rates & extents of reactions. rates & extents of rates & extents of reactions quiz a...
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Rates & Extents ofReactions
Rates & Extents ofReactions
Quiz AWhat prevents reaction from
going?Increasing the Rate of
reaction
Measuring Change
Establishing EquilibriumThe Equilibrium ConstantLe Chatelier’s Principle
Industry
Temperature
Catalysis Collisions
Quiz B
Temperature
Pressure Mass ofSystem
Concn.
Quiz C
DynamicEqm. Quiz DQuiz EQuiz F
Quiz G
Overview
Quiz A- Common misconceptions
1. When solid copper carbonate reacts with excess acid, carbon dioxide is produced. The curves shown were obtained under two different sets of conditions.
Volu
me
of
CO
2time
X
Y
A. increasing the concentration of the acid.
B. decreasing the particle size.
C. adding a catalyst.
D. increasing the temperature.
E. decreasing the mass of copper carbonate.
The change from X to Y could be brought about by :
2. Chemical reactions are in a state of dynamic equilibrium only when:
A. the rate of the reverse reaction is equal to the rate of the forward reaction.
B. the reaction involves a zero enthalpy change.
C. the activation energy of the forward reaction equals the activation energy of the reverse reaction.
D. the reaction goes to completion.
E. the ratio of the products to reactants is equal to exactly 1.
Quiz A- Common misconceptions
3. X(g) is placed in a flask and the following reaction proceeds to equilibrium
X(g) ⇌ Y(g) ΔH = +ve
Which of the following statements is correct?
A. The forward reaction rate increases as the reaction “gets going”.
B. The forward reaction rate always equals the reverse reaction rate.
C. The activation energy of the forward reaction of the system considered willalways be higher than the activation energy of the reverse reaction no matterthe reaction conditions.
D. When equilibrium is re-established after a disturbance (e.g. adding more Y) the rate of the forward and reverse reactions will be equal to those at the initial equilibrium.
E. The forward reaction is completed before the reverse reaction begins.
Quiz A- Common misconceptions
4. The graph below shows the variation of concentration of a reactant , X, with timeas the reaction proceeds.
A 0.0005
B 0.005
C 0.015
D 0.020
E The slope of the tangent to the curve at t = 30s
[X]
/ m
ol.d
m-3
Time /s
30
60
0.020
0.005
What is the average reaction rate (mol dm-3 s-1) during the first 30 seconds?
Quiz A- Common misconceptions
5. Excess marble chips (CaCO3) were added to 50 cm3 of 1 M HCl. The experiment was repeated using the same mass of marbles chips and 50 cm3 of 1M CH3COOH.
Which of the following would have been the same for both experiments?
A The average rate of reaction.
B The rate at which the first 2 cm3 of gas was evolved.
C The time taken for the reaction to be complete.
D The mass of the marble chips remaining after the reaction had stopped.
E None of the above.
Quiz A- Common misconceptions
What prevents reactions from going?Po
tenti
al Energ
y
Reaction co-ordinate
Ea
N2 + O2
2NO
Energy Profile
ΔH
2NO N2 + O2
Increasing the Rate of Reaction
Rate = k [A]x [B]y
Rate constant
Orders of reactants
Non-examinable ! But useful for understanding.
Increasing the Rate of Reaction
A. Temperature
Everyone knows that as we increase the temperature, the reaction rate goes up……
But how do we explain this observation scientifically?Q AThe answer lies in the Maxwell Distribution where the idea of
a range of molecular speeds is introduced.
What makes reactions go faster?
Fract
ion o
f m
ole
cule
s
Molecular Velocity
Maxwell Distribution of velocities
KEmin = ½mv2min
T1
T1
T2
T2
vmin
T2 > T1
Increasing the Rate of Reaction
If we increase the temperature we will increase the fraction of the molecules with the energy to overcome the activation energy.
f =
e
–Ea/RT
If T then f
If f then k
If k then Rate
Note : If Ea due to a catalyst then f and the Rate
Where f = fraction of molecularcollisions with energy greater orequal to Ea
Increasing the Rate of Reaction
B. Catalysis
Catalysts are employed to speed–up the attainment of equilibrium.
Catalysts do not change the position of equilibrium .
Catalysts offer a different mechanism for the reaction to occur .
· For the catalyst to be successful it must offer a mechanism with a lower activation energy, Ea. · Catalysts can either be in the same phase as the reactant – homogeneous catalysts -or a different phase –heterogeneous catalysts.
N
O
N
O
Increasing the Rate of Reaction
NN
OO
NN
OO
N
O
N
O
Pote
nti
al Energ
y
Reaction co-ordinate
Ea
N2 + O2
2NO
Ea
Energy Profile
Non – catalyzed Catalyzed
N2(g) + O2(g) 2NO(g)
ΔH
Increasing the Rate of Reaction
C. Increasing collisions
The more collisions there are the more likely reactants are to react.
