A chemistry course to cover selected topics covered in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society.
Prerequisites: Students should have a background in basic chemistry including nomenclature, reactions, stoichiometry, molarity and thermochemistry.
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Acid-Base Equilibria
The concept of equilibrium is applied to acid and base solutions. To begin, the idea of weak acids and bases is explored along with the equilibrium constants associated with their ionization in water and how the value of the equilibrium constant is associated with the strength of the acid or base. The autoionization of water is discussed and how temperature affects this process. A variety of problem types are covered including calculations of pH, pOH, [OH-], and [H+] for both strong and weak acids and bases.
Aqueous salt solutions are classified as acids and bases and the multi-step ionization of polyprotic acids is discussed. Finally, the concept of Lewis acids and bases is discussed and demonstrated through examples.
In this module we're going to look at strong and weak bases
in many of the same ways that we look at strong and weak acids in the previous modules.
Our objective is to be able to write reactions associated with the
dissociation of both strong and weak bases
as well as to calculate the pH and pOH values
of the week and strong base solutions. Just like
Just like we talked about strong acids and weak acids being strong and weak electrolytes
we can talk about bases in the same way.
Strong bases are strong electrolytes, they dissociate completely in water
such as what we see here with KOH. If I prepare solution a potassium hydroxide
when I look at the solution
what I see are potassium ions and hydroxide ions.
Not KOH molecules. Weak bases
are weak electrolytes. Ammonia NH_3
is one of the most common examples of a weak base. When I put NH_3 in water
what I get are
primarily NH_3 molecules in solution
and a small amount of NH_4+ ions and hydroxide ions.
We can look at this list to look at some examples
and make some generalizations about the type of compounds that are strong bases
and those that are week bases. Shen I look at the hydroxide compounds here we
have lithium, sodium, calcium, we've already seen potassium hydroxide
but there are others. These are our strong bases
When I put those compounds in water they completely dissociate. So I end up with
my ions present in solution and none of the original compound.
When I put those compounds in water they
completely dissociate. So I end up with
my ions present in solution and none of the original compound.
When I look at my week basis, they're kinda two kind of cases of week bases that I see.
my ions such as carbonate and bicarbonate ions.
and amean base compounds
such as ammonia NH_3 or pyridine.
when I look at the K_b values
have those weak bases. I see that the K_b values differ.
So if I look at the K_b values the higher the K_b value is
the stronger the base. So the 4 listed in this table, carbonate
will be my strongest base because it has the highest K_b value.
pyridine has the lowest K_b value
and therefore will be the weakest base up these four.
So let's look at an example of how we can figure
out what the pH is of a solution of a strong base.
For a strong base we can assume complete dissociation.
This lets us write the reaction as calcium hydroxide
completely dissociating or ionize into the calcium and hydroxide ions.
Note that we do have a 2 as a coefficient in front of the hydroxide
because we need a balanced chemical equation.
Now I need to look at how much hydroxide I'm going to have in this particular solution.
If my concentration of calcium hydroxide is equal to 0.250 molar
Now I need to look at how much hydroxide
I'm going to have in this particular solution.
If my concentration of calcium hydroxide is equal to 0.250 molar
Then what I find is that my hydroxide concentration is actually twice that.
Because for every one unit of calcium hydroxide
I end up with two units hydroxide ions.
So I take my 0.250 multiply it by 2
and get 0.5000 molar for my concentration of hydroxide.
Now I can used what I learned in an earlier module
about finding pOH and say that pOH is equal ^ negative log the OH-
concentration which in this case is 0.500.
And my pOH is going to be equal to 0.301.
I can then find the pH, because I know I'm a 25 degree Celsius
so I know that pH plus pOH is equal to 14.
I plug in there pOH value of 0.301
and I end up with a pH equal to 13.699.
When we look at the amine base compounds we can see a variety of options.
We see our ammonia which has 3 hydrogen groups.
We see ethyl amine which has it ethyl group which is that 2 carbon group
and two hydrogens or we can see something such as a pyridine
which is a ring of carbon atoms. All these are considered weak bases.
Let's look at an example of how we use this. Aniline is a weak base
the K_b value given as 3.8 x 10 ^10. Let's look at an example of how we use
this. Aniline is a weak base
the K_b value given as 3.8 x 10 ^10.
And we're looking to find the hydroxide concentration of 0.20
molar aniline solution. Sow I'm not worried about what the specific formula
is for aniline.
