Transcript Chemistry: Matter and Change

Section 17.1 A State of Dynamic Balance
• List the characteristics of chemical equilibrium.
• Write equilibrium expressions for systems that are at
equilibrium.
• Calculate equilibrium constants from concentration
data.
What is equilibrium?
• Chemical reactions often reach a balancing point, or
equilibrium.
•
N2 (g) + 3H2 (g)
 2NH3 (g)
∆G° = -33.1 kJ
• The concentrations of the reactants ( H2 and N2 )
decrease at first, while the concentration of the product
(NH3) increases. Then, before the reactants are used up,
all concentrations become constant.
What is equilibrium? (cont.)
• A reversible reaction is a chemical reaction that can
occur in both the forward and reverse directions, such
as the formation of ammonia.
• Forward: N 2(g) + 3H2(g) → 2NH3(g)
• Reverse: N2 (g) 3H2 (g)  2NH3 (g)an office.
N2(g) + 3H2(g) ↔ 2NH3(g)
The reactants in the forward reaction are on the left of the
arrows. The reactants in the reverse reaction are on the
right of the arrows. In the forward reaction, hydrogen and
nitrogen combine to form the product ammonia. In the
reverse reaction, ammonia decomposes into the products
hydrogen and nitrogen.
What is equilibrium? (cont.)
• The reaction reaches equilibrium in
figure d.
• Study the diagrams to answer the
following questions. In a, how do you know
that the reaction has not yet begun?
• Only reactants are present.
•
• In b, what evidence indicates that the
reverse reaction has begun?
• More than two ammonia molecules are
present, so they can react in the reverse
direction
• Compare c with d. How do you know that
equilibrium has been reached?
• The number of reactant and product
molecules are the same in c and d, so
equilibrium is established.
What is equilibrium? (cont.)
• Chemical equilibrium is a state in which
the forward and reverse reactions balance
each other because they take place at
equal rates.
• Equilibrium is a state of action, not inaction.
• Radioactive iodine molecules in the solid in
the flask on the right are separated from
nonradioactive iodine in the flask on the left.
Note the readings on the radiation monitors.
b. After the stopcock has been open for a
time, the radiation monitors show that
radioactive molecules are in both flasks.
The particles must have moved back and
forth between the flasks and between the
solid and the gaseous phases.
• the constancy does not mean that the
reactions have stopped, but that a state of
equilibrium has been established
Equilibrium Expressions (cont.)
• The law of chemical equilibrium states
that at a given temperature, a chemical
system might reach a state in which a
particular ratio of reactant and product
concentrations has a constant value.
• The Equilibrium Constant Expression
Equilibrium Expressions (cont.)
• The equilibrium constant, Keq, is the
numerical value of the ratio of product
concentrations to reactant concentrations,
with each concentration raised to the
power equal to its coefficient in the
balanced equation.
• The value of Keq is constant only at a specified
temperature.
Equilibrium position:
Keq > 1: Products are favored at equilibrium
Keq < 1: Reactants are favored at equilibrium
Equilibrium Expressions (cont.)
H2(g) +I2(g) ↔ 2HI(g)
• This reaction is a homogeneous
equilibrium, which means that all the
reactants and products are in the same
physical state.
Equilibrium Expressions (cont.)
• When the reactants and products are
present in more than one physical state,
the equilibrium is called a heterogeneous
equilibrium.
• Ethanol in a closed
flask is represented
by C2H5OH(l) ↔
C2H5OH(g).
Equilibrium Constants
• For a given reaction at a given
temperature, Keq will always be the same
regardless of the initial concentrations of
reactants and products.
• Equilibrium characteristics:
• To attain equilibrium, a reaction mixture
must be in a closed container, it must be
at constant temperature, and all reactants
and products must be present
Q1: Explain the meaning of a double arrow
in chemical equations.
• The double arrow means that a reaction is
at equilibrium and that the forward reaction
and the reverse reaction are occurring at
the same rate.
Q2:Explain how the size of the equilibrium
constant relates to the amount of product
formed at equilibrium.
• The larger the equilibrium constant the greater
the amount of products formed at equilibrium.
• Q3:Compare homogeneous and
heterogeneous equilibria.
• In a homogenous equilibrium, all the reactants
and products are in the same physical state. In
a het- erogeneous equilibrium, reactants and
products are in different physical states.
• Q4:List three characteristics a reaction
mixture must have if it is to attain a state of
chemical equilibrium.
• To attain equilibrium, a reaction mixture
must be in a closed container, it must be
at constant temperature, and all reactants
and products must be present
Section 17.1 Assessment
A reaction is in equilibrium when:
A. there are more products than reactants
B. the amount of products equals
the reactants
D
C
0%
A
D. the rate of the forward and
reverse reactions are equal
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. the rate of the forward reaction
is greater than the reverse
reaction
Q5:Interpret Data The table below shows the
value of the equilibrium constant for a reaction
at three different temperatures. At which
temperature is the concentration of the products
the greatest? Explain your answer.
• 373 K; The products are in the numerator of the
ratio, so the larger the Keq is, the greater the
concentration of the products.
Section 17.1 Assessment
The value of the equilibrium constant is
constant for a given ____.
A. temperature
B. pressure
D
A
0%
C
D. density
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. volume
A change in ____ alters both the
equilibrium position and the equilibrium
constant.
A. pressure
A
0%
D
D. density
C
C. volume
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. temperature
A(n) ____ equilibrium is when all reactant
and products are in the same physical
state.
A. endothermic
A
0%
D
D. homogeneous
C
C. heterogeneous
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. exothermic
Which is NOT an example of a
homogeneous equilibria?
A. H2(g) +I2(g) ↔ 2HI(g)
B. 2H2(g) +O2(g) ↔ 2H2OI(g)
D
A
0%
C
D. C2H5OH(l) ↔ C2H5OH(g)
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. HCl(aq) + NaOH(aq) ↔
NaCl(aq) + H2O
The state in which the forward and
reverse chemical reactions take place at
the same rate is called ____.
A. Le Châtelier’s Principle
A
0%
D
D. chemical equilibrium
C
C. Charles’s Law
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. Henry’s Law
Which does NOT describe a system that
has reached chemical equilibrium?
A. No new product is formed by the
forward reactions.
D. All the reactants have been used up.
D
A
0%
C
C. The concentration of products
is equal to the concentration
of reactants.
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. The forward and reverse
reactions occur at equal rates.
A(n) ____ equilibrium is when all reactant
and products are present in more than
one physical state.
A. endothermic
A
0%
D
D. homogeneous
C
C. heterogeneous
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. exothermic
End of section 17.1