7.1

Dynamic Equilibrium

The word equilibrium often brings to mind the concept of balance, for example the balancing of two people on a see saw. However, equilibrium in chemistry has a very different meaning. Dynamic equilibrium is like trying to remain on a fixed point on an escalator. The only way to do this is to walk upwards at the same rate as which you are walking downward. Many chemical and physical changes in chemistry are reversible, particularly if they take place in a closed system where the reactants and products can’t escape. Dynamic Equilibrium in Physical Changes Dynamic equilibrium occurs in physical changes (like a change in state) that take place in closed system (container) so the reactants and products can’t escape. The change is reversible. The reactants change into products at the same rate as the products change into reactants. The change appears to have stopped because no more changes will be visible to the eye. A double headed arrow is used to represent this. A

B

The two changes are called • Forward reaction - proceeds to the right, reactants to products • Reverse reaction - proceeds to the left, products to reactants NOTE: A closed system is when reactants, products (matter) and energy can’t be lost or gained from the system. If the system is open, some of the products of the reaction could escape (gas) and dynamic equilibrium would never be reached. Consider a closed flask containing water, equilibrium will be reached between the liquid water and water vapor. During vaporization/evaporation the molecules on the surface of the water with the greater kinetic energy will break their intermolecular forces and escape from the surface of the liquid into a gaseous vapor. The slower moving molecules of water vapor will condense back into liquid water. Equilibrium is established when the rate of the forward reaction, vaporization equals the rate of the reverse reaction, condensation. vaporization H2O(l)

H2O(g) condensation

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Note: During vaporization (before equilibrium is established) the temperature of the liquid decreases because as the more energetic molecules escape from the liquid, the remaining molecules in the liquid have a lower average kinetic energy and hence lower temperature. Remember that temperature is a measure of the average kinetic energy. Bromine gas is an example of another physical change that is in a state of dynamic equilibrium. When liquid bromine is placed in a container and sealed, the liquid will start to evaporate and form a brown gas because the molecules on the surface of the bromine liquid with the greatest kinetic energy break their intermolecular forces and escape into a gaseous vapor. The slower moving molecules of bromine vapor condense back into liquid bromine. Eventually the color and concentration of the liquid and the gas will not appear to change. At this point dynamic equilibrium is established because the rate of vaporization equals the rate of condensation vaporization Equilibrium change

Br2(l)

Br2(g) condensation

Forward change Reverse change

Br2(l)

Br2(g)

Br2(g)

Br2(l)

A closed system containing a saturated solution of sodium chloride is another physical change that establishes dynamic equilibrium. dissolving NaCl(s) Na+(aq) + Cl-(aq) precipitation Equilibrium is established when the rate of dissolving equals the rate of precipitation.

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Dynamic Equilibrium in Chemical Reactions All chemical reactions have a tendency to form dynamic equilibrium. The reactants are changing into products at the same rate as the products are changing into reactants. The change appears as though it has stopped as no more changes will be visible to the eye. For example: I2 (g)

+

3Cl2 (g)

N2O4(g)

2 ICl3(g)

2 NO2(g)

However, many chemical reactions ‘go to completion’. A single arrow, → is used to show that the reactants have been converted to products and the reaction is complete. For example:

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2 Mg

+

O2



2 MgO

NaOH

+

HCl



NaCl

+

H 2O

Establishing Dynamic Equilibrium Consider the reaction A

+

B

C

+

D

The change in concentration with time in establishing equilibrium

Reactants

Products

Initially at time zero, before mixing there is only reactants and their concentrations are high. The concentration of the products is zero. When the reactants A and B are mixed together, they combine to form the products C and D. As the reactants A and B are turned into products, their concentration decreases and the concentration of the products C and D increases. This will happen until a certain time is reached in the reaction when the concentration of the reactants and products no longer changes, it remains constant. At this point the reaction has reached equilibrium.

The change in the rate of reaction with time in establishing equilibrium

The reactants A and B have the highest concentration initially at t = 0 and so the rate at which they are turned into products will be high. The products C and D will have a rate of zero since there are no products formed yet. With time A and B combine to form the products C and D and their concentration begin to decrease, decreasing the rate at which they are turned into products (forward reaction). While the rate of the forward reaction decreases the rate of the reverse reaction, the conversion of product back into reactants will increase because the concentration of the products C and D is increasing with time.

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Forward reaction A

+

B

C

+

D

C

+

D

A

+

B

Reverse reaction

This will continue until a time is reached in the reaction when the rate of conversion of the reactants into products is the same as the rate of conversion of products into reactants. At this point the reaction has reached equilibrium. Increasing the temperature by heating the equilibrium mixture will cause the position of equilibrium to be established faster (in a shorter time). This is because the rate of the reaction increases because a greater proportion of the reacting molecules will collide with the correct orientation and with sufficient energy, E to overcome the activation energy, Ea. Decreasing the temperature by cooling will increase the time at which the position of equilibrium is reached. Consider the concentration vs. time graphs for the following reaction 2 SO2 (g)

+

O2(g)

Forward reaction 2 SO2 (g)

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+

O2(g)

2 SO3 (g)

Reverse Reaction 2 SO3 (g)

2 SO3 (g)

2 SO2(g)

+

O2(g)

Summary The characteristics of a reversible reaction at equilibrium are:  All the reactants and products are present.  The concentration of the reactants and products does not change (remains constant).  Both the forward and reverse reactions occur at the same rate.

Questions 1.

2.

In a closed container, liquid bromine is in dynamic equilibrium with its vapor. a)

Define the term dynamic equilibrium.

b)

Explain why there would no longer be at equilibrium if the stopper was removed from the bottle.

For the Haber process: N2 (g) a)

3.

+

3 H2(g)

2 NH3 (g)

Write down chemical equations for: i) forward reaction ii) reverse reaction

(M03/H) The following graph represents the change in concentration of reactant and product during a reaction.

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a)

Calculate the average rate of reaction over the first 15s, stating the units. [3]

b)

After 19s the concentration of the reactants and products do not change. State what this indicates about the reaction. [1]

c)

Sketch a graph to show the change in the rate of the forward and reverse reaction with time during the establishment of equilibrium and shortly afterwards. Explain the shape of the curve [8]

4.

(M02/S) State two characteristics of a reversible reaction. [2]

5.

(N04/S) State the meaning of the

6.

Since we cannot see any changes occurring when a system is at equilibrium. How do scientists know that a reaction is reversible?

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sign. [1]