I know an animen base compound but more importantly I know it's behaving as
a weak base.
So I'm going to use the abbreviation B to represent that weak base.
So I have B standing for base, plus water
and I know that bases are going to act as a proton acceptor
so I am going to make my product BH+
plus OH-. My law mass action then becomes BH plus
times hydroxide concentration divided by my base concentration
at equilibrium. I don't include the water because it's in the liquid phase in pure
liquids are not included in the law of mass action.
Now to solve this what I want to look at is setting up a ICE Table.
I know that I'm going to have an initial concentration
a change in concentration and an equilibrium concentration.
My initial concentration of my bass is given as 0.200 molar.
For water I don't worry about that it doesn't have a concentration because it's a pure substance.
For BH+initially is zero for OH-
is initially 0. My change with be -X
plus X and plus X.
My equilibrium concentration is 0.200 - X. X and X.
Now, I can use the information I'm given
to solve for my value of the OH- concentration.
If I look at my law of mass action, I can substitute in X for both BH+
and for OH- so I have X^2
over 0.200 - X.
And this is equal to our K_b value of 3.8 x 10 ^ -10.
I can make the same simplifying assumption that I did an earlier modules
and say that X is much much less than 0.200.
Given the value of the K_b and that its 10 ^-10
this is going to be a very safe assumption. Such a small K_b value
indicates that the reactants are heavily favored.
So now I have 3.8 x 10 x 10 ^ -10
equals X squared over 0.8200.
When I solve for X^2 I where to end up with X squared
equals 7.6 x 10 ^ -11.
I take the square root of that
and I find that I get X equal to
8.7 x 10 ^ -6.
It is that X value, which is also my
hydroxide concentration.
Now by knowing this information I can then find the pOH of
the solution but I can also find the pH or
the H+ ion concentration by using the calculations we used with the K_q value.
Now that we have the OH- concentration we can look at a problem
that will allow us to find the pH of that solution.
You should have gotten a pH 8.94.
Now we can use that OH- concentration that we found earlier
which was 8.7 times 10 ^ -6 molar.
And we can take that information and find the pH.
We can take the pOH
first which is the negative log
of 8.7 x 10 ^ -6.
So pOH will be equal to
5.06 and pH
plus pOH which is equal to 14
so I know that pH plus
5.06 is equal to 14
giving me a pH value of 8.94.
This makes sense because we're talking about a weak base
giving me a pH value of 8.94.
This makes sense because we're talking about a weak base
we are expecting a pH greater than 7
which is what we see for a basic solution.
If we chose we can also take are hydroxide concentration
and say that K_w is equal to H+ concentration
times the OH- concentration
and we get 1.0 x 10 ^-14 equals
are H+ concentration x what we calculated which is 8.7 x 10 ^ -6.
From this we can get our H+ concentration
which is 1.1 x 10 ^ -9.
And I can then take the negative log of that value
and get that we have 8.94 as our pH.
Which is what we would expect since we are calculating the pH of the same solution.
Now we can look at the relative strength have an acid and its conjugate base.
What we find is that the weaker the acid the stronger its conjugate base.
When we look at our spectrum acid here
when we get to something down here like NH_4+ or
HCO_3- we see that our conjugate
bases of these substances are on the strong in of the bases.
When we look at a stronger acid we see
the weaker its conjugate base is, so for HCl
When we look at a stronger acid we see
the weaker its conjugate base is, so for HCl
we see its conjugate base Cl- is actually so weak that it's considered
neutral. One place where this will come into play is when we start to look at salts
and trying to determine the acidity or basicity
of a particular salt. Let's look at an example to determine which
anion is the weakest base. Here were given
4 anions and we are 4 k values.
But note that we have three K_a value and 1
K_b value.
For an answer we actually get something that such a weak base that it is in fact considered
neutral. When we look at this
what we want to look for is the substance that is the strongest
conjugate acid
because we want to find the weakest base. So in comparing our acids
HCl, HF and HCN we see that HCl is the strongest
acid, because it has a K_a value much much greater than 1
and so Cl- will be the weakest base.
Remember that the stronger the acid
the weaker the conjugate base.
In the next module we are going to look at the acidity and basicity of ions
and how they behave as salts.
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Dr. Allison SoultLecturer
Dr. Kim WoodrumSr. Lecturer
