strong acid-strong base titrations?

Question

WHy do strong acid-strong base titrations curves look different in shape than weak acid-strong base titration curves?

Answer 1

These are the ones that are virtually symmetrical about the pH = 7 point. They also have the longest vertical portion at the equivalence point. They don't have a buffer region.

Answer 2

Think about what you're adding together. For the sake of simplicity, let's define an acid as anything that gives a proton to a solution, and a base as anything that gives an OH to a solution. Acids and bases are termed "strong" or "weak" based on their ability to give away their H+ or OH- groups. A strong acid gives away its H+ as soon as it comes into contact with water, and cannot take it back. A strong base steals and H+ from the water, and makes an OH- in solution, but is really bad about returning the OH-. A weak acid gives H+, but it can also take it back in a reversible reaction. A weak base takes an H+ to make OH-, but can also give backs its H+ in a reversible reaction. Every weak acid has a twin that is a weak base, just missing its proton. So, if you put strong and strong, they react violently. If you put in an equal number of moles of each, the moles of H+ and the moles of OH- will get together and make a mole of water. So, every acid molecule and every base molecule have a partner, the pH will be neutral (7) 'cause they've cancled everything else out. With a strong/weak titration, things go a little bit differently. The two are essentially the same, though, in principle, and work by the same mechanism. Let's look at an strong base/weak acid titration, 'cause they're really common. As you add the strong base to the mixture, it pulls H+ away from the acid, and makes the conjugate base. But, that weak base pulls H+ from water, and makes OH-! So, when you have neutralized all the acid in the mixture, your final pH will actually be a little bit higher than 7 because of the equilibrium. When you have a weak acid and a strong base, the final pH is lower than 7. When you do a weak/strong titration, you can find the pKA, or the pH at which the concentration of acid/base conjugate pair twins are equal. This happens at the pH which corresponds to exactly half the volume of acid added. If you're in a lab working with a burette and color indicator, the point can sometimes be hard to find. Colorimetric titration is an art-form. If you have a pH meter, make a plot of pH vs. volume added. Then, take the second derivative of the line. The second derivative of this plot is 0 at the equilivance point.