Acids And Bases

What is an Acid and A Base? (SL)

An acid is generally defined as a substance that dissociates to form H+ ions, and a base as a substance that dissociates to form OH- ions. There are 2 main theories you need to know for IB, Bronsted-Lowry and Lewis, but to understand these, you need to meet a new ion, the hydronium ion.

In GCSE you were lied to. Unfortunately, the H+ ion does not really exist. For it to exist, water would need to spontaneously just break apart, and that doesn't really make sense.

It makes much more sense for the hydronium ion to exist, and the dissociation of water to look like this

This way, it's just a collision between 2 water molecules that results in a hydrogen (proton) swapping waters.

​Now, on to our acid base theories

Bronsted - Lowry (SL)

The Bronsted-Lowry theory of Acids and Bases says that an Acid is a proton donor. Here's an example of an acid 'donating' a proton.

Here, the HCl molecule donates its proton (hydrogen) to water to become Cl-.
A base is defined as a proton acceptor in the Bronsted Lowry theory of acids and bases. Here, NH3 accepts a proton from water to form NH4+ and OH-

Lewis (HL)

The Lewis theory of acids and bases defines an acid as an electron pair acceptor, and a base as an electron pair donor. There are a couple of examples of this but they're annoyingly complicated and confusing (mostly because I haven't learnt bonding yet) so I'm going to leave them out because I suggest you just memorise those definitions.

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The pH Scale (SL)

pH stands for 'power of hydrogen', and is calculated using only the concentration of H3O+ ions. It is a logarithmic scale, the pH is the number you raise 10 to to get the H3O+ concentration, with the opposite sign. This sounds complicated but it's actually quite simple, to get the H3O+ concentration, just raise 10 to the negative of the pH. If the pH is 3 then the H3O+ concentration is 10 to the power of -3 mol/dm^3

And yes, there is a formula for it

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Strong and weak acids and bases (SL)

In Chemistry (the superior science), there are such things as strong and weak acids. Strong and weak often get confused with concentrated and dilute - they are NOT the same thing!!! While concentrated and dilute relate to the amount of your acid or base in a certain amount of water, strong and weak relate to the extent of dissociation of acids and bases in water.

A strong acid or base fully dissociates in water
A weak acid or base does not fully dissociate in water

The following is an example of a strong acid. Here, H+ is used to simplify the equation.

​Hydrochloric acid is a strong acid as it fully dissociates into H+ (H3O+) and Cl- in water, as can be seen by the use of the arrow in the equation, suggesting that the equation goes to completion.

Ammonia is a weak acid because it doesn't fully dissociate, as can be seen by the equilibrium, showing that the reaction never goes to completion.

How can we tell the difference between strong and weak acids and bases of the same concentration? (SL)

Here's a situation. Let's say you add 1 mole of substance X into water, and substance X fully dissociates in water to form a strong acid. The amount of H3O+ ions in the water once it is all dissolved will be 1 mole. Now imagine you dissolve 1 mole of substance Y into water, and substance Y doesn't fully dissociate in water and forms a weak acid. What will the amount of H3O+ ions in water be? It will definately be less than 1 mole (in fact, the amount is different for every weak acid).
From this, we can figure out some ways to distinguish between them. The concentration of H3O+ ions is related to pH, the more H3O+ ions the lower the pH. So one way to distinguish between a weak and a strong acid is to test the pH with a pH meter or an indicator (usually universal indicator).
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We can deduce even more ways from this. How does pH affect rate of reaction? The further away the pH is from 7, the faster the reaction will take place. So another way to distinguish between them would be to add a metal to each and see which would react quicker.
Another way? Yep! As we discussed before, a strong acid dissociates fully while a weak acid doesn't. How does this affect the amount of ions in each solution? The strong acid will have more right? Great! there's a way we can measure that - with a conductivity meter!

