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.
