Factors affecting equilibrium

Factors Affecting

Equilibrium

Reviewing Prior Knowledge

✓ When a reaction results in an almost complete conversion of reactants to
products, chemists say that the reaction goes to completion, but most reactions
do not go to completion.

✓ The reactions appear to stop because they are reversible.

✓ A reversible chemical reaction can occur in both the forward and the reverse
directions.

✓ Chemical equilibrium is a state in which the forward and reverse reactions
balance each other because they take place at equal rates.

Rateforward reaction = Rate reverse reaction

Le Chatelier’s Principle

In 1888, French chemist Henri- Louis Le Chatelier discovered
that there are ways to control equilibria to make reactions more
productive. He proposed what is now called Le Chatelier’s
principle.

If stress is applied to a system at equilibrium, the system shifts in the direction that relieves that stress. Stress is any kind of change in a system at equilibrium that upsets the equilibrium

Applying Le Chatelier’s Principle

✓ Suppose that the products of an industrial process are the gases carbon
monoxide and hydrogen, a company chemist believes that these gases can be
combined to produce the fuel methane.

✓ Carbon monoxide and hydrogen are placed in a closed vessel at 1200 K,
this exothermic reaction establishes equilibrium.

✓ A methane concentration of 0.05900 M in the equilibrium mixture is too
low to be of any practical use.

Multiple Choice

Reversible reaction -----

• the reactants will produce products from products it can go back to be reactants

• cannot be changed

• stays the same

Multiple Choice

This Law "states that any change to system at equilibrium will result in an expected shift that counter act the change"

• Le Chatelier Principle

• Boyles Law

• Law of thermodynamics

Multiple Choice

The following factors affect the position of equilibrium EXCEPT

• concentration

• temperature

• pressure

• states of matter

Applying Le Chatelier’s Principle to
increase the concentration of methane.

Change in Concentration

Either the reactants or the products will put stress on the equilibrium.

Recall the collision theory, which states that particles must collide to interact. The

number of collisions between the reacting particles depends on the concentration of

the particles, so the equilibrium can be changed by changing concentrations.

Change in Concentration

Additional carbon monoxide (CO)→
raised methane’s concentration from
0.30000 M to 1.0000 M.
The increase in the concentration of (CO)
immediately increases the number of
effective collisions between the carbon
monoxide (CO) and hydrogen (H2 ) and
upsets the equilibrium and the rate of the
forward reaction increases.
With time, the rate of the forward reaction
decreases as the concentrations of carbon
monoxide (CO) and hydrogen (H2 )
decrease.
The new equilibrium position results in
the desired effect; an increased
concentration of methane (CH4 ).

Removing a product (like water) by
adding a desiccant (solid material that
absorbs water). →
Removing or decreasing the
concentration of a product is stress on
the equilibrium. The equilibrium system
reacts to this stress by producing more
water by shifting to the right i.e. in the
direction that will tend to bring the
concentration of water back.

Change in Concentration

The increased concentration of carbon
monoxide (CO ) is a stress on the
equilibrium. The equilibrium system reacts
to this stress by consuming carbon
monoxide (CO) at an increased rate i.e. the
equilibrium shifts to the right forming more
methane (CH4 ) and water (H2O).

An increase in the concentration of
reactant results in a shift to the right and
additional product

The equilibrium shifts to the right and
results in the desired effect; the
production of more products.

Change in Concentration

According to Le Chatelier’s principle
adding or increasing the concentration of a
product (like water) is stress on the
equilibrium. The equilibrium system
reacts to this stress by converting products
to reactants.
The equilibrium shifts to the left,
toward the reactants.

According to Le Chatelier’s principle
removing or decreasing the concentration
of a reactant is stress on the equilibrium.
The equilibrium system reacts to this
stress by converting products to
reactants.
The equilibrium shifts to the left,
toward the reactants.

Multiple Choice

Equilibrium for 2CrO₄^2- + 2H⁺ → Cr₂O₇^2- + H₂O. What happen when H⁺ ion are added to the system?

• Position of equilibrium will shift to left to increase the H concentration

• Color of system will turn from orange to yellow

• K_eq will be higher

• Equilibrium will shift to right to decrease the added H⁺ ion

Multiple Choice

For CoCI₄^2- +6H₂O →Co(H₂O)₆²⁺ + 4CI^-. What happen when CI^- ions are added?

