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Chapter 16 – Controlling the Yield of Reactions
The stage when the quantities of reactants and products remains
unchanged is called the chemical ____________________. Reaction yield is the
extent of conversion of reactants into products, or how far the reaction
will go.
Some ____________________. and chemical changes can be reversed. For
example ice to liquid water and back again. A double arrow
____________________.) is used to indicate a ____________________. reaction.
We can use the idea that processes can be reversed to understand why
some reactions reach an ____________________.. Chemists have shown that in
these reactions the forward and reverse reactions occur
____________________..
For example, nitrogen gas and hydrogen gas were added to a sealed
container at a constant temperature. The nitrogen and hydrogen start to
react immediately, forming ammonia. The following sequence of events
then occurs:
–As the ____________________. reaction proceeds, the concentration of
nitrogen and hydrogen decrease, so the rate of ammonia production
decreases.
–At the same time the ammonia is being formed, some ammonia
molecules react to re-form nitrogen and hydrogen. The rate of this
____________________. reaction increases as the concentration of ammonia
increases.
–Eventually the ____________________. and back reactions proceed at the
same ____________________.. When this situation is reached, the ammonia is
formed at exactly the same rate it is breaking down.
–The ____________________. of the hydrogen, nitrogen and ammonia will
remain constant.
–At the ____________________. position no further change will take place in
the rate of either the forward or back reaction. The reaction has reached
a point of ____________________., an equilibrium.
Draw an example of the establishment of equilibrium
Equilibrium is a ____________________. state, since the forward and back
reactions have not ceased. They occur simultaneously at the same rate.
During dynamic equilibrium:
–The ____________________. and concentrations of chemical substances
remain constant.
–The total gas ____________________. is constant (if gases are involved)
–The ____________________. us constant
–The reaction is ____________________. (all of the substances are present in
the equilibrium mixture).
Different reactions proceed to ____________________. extents. For example a
____________________. acid, like HCl, almost completely ionises. Whereas a
____________________. acid, like ethanoic acid, does not completely ionise.
This is shown by their electrical conductivity. As such their point of
____________________. is different.
K is known as the ____________________. constant. K can be found by finding
the concentration fraction, also known as the reaction quotient. While
the concentration fraction can be ____________________. for any mixture of
reactants and products, it is only at equilibrium that it has a constant
value.
K = [products]
[reactants]
From studies of equilibria, chemists have found that:
–Different chemical reactions have ____________________. values of K
–For a particular reaction, K is constant for all equilibrium mixtures at a
fixed ____________________..
The equilibrium constant gives us an indication of the extent of the
reaction. For values of K that are between 10-4 and104, there will be
significant amounts of reactants and products present at equilibrium.
For values of K that are ____________________. (>104), the equilibrium
mixture consists mostly of ____________________. with small amounts of
reactants. For values of K that are ____________________. (<10-4) the
equilibrium mixture will consist mostly of ____________________. with small
amounts of products.
The value of K, for a particular reaction, depends only on
____________________.. It is not affected by addition of reactants or products,
changes in pressure or the use of catalysts. As temperature increases:
–For ____________________. reaction, the amount of products decreases so
the value of K ____________________..
–For ____________________. reactions, the amount of products increases and
so the value of K ____________________..
The equilibrium position of a reaction may be changed by:
–____________________. or removing a reactant or product
–Changing the ____________________. by changing the volume (equilibria
involving gases)
–____________________. (for equilibria in solution)
–Changing the ____________________
Suppose a vessel contains an equilibrium mixture represented by the
equation:
________________________________________________________________________
If extra nitrogen gas were added to the vessel without changing the
volume or temperature, the mixture would momentarily not be in
equilibrium. The following events occur as the composition of the
mixture adjusts to return to the equilibrium:
–The ____________________concentration of nitrogen gas causes the rate of
the forward reaction to increase and more ammonia is formed.
–As the ____________________of the ammonia increases, the rate of the back
reaction to reform N2 and H2 increases.
–Ultimately the rates of the forward and back reaction become equal
again and a new ____________________is established.
When equilibrium is re-established the concentration of all substances
have changed. The overall effect of ____________________nitrogen is to
increase the concentration of ammonia at equilibrium – a net forward
reaction. Addition of more NH3, increases the rate of the back reaction.
To summarise:
–____________________of a reactant leads to the formation of more products
(a net ____________________reaction).
–Addition of a ____________________leads to the formation of more reactants
(a net ____________________reaction).
Le ____________________Principle: if an equilibrium system is subjected to a
change, the system will adjust itself to partially oppose the effect of the
change.
The ____________________of gases in an equilibrium mixture can be changed
by increasing or decreasing the ____________________of the container while
keeping the temperature constant.
In general the effect of a change in ____________________, by changing the
container volume, depends on the relative ____________________of gas
particles on both sides of the equation.
The total ____________________of an equilibrium mixture of gases may also
be changed by adding a non-reacting gas such as helium, neon or argon.
Despite the increase in total pressure that occurs when this gas is
added, the concentration of the individual chemicals involved in the
equilibrium are not affected by the presence of the extra gas. The
system therefore stays in ____________________and there is no net
____________________or back reaction.
The effect of ____________________a solution by adding water is similar to
changing the volume in gaseous equilibria. Where possible, a net
reaction occurs in the direction that produces the greater
____________________of particles. For example, dilution of a solution
containing the equilibrium system:
Fe3+(aq) + SCN-(aq)
2 particles in solN

Fe(SCN)2+(aq)
1 particle in solN
Results in an ____________________in the amount of Fe3+ and SCN- ions. In
terms of Le Chatelier’s principle, a net back reaction increases the total
concentration of particles in solution, offsetting the effect of dilution.
Despite the net back reaction, the concentration of Fe3+ and SCN- at the
new ____________________will be lower than their concentration prior to the
dilution as the equilibrium shift partially offsets the change.
____________________increases the energy of the substances in the mixture.
Applying Le Chatelier’s Principle to an ____________________reaction, the
reaction opposes an increase in energy by removing energy – that is a
net ____________________reaction occurs. For an ____________________reaction,
net forwards reactions occur as the temperature rises. An
____________________in temperature in an equilibrium mixture reaction
results in:
–A net ____________________reaction (less products) for exothermic
reactions
–A net ____________________reaction (more products) for endothermic
reactions.
____________________increase the rate of reactions. It has been shown that
they increase the rate of forward and back reactions equally. As a
consequence, the presence of catalyst does not change the
____________________of equilibrium. A catalyst may greatly
____________________the rate at which equilibrium is reached.
Many reactions can be regarded as continuing until they are complete.
These include:
–Reactions that produce ____________________such as gases that escape
from the reaction mixture as they are formed. Continual loss of products
drives these reactions ____________________.
–Reactions that form ____________________in which only minute quantities
of reactants are present.
Reactants
Products
Add reactants:

Add products:

Increase pressure:
Eg 2 reactant particles
particle
1 product

Eg 1 reactant particle
2 product
particles

Increase
temperature:
Exothermic
reactions

Endothermic
reactions

Add a catalyst:
Does not alter the extent of a reaction