Download Reversible reaction

Tuesday, April 5th: “A” Day
Agenda
ACT sample test #3
Begin chapter 14: “Chemical Equilibrium”
Homework:
Section 14.1 review, pg. 501: #1-5
Concept Review: “Reversible Reactions and
Equilibrium”
Be ready for a quiz covering section
14.1 next time!
Completion Reactions
If enough oxygen gas is provided for the
following reaction, almost all of the sulfur will
react:
S8(s) + 8 O2(g) → 8 SO2(g)
Reactions such as this one, in which almost all
of the reactants react, are called completion
reactions.
Reversible Reactions
In other reactions, called reversible reactions,
the products can re-form reactants.
Reversible reaction: a chemical reaction in
which the products re-form the original
reactants.
Another way to think about reversible
reactions is that they form both products and
reactants.
Reversible Reactions Reach Equilibrium
One reversible reaction occurs when you mix
solutions of calcium chloride and sodium
sulfate:
CaCl2(aq) + Na2SO4(aq) → CaSO4(s) + 2 NaCl(aq)
Because Na+ and Cl- are spectator ions, the net
ionic equation best describes what happens.
Ca 2+ (aq) + SO4 2- (aq)
CaSO4 (s)
Reversible Reactions Reach Equilibrium
Solid calcium sulfate, the product, can break
down to make calcium ions and sulfate ions in
a reaction that is the reverse of the previous
one.
CaSO4 (s)
Ca2+ (aq) + SO42- (aq)
Use arrows that point in opposite directions
when writing a chemical equation for a
reversible reaction.
Ca 2+ (aq) + SO4 2- (aq)
CaSO4 (s)
Chemical Equilibrium
The reactions occur at the same rate after the initial
mixing of CaCl2 and Na2SO4.
The amounts of the products and reactants do not
change.
Reactions in which the forward and reverse reaction
rates are equal are at chemical equilibrium.
Chemical Equilibrium: a state of balance in which
the rate of a forward reaction equals the rate of the
reverse reaction and the concentrations of products
and reactants remains unchanged.
Opposing Reaction Rates are Equal at Equilibrium
The reaction of hydrogen, H2, and iodine, I2, to
form hydrogen iodide, HI, reaches chemical
equilibrium.
H2 (g) + I2 (g)
2 HI (g)
At first, only a very small fraction of the
collisions between H2 and I2 result in the
formation of HI.
H2(g) + I2(g) → 2 HI(g)
Opposing Reaction Rates are Equal at Equilibrium
After some time, the concentration of HI goes
up.
As a result, fewer collisions occur between H2
and I2 molecules, and the rate of the forward
reaction drops.
Similarly, in the beginning, few HI molecules
exist in the system, so they rarely collide with
each other.
Opposing Reaction Rates are Equal at Equilibrium
As more HI molecules are made, they collide
more often and form H2 and I2 by the reverse
reaction.
2 HI(g) → H2(g) + I2(g)
The greater the number of HI molecules that
form, the more often the reverse reaction
occurs.
Rate Comparison for H2(g) + I2(g) 2 HI(g)
Opposing Reaction Rates are Equal at Equilibrium
When the forward rate and the reverse rate are
equal, the system is at chemical equilibrium.
If you repeated this experiment at the same
temperature, starting with a similar amount of
pure HI instead of the H2 and I2, the reaction
would reach chemical equilibrium again and
produce the same concentrations of each
substance.
Chemical Equilibria are Dynamic
If you drop a ball into a bowl, it will bounce.
When the ball comes to a stop it has reached
static equilibrium, a state in which nothing
changes.
Chemical equilibrium is different from static
equilibrium because it is dynamic.
In a dynamic equilibrium, there is no net change
in the system.
Two opposite changes occur at the same time.
Chemical Equilibria are Dynamic
In equilibrium, an atom may change from being
part of the products to part of the reactants many
times.
But the overall concentrations of products and
reactants stay the same.
For chemical equilibrium to be maintained, the
rates of the forward and reverse reactions must
be equal.
Arrows of equal length also show equilibrium.
Reactants
Products
Chemical Equilibria are Dynamic
In some cases, the equilibrium has a higher
concentration of products than reactants.
This type of equilibrium is also shown by using
two arrows.
Reactants
Products
The forward reaction has a longer arrow to
show that the products are favored.
Another Example of Equilibria
Even when systems are not in equilibrium, they are
continuously changing to try to reach equilibrium.
For example, combustion produces carbon dioxide,
CO2, and poisonous carbon monoxide, CO. As CO and
CO2 cool after combustion, a reversible reaction
produces soot, solid carbon.
2 CO(g)
C(s) + CO2(g)
This reaction of gases and a solid will reach
chemical equilibrium.
Equilibria can involve any state of matter, including
aqueous solutions.
Equilibria Involving Complex Ions
Complex ion, or coordination compound: the
name given to any metal atom or ion that is
bonded to more than one atom or molecule.
Ligands: a molecule or anion that readily
bonds to a metal ion. (Ex: NH3, CN-)
Complex ions may be positively charged
cations or negatively charged anions.
(Remember, in order to be an ION, an atom or
group of atoms has to have a CHARGE.)
Equilibria Involving Complex Ions
In this complex ion, [Cu(NH3)4]2+, ammonia
molecules bond to the central copper(II) ion.
What is the
ligand in this
complex ion?
NH3
Equilibria Involving Complex Ions
 Complex ions formed from transition metals
are often deeply colored.
Equilibria Involving Complex Ions
The charge on a complex ion is a sum of the
charges on the species from which the complex
ion forms.
For example, when the cobalt ion, Co2+,
bonds with four Cl− ligands, the total charge
is (+2) + 4(−1) = −2.
Metal ions and ligands can form complexes that
have no charge. These are not complex ions.
Why not?
Complex ions often form in systems that reach
equilibrium.
Equilibria Involving Complex Ions
Consider zinc nitrate dissolving in water:
Zn(NO3)2 (s)
Zn2+ (aq) + 2 NO3- (aq)
In the absence of other ligands, water molecules
bond with zinc ions. So, this reaction can be
written:
Zn(NO3)2(s) + 4H2O
[Zn(H2O)4]2+ (aq) + 2NO3- (aq)
(complex ion)
Equilibria Involving Complex Ions
If another ligand, such as CN−, is added, the new
system will again reach chemical equilibrium.
Both water molecules and cyanide ions “compete”
to bond with zinc ions, as shown in the equation
below.
[Zn(H2O)4]2+(aq) + 4CN-(aq) [Zn(CN)4]2-(aq) + 4H2O(l)
All of these ions are colorless, so you cannot see
which complex ion has the greater concentration.
Equilibria Involving Complex Ions
In the chemical equilibrium of nickel ions,
ammonia, and water, the complex ions have
different colors.
You can tell which ion has the greater
concentration based on color:
[Ni(H2O)6]2+(aq) + 6NH3(aq)
Green
[Ni(NH3)6]2+(aq) + 6H20(l)
Blue-violet
The starting concentration of NH3 will determine
which one will have the greater concentration.
Homework
Section 14.1 review, pg. 501: #1-5
Concept Review: “Reversible Reactions and
Equilibrium”
Quiz over this section next time…