• Position of equilibrium will shift to right

• Concentration of reactants and products remain unchanged

• Equilibrium shift to left to reduce the added CI^- ions

• Color of system will turn from blue to pink

Multiple Choice

C₂H₆(g) <---> C₂H₄(g) + H₂(g) H=+ve

Suggest a way to increase the concentration of C₂H₆.

• decrease concentration of C₂H₆

• decrease the concentration H₂

• increase the concentration C₂H₄

• decrease concentration of C₂H₄

Multiple Choice

Predict the colour of solution if SCN^- removed from the test tube.

• yellow

• colorless

• blood red

• yellowish green

Change in Temperature

Chemical reaction

Endothermic reaction

Exothermic reaction

Consider heat as a reactant

Consider heat as a Product

The reaction for making methane (forward reaction) is exothermic, while the reverse
reaction is endothermic.

According to Le Chatelier’s principle,
if heat is added to an equilibrium
system, the equilibrium shifts in the
direction in which heat is used up, i.e.
the equilibrium shifts to the left and
decreases the concentration of
methane (CH4 )

According to Le Chatelier’s
principle, lowering the temperature
shifts the equilibrium to the right as
the forward reaction gives off heat
and relieves stress. Shifting the
equilibrium to the right produces
more methane (CH4 ).

Change in Temperature

Consider heat as a reactant. The conversion between dinitrogen tetroxide (N2O4 ) and
nitrogen dioxide (NO2 ) responds to changes in temperature in an observable way.
N2O4 is a colorless gas, and NO2 is a reddish-brown gas. The color of the equilibrium
mixture in ice is much lighter than when the mixture is heated in boiling water.

Change in Temperature

Adding heat shifts the equilibrium to
the right and creates more reddish-
brown NO2 .

The removal of heat by cooling shifts
the equilibrium to the left and creates
more colorless N2O4 .

Multiple Choice

When heat is released, what type of reaction takes place?

• Exothermic

• Endothermic

• Osmosis

Multiple Choice

Heat is the reactant in the system, Δ  H=+

• Exothermic

• Endothermic

• Osmosis

Multiple Choice

Br2 <-----> 2Br H=+ve

What happened to the equilibrium position when the temperature decreased.

• Shift forward

• Shift reverse

• remain unchanged

Multiple Choice

For N₂O₄ → NO₂ (ΔH = +ve). What will happen when temperature is increased?

• Position of equilibrium will shift to left

• Position of equilibrium will shift to right

• No change in position of equilibrium

• Equlibrium will not be affected

Change in Volume and Pressure

Using a piston-like device

Force the piston downwards; the
volume of the piston will decrease.
Recall Boyle’s law, which states that
decreasing the volume at a constant
temperature increases the pressure.

The pressure exerted by a
gas depends on the number
of gas particles that collide
with the walls of the vessel.
The more gas particles in
the reaction vessel, the
greater the pressure will be

According to Le Chatelier’s principle,
the increased pressure is a stress on the
equilibrium.

For the reaction between carbon
monoxide (CO) and hydrogen (H2 ) at a
constant temperature, changing the
volume on the reaction vessel changes
the concentrations of gaseous reactants
and products.

Increasing the pressure shifts the
right equilibrium and increases the
amount of the product.

Moles of Reactants versus Moles of Products

Compare the number of moles of gaseous reactants in the equation to the number
of moles of gaseous products.

4 moles of gaseous reactants

2 moles of gaseous reactants

According to Le Chatelier’s principle, the equilibrium will relieve the stress of
increased pressure by shifting to the right, as this shift decreases the total number of
moles of gas, and thus the pressure inside the reaction vessel decreases.

Moles of Reactants versus Moles of Products

The shift does not decrease the pressure to its original value, however, it has the desired
effect; more methane (CH4 ) is produced.

Changing the volume and pressure of an equilibrium system shifts only the

equilibrium only if the number of moles of gaseous reactants is different from the

number of moles of gaseous products. If the number of moles of gaseous

reactants is the same as the number of moles of gaseous products, changes in

volume and pressure have no effect on the equilibrium.

•An increase in pressure shifts the equilibrium towards the side with fewer
gaseous moles, decreasing the pressure.

•A decrease in pressure shifts the equilibrium towards the side with the greater
number of gaseous moles, increasing the pressure

Multiple Choice

3. In change in pressure and volume, if the pressure decreases the system will shift to the?

• location of more moles

• location of less moles

• shift to left

• shift to right

Multiple Choice

For N₂O₄ (colorless) → 2NO₂ (brown). What happens when pressure is increased?