1. Increasing the concentration of reactants (pressure in the case of gases).
2. Increasing the surface area of solid reactants (or solid catalysts).
We can influence the amount of collisions by:
1. A catalyst lowers the activation energy of a reaction from 100 kJ/mol to 50 kJ/mol. What increase in f will result?
Quiz B- Increasing reaction rates
2. The activation energy for a reaction is known to be 50 kJ/mol of reactant. If the temperature is increased from 300 K to 310 K what increase in the rate of reaction would you expect?
Measuring ChangeConsider the following reaction:
How could we measure the rate of this reaction?
Well let’s consider what might change during this reaction.
QA
TemperaturePressure
Mass of system
Concentration
A (g) B(g)
Im bending your spoon with my mind! ! ! ! !
Measuring ChangeA. Temperature
If the reaction is exothermic or endothermic then the temperature will change.
But if the temperature changes then the rate of reaction will change! Why? *
* See Maxwell Distribution
Therefore if we want to measure the rate of reaction we normally keep the temperature of the reactor constant. This is called thermostatting the reactor.
Exothermic; ΔH is –ve; T Endothermic; Δ H is +ve; T
T Rate of reaction
THEREFORE WE CANNOT MEASURE THE RATE OF A REACTION USING THE CHANGE IN TEMPERATURE
Measuring ChangeB. PressureWe know from Grade 11 that :
PV = nRT
Therefore if we keep the volume of the thermostatted reactor constant then:
RT V
× n
P =
= const × n P n
Will PA change during the reaction?Q AOf course : PA as nA
Will PB change during the reaction?Q AOf course : PB as nB
Will PTOTAL change during the reaction.Q ANO! PTOT nTOT and nTOT won’t change for this reaction .
Measuring Change
THEREFORE WE CANNOT MEASURE THE RATE OF THIS REACTION USING PTOTAL
Measuring Change
C. Mass of the system
This one is easy – if the reactor is a closed system then the mass cannot change. Mass must be conserved.
If the reaction is taking place in an open system then gases might leave the system and the mass of the system will decrease.
In this particular case we have a closed system.
THEREFORE WE CANNOT MEASURE THE RATE OF THIS REACTION USING THE MASS OF THE SYSTEM
Measuring ChangeD. ConcentrationThis is by far the most common means of measuring rates, because concentrationsALWAYS change during a reaction. In this example :
We need to be able to measure concentrations and the most used techniqueis SPECTROSCOPY where a reactant or product selectively absorbs or emitselectromagnetic radiation, leaving it coloured.
The intensity or amount of the radiation absorbed (A) is proportional to the concentration.
A [ ]
Normally chemists measure concentrations to determine reaction rates
A substance does not need to be coloured to absorb electromagnetic radiation. CO2 for instance is colourless as it does not absorb visible radiation but it doesabsorb infra-red radiation (the cause of global warming!). Therefore we could use infra-red spectroscopy to monitor [CO2].
[A] with time while [B]
1. Which of the following reactions could we use PTOTAL for the measurement of the reaction rate?
a) 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)
Quiz C- Measuring change
b) O3(g) + NO(g) NO2(g) + O2(g)
c) 2CaO(s) + 5C(s) 2CaC2(l) + CO2(g)
2. Predict the necessary experimental conditions to be able to measure the rate of the following reaction by monitoring the mass of the system.
2CaO(s) + 5C(s) 2CaC2(l) + CO2(g)
3. Sulfur dioxide gas can be oxidized to sulfur trioxide gas by oxygen gas. In aparticular experiment a stoichiometric amount of sulfur dioxide and oxygen
are allowed to react in a glass vessel. If the initial pressure in the flask is 3 atm ,what would be the predicted final pressure if the reaction goes to completionand the temperature remains constant.
Quiz C- Measuring change
4. Calculate the rate of CO2 production (moles of CO 2 / dm3 sec) from an industrial process given the data in the two graphs shown below.
Solution A at t = 0 sec
A ⇌ B
5
10
15
20
Mole
cule
s /c
m3
time
Establishing Equilibrium
K = [B]
[A]= 3
Quiz D- Establishing Equilibrium
1. Which of the following systems are at equilibrium ?(Assuming they have been left longenough to establish equilibrium.)
a) A boiling kettle.
b) A sealed thermostatted test-tube containing a drop of water.
c) A completely flat battery.
d) A water tank under the following conditions:
WaterH2O H2O
5 L s-1 300 L min-1
The Equilibrium Constant
The Law of Mass action, which is independent of kinetic theory, states that for a reaction-
aA + bB ⇌ cC + dD the ratio
[C]c [D]d
[A]a [B]b
will be a constant when the system is at equilibrium. This constant is known as the equilibrium constant, Kc.
