3.05 Strong and Weak Bases

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Из курса от партнера University of Kentucky

Advanced Chemistry

388 оценки

University of Kentucky

Advanced Chemistry

388 оценки

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.

Из урока

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.

3.01 Acids and Bases9:15

3.02 pH and Kw13:32

3.02a Finding Kw2:10

3.03 Acid Strength6:34

3.04 Finding pH10:25

3.05 Strong and Weak Bases12:51

3.05a Calculating equilibrium concentrations of a weak base, part 12:51

3.05b Calculating equilibrium concentrations of a weak base, part 21:36

3.06 Ions as Acids and Bases18:15

3.06a Acid-base properties of aqueous salts, part 11:43

3.06b Acid-base properties of aqueous salts, part 21:07

3.06c Acid-base properties of aqueous salts, part 34:13

3.07 Types of Acids12:06

3.08 Polyprotic Acids4:46

Познакомьтесь с преподавателями

Dr. Allison Soult
Lecturer
Chemistry
Dr. Kim Woodrum
Sr. Lecturer
Chemistry

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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