• Position of equilibrium will shift to right

• Color of system will turn from colorless to brown

• Color of system will turn from brown to colorless

• Position of equilibrium will not change

Multiple Choice

For N₂O₄ (colorless)→ 2NO₂ (brown). By reducing pressure, equilibrium is shift to right. WHY?

• Shift to the right will increase the rate of reaction

• Color of system will turn from colorless to brown.

• More particles on the right, so K_eq will increase

• More particles on the right, so pressure can increase again

Multiple Choice

For N₂ + 3H₂ →2NH₃ . When pressure is increased the equilibrium shift to right. Why?

• To increase the amount of products

• To reduce the pressure, as right has less number of molecule

• K_eq will increase when it is shifted to the right

• So it will increase the rate of reaction

True or False (Appetizer-pick 3)

1.  A reaction is at equilibrium when the rate of the forward and reverse reactions is unequal.

2.  When changes are made to a system at equilibrium, the system shifts to a new equilibrium position.

3.  Adjusting the concentrations of either the reactants or products in a reaction puts stress on the equilibrium

4.  At equilibrium, the concentrations of reactants and products are not constant.

5.  In an exothermic reaction, an increase in temperature favors the formation of products.

6.  When a reaction results in an almost complete conversion of reactants to products, chemists say that the reaction goes to completion.

Short answer (main course-pick 5)

1. For each reaction, figure out how its equilibrium could be shifted in the indicated direction.

a.  H₂(g) + Cl₂(g)   2HCl(g) + thermal energy: shift to the left

b.  C(s) + O₂(g)    CO₂(g) + thermal energy: shift to the right

c.   Thermal energy + 8SO₂(g)    S₈(s) + 16O₂(g): shift to the right

d.  PCl₅(g)  PCl₃(g) + Cl₂(g) + thermal energy: shift to the left

 

2. If you wished to maximize the products of the following reactions, determine which concentrations would you lower or raise.

a.  H₂(g) +Br₂(g)     2HBr(g)

b.  CO₂(g) + H₂(g)   CO(g) + H₂O(g)

3. For each reaction, state whether increasing or decreasing the volume of the reaction vessel would yield more product at equilibrium. Evaluate your choice.

a.  N₂O4(g) 2NO₂(g)

b.  2SO₃(g)  2SO₂(g) + O₂(g)

c.   CH4(g) + 2O2(g)   CO2(g) +2H2O(g)

d.  2CO(g) + O₂(g)  2CO₂(g)

4. Conclude the effect that an increase in temperature would have on these reactions at equilibrium.

Why?

a.  Heat + H₂(g) + I₂(g)  2HI(g)

b.  CH₄(g) + 2O₂(g)  CO(g) + 2H₂O + heat

 

 

5.  Determine the principle that states that if stress is applied to a system at equilibrium, the system shifts in the direction that relieves that stress.

 

6.  Methanol may be produced by the exothermic reaction of carbon monoxide gas and hydrogen gas.

CO(g) + 2H₂(g)    CH₃OH(g) ∆HO = –103 kJ

State and explain the effect of changing the following conditions on the amount of methanol present at equilibrium:

a.  increasing the temperature of the reaction at constant pressure.

b.  increasing the pressure of the reaction at constant temperature.

Multiple choice (salad-pick 4)

1. A decrease in the concentration of reactants causes the rate of the _____ reaction to slow.

a. complete           c.         reverse

b. forward             d.         incomplete

 

2. In this reaction, which of the following factors will NOT change the concentration of CO₂ in the reaction?

a.  a decrease in the volume of H₂O  

b.  a decrease in pressure

c.   an increase in the amount of catalyst      

d.  a decrease in temperature

 

3. In an exothermic reaction, equilibrium shifts _____ when temperature is lowered.

a.      to the left          c. to the centre

b.     to the right           d. none

4. In an exothermic reaction, equilibrium shifts _____ when temperature is raised.

a.     to the left c. to the centre

b.     to the right           d. none

5. What change can result in a shift in equilibrium during a reaction?

a.     change in concentration c.  change in volume and pressure

b.     change in temperature   d. all of the above

 
6. According to Le Chatelier’s principle, when the volume is increased, the equilibrium shifts to the right for only one of the following. For which of the following reversible reactions is this true?
a. H₂(g) + I₂(g) <--> 2HI (g)
b. 2CO(g) + O₂(g) <--> 2CO₂(g)
c. S(g) + O₂(g) <-->SO₂(g)
d. PCl₅(g) <--> PCl₃(g) + Cl₂(g)