One must remember that the value of Kc is specific to a particular reaction equation(i.e. one in which the stoichiometry is fixed) and that it is specific to a given temperature.
The Equilibrium Constant
For a gaseous reaction the equilibrium constant can be expressed as a ratio of the partial pressures of the productsand reactants. For example the equilibrium constant for thereaction :
aA(g) + bB(g) ⇌ cC(g) + dD(g)
could be expressed as
(PC)c (PD)d
(PA)a (PB)bKP
=Note that if one uses this expression the pressures must be quoted in atmospheres. Also note that the numerical value of KP might not be the same as the numerical value of KC.
Quiz E- Equilibrium Constants1. The reaction between nitrogen gas and oxygen gas to form nitrogen dioxide gas
is shown below.N2(g) + 2O2(g) ⇌ 2NO2(g)
a) Write down an expression for the equilibrium constant, Kc, for this reaction.
b) Kc is equal to about 2.6 × 10-15 for this reaction at 25 oC. In a 1 L flask at 25 oC there are 1.0 × 10 13 molecules of N2, 3.0 × 1015 molecules of O2 and 1.0 × 1012
molecules of NO2. i) Is this system at equilibrium?
ii) If this system is not at equilibrium, in what direction will the reaction proceed?
c) What is the numerical value of the equilibrium constants for each of the followingreactions?
i) ½N2(g) + O2(g) ⇌ NO2(g)
ii) 2NO2(g) ⇌ N2(g) + 2O2(g)
Quiz E- Equilibrium Constants
2. At a given constant temperature, a 1 L flask initially containing 0.100 mol of SO2 and0.150 mol of O2, is allowed to come to equilibrium. 80% of the SO2 is found to have reacted to form SO3. Calculate the equilibrium constant for the reaction-
O2(g) + 2SO2(g) ⇌ 2SO3(g)
3. For the endothermic reaction :
2SO3(g) ⇌ O2(g) + 2SO2(g)
state the effect on the equilibrium constant that the following disturbances will have.
a) Increasing the concentration of SO3(g).
b) Decreasing the concentration of SO2(g).
c) Doubling the size of the reaction flask.
d) Decreasing the temperature.
Le Chatelier’s Principle
If a change is imposed upon a system at equilibrium the position of the equilibrium will shift in a direction that tends to reduce that change.
Note : By changing the position of equilibrium we are not changing the equilibrium constant if the temperature remains constant. Equilibrium constants can only change if one changes the temperature of the reaction vessel.
Quiz F- Influencing the Equilibrium
1. Consider one of the gaseous equilibria involved in the industrial preparation of nitric acid by the Ostwald process.
2NO(g) + O2(g) ⇌ 2NO2(g) ΔH = –ve
What qualitative effect would the following disturbances have on the position of the equilibrium?
a) An increase in PNO.
b) An increase in temperature.
c) A decrease in reactor volume.
d) An increase in pressure via the addition of an inert gas e.g. Ar.
Quiz F- Influencing the Equilibrium
2. Consider the following equilibria below :
BaCO3(s) + (aq) ⇌ Ba2+(aq) + CO32- (aq)
CO32- (aq) + H2O(l) ⇌ CO2(g) + 2OH-(aq)
What qualitative effect would the following disturbances have on the positionof the equilibrium?
a) Making the particle size of the BaCO3 smaller.
b) Decreasing the pH of the aqueous solution.
Industry
They want to do things as quickly and as efficiently as possible. However, sometimes, doing something quickly might not mean doing the same thing efficiently.
Thermodynamic (i.e. enthalpy ) considerations might sometimes clash with kinetic (i.e. rate) considerations.
Industries want to make money!
Quiz G- Equilibrium & rates in industry
1. Predict the conditions of temperature and pressure required to increase productivity in the following industrial processes.
a) N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -ve
b) 4NH3(g) + 5O2(g) ⇌ 4NO (g) + 6H2O(g) ΔH = -ve
c) 2CaO(s) + 5C(s) ⇌ 2CaC2(l) + CO2(g) ΔH = +ve
Quiz G- Equilibrium & rates in industry
2. An industrial process to convert X into Y has the following stoichiometry
2X(g) ⇌ Y(g) ΔH = -ve
The reaction is catalysed by a solid heterogenous catalyst. Which of the following setof experimental conditions would an industrial chemist choose to optimize the reaction?
X Y + X
T = 300K P = 10 atmA)X Y + X
T = 600K P = 1 atmB)
X Y + X
T = 300K P = 10 atmC)X Y + X
T = 300K P = 1 atmD)
Catalyst