A)

B)

C)

D)

EA is the lowest, so

probably reachedfastest!



The graph shows the energy distribution of thereactants of a reaction at a certain temp

Number of

E

Threshold energy

( )K

E average

molecules

Kinetic Energy

Based on the next graph, explain what has broughtabout an increase in the rate of the chemical reaction.

Number of

Threshold energy

( )

E average

molecules

Kinetic Energy E K

A catalyst has been added.The catalyst lowers the threshold energy so that there

are more effective collisions between molecules



The energy distribution graph for a givenreaction is shown below

Which graph belowshows the affect of

increasedtemperature?

EA is same, but lessmolec needed to

reach EA



Pg 424 #18, 20 & 23

Old Exam Question



The spon reaction of a solid piece of P4 with O2 in air has

an activation energy of 30 kJ/mol and is represented bythe following equation.

P4(s) + 5 O2(g) → P4O10 (s) ∆ H = −700 kJ/mol

Here is a sketch of the kinetic energy distribution curvethat corresponds to this reaction at 20°C and 101 kPa.

Kinetic energy (E k )

Number of moles of

reactant molecules

(n)

a

umber of



Below is a list of 5 changes in reaction conditions, alongwith 5 graphs. Which graph best corresponds to each

change in conditions listed below?Changes in conditions:

1. The conc of oxygen gas is increased2. An inhibitor is added 3. The temp is lowered

4. A catalyst is added 5. The temp is raised

A B C

D E

E k

n

E k

n

E k

n

E k

n

E k

n

Kinetic energy (E k )

umber of

moles of

reactant molecules

(n)

a

1

234

5



Rate Laws & Estimating Rate



Rate Law:

An expression relating the rate of reaction to theconcentration of the reactants



For: aA + bB → cC +

dD

Rate equation: Rate = k [A]a [B]b

Rate constant:

•determined experimentallyfor each reaction at given

temp

•L/mol•s or s-1

•Large k = fast reaction

Coefficients:

•Coefficientsbecome theexponents



REMEMBER:Rate in this equation

is dependant on theconcentration of reactants only.



NO2(g) + O3(g) → NO3(g) + O2(g)

What is the rate equation for the formation of

NO3(g) and O2(g)?Rate = k [NO2]

[O3]If the initial conc of NO2(g) and O 3(g) is

1.0M, what would happen to the rate if weincreased the conc of NO3 to 2.0M?

Rate = k [1][1]

Rate =

Rate = k [2][1]

Rate =

Ratedoubles!



2NO + O2

→ 2NO2

What is the rate equation for the formation of NO2? Rate = k

[NO]2[O2]If the initial conc of both reactant is 1.0M,what would happen to the rate if weincreased the conc of O2 to 2.0M?

Rate = k

[1]2[1]Rate =1k

Rate = k

[1]2[2]Rate =2k

Rate

doubles!

NO to

2.0M?Rate = k2 Rate =

Rate

quadruples



Reactions that have reactants:

With oppositely charged ions are very fast

With few or weak bonds are faster than those

with many or strong bonds

Predicting Rates of Reactions:

Ag+ + Cl- AgCl

Mg + HCl MgCl+ H2

C2H5OH + 3O2 2CO2 + 3H2O



In the same phases are faster that those indifferent phases

Undergoing a two particle collision are fasterthan those requiring many particle collisions

Predicting Rates of Reactions:

H2 (g) + F2 (g) 2HF (g)

Mg (s) + 2H2O (l) 2Mg2+ (aq) + 2OH-

(aq) + H2 (g)

2H2 (g) + O2 (g) 2H2O (g)

4NH3 (g) + 7O2 (g) 4NO2 (g) +



Rank the following from fastest to slowest.

H2 (g) + F2 (g) 2HF (g)

Ag+

+ Cl-

AgCl

C2H5OH + 3O2 2CO2 + 3H2O

1.

2.3.

2 1 3



Which would have the greatest rate?

Hg (l) + Br2 (g) HgBr2 (s)

2H2 (g) + O2 (g) 2H2O(g)

4NH3 (g) + 7O2 (g)

4NO2 (g) +6H2O (g)

A.

B.

D.

C

C. Ag+ (aq) + Cl- (aq) AgCl (s)



Calculating Rate of Reaction



Calculating Rate:

Recall, rate is the change insomething over time

You can calculate rate if:

1. Given info

2. Given a set of Data

3. Given a Graph



A bottle of wine contains approximately 72 g of

ethyl alcohol, C2H5OH. When left open, the ethylalcohol changes into acetic acid according to thefollowing equation:

C2H5OH + O2 →CH3COOH + H2O

The ethyl alcohol changed completely into aceticacid after 60 days (d).

What is the average reaction rate, in molesper day (mol/d), of the transformation of thealcohol?



Calculate the number of moles of alcohol:

moles = 72/ 46moles = 1.57 mol of alc.

Calculate the average rate:

Rate =(d) days of number

(mol) moles of number

Rate = 1.57 / 60

Rate = 0.026 mol/day



Below is a balanced equation:A + B → C + 2D

A student dissolved 0.15 mol of substanceA in solution B. After a ten-minute periodof reaction, 0.05 mol of A is recovered.

What is the average rate of formation of D in mol/sec?



Calculate the number of moles of A used in rxn:

Moles used of A = 0.15 – 0.05 mol

moles of A used = 0.1 mol

Calculate the number of moles of D produced:

moles of D produced = 0.2 mol



Calculate the length of rxn:

10 min = ? sec

time = 600 sec


Rate =)( sec of number

(mol) D of moles of number

s

Rate = 0.2 / 600

Rate =3.3 x 10-4 mol/sec



The combustion of a candle is recorded in

the table below.Before

combustionAfter

combustion

Mass of candle

(g)

165.5 162.0

Time (h:min) 1:00 6:00

The equation is:C25 H52 + 38 O2 →25 CO2 + 26 H2O

Calculate the rate of this combustion

reaction in moles/hour



Calculate the # moles burned:

moles = 3.5/ 352moles = 0.01 mol of candle burned


Rate =)(hr hours of number


Rate = 0.01 / 5

Rate = 0.002 mol/hr are burned



G ( ) ( ) C( )



The graph below shows the # of moles of C

formed by the reaction, as a function of time.

What is the average rate for the formation of

C during the first three seconds?

0.40

0.30

0.20

0.10

1 2 3 4 5

Time (s)

Number of moles of substance C

Given: A(g) + B(g) → C(g)



Calculate the # C moles formedin first 3 seconds:

moles = 0.25 mol of C formed

Calculate the average rate of C:

Rate =)(s sec of number


Rate = 0.25 / 3

Rate = 0.083 mol/s of C are

formed



Zinc reacted with HCl, to produce H2 &

MgCl2. The following table shows thevolume of H2 formed during the reaction as

a function of time.Time (s) Volume of H

2(mL)

01050100

150

083245

50

What is the average rate of formation of H2

between the 25th and 75th second?

Vol of H = 39mL 19mL



1400 40 60 80 100 120 140 Time (s)160

Volume

of H 2

10

20

(mL)

20

10

30

40

50

60

1400 40 60 80 100 120 140 Time (s)160

Volume

of H 2

10

20

(mL)

20

10

30

40

50

60

19mLof H2

39mL

of H2

Vol of H2 = 39mL – 19mL

Vol of H2 = 20 mL

Rate = (s) sec of number

(mL) H of vol 2

Rate = 20 / 50

Rate = 0.40mL/s

reaction rate notes complet

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