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Oh No, what are conjugate acids and bases? It sounds complicated and scary (SL)

They're not too bad. They come in pairs, each acid has a conjugate base and each base has a conjugate acid. These pairs always differ from each other by a proton.

In this reaction of HCl with water, there are 2 conjugate pairs, and as I said earlier, they differ only by a proton.

Relative strengths of conjugate acids and bases (SL)

Again using HCl as an example, let's have a look at the relative strengths of conjugate pairs

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Water is a much stronger base than the chloride ion, meaning it has a much higher tendency to accept a proton. This can also be described as the water molecule having a much stronger pull on the proton than the chlorine, so when the chlorine ion and water go head to head for a proton, the water wins because it is a much stronger conjugate base. Because the water wins the fight for the proton 99.99999 times out of 100, you can see why HCl is a strong acid and why it fully dissociates. It's a different story for weak acids and bases.

Here are the conjugate pairs in the dissociation of ammonia. When ammonia (NH3) and the OH- ion go head to head for a proton, it's not so obvious who will win, instead of ammonia winning it 99 times out of 100, it could be 60, or 50, or 40 times out of 100. This means that a weak acid never fully dissociates.

THERE'S a way to know how much dissociates (HL)

Ka (constant of dissociation of acid) and Kb (constant of dissociation of base) values tell us how much acid and base dissociate. Each acid or base has a different number - it's a bit like an equilibrium constant.

 How do they calculate the value of Ka and Kb? (HL)

There are 2 different formulas, 1 for Ka and one for Kb.

​If the chemical equation for the dissociation of your weak acid looks something like this : 

Where HA is your acid and A- is your conjugate base, then the equation for Ka is:

Keep in mind that the square brackets mean the concentration of, so [HA] for example means the concentration of HA.

Remember the positions of the positive and negative ions in this equation. This will help you remember where everything goes in the equation

The equation for Kb is quite similar, if the chemical equation for the dissociation of your weak base looks something like this:

Here, B is your weak base and BH+ is the conjugate acid. The Kb equation is:

Again take note of the position of the positive and negative ions here. They're in the same positions as the Ka formula. This is a nifty way to remember which way around they go is by remembering that the + ion goes in front, and the - ion goes on top because it applies to both rules.

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Kw (SL)

Kw is the ionic product of water. Remember before we said that water dissociates in a reversible reaction like this?

Well let's calculate the equilibrium constant for this. Products over reactions etc - we get

Because the equilibrium lies so far to the left, and so little water is dissociated, the concentration of water changes so little that it can be ignored and taken out of the equation which gives us the following equation;

At 298K, the concentrations of H3O+ and OH- ions are both 10 to the power of -7, so Kw is 10 to the power of -14.

Kw will always be 10 to the power of -14 at 298K, but it changes at different temperatures. At a higher temperature, more water dissociates.

pKa, pkb and pkw (HL)

To make life easier, you can put a p in front of some of your numbers. But p isn't a constant. Let me explain. 

pKa for example is the negative of the power you raise 10 by to get Ka. For example, if Ka is 10 to the power of negative 6, then PKa is 6. It just makes it easier to compare acid and base strengths.

There are some really handy things you can do with these calcs, like this:

We know from before that Kw is 10 to the negative 14, so pKw must be 14. So pKa + pKb = 14. This is so so useful. Never forget this!!!

Buffer solutions

Buffer solutions are the coolest! They're solutions that resist changes in pH. To make them you have to get a certain amount of a weak acid with a dissociation formula like this

Still editing this part. Come back later to see if i've updated it

PH curves (HL)

So yeah here's the graph for a strong acid and strong base titration.

Notice that the gradient of the curve before 45 and after 55 ml is very small (it's very flat). This is a way you can tell between a strong and weak acid or base.

Next let's look at a weak acid and strong base pH curve

Now you see that the equivalence point has been shifted up, and the gradient of the graph before the equivalence point isn't as small as before (it's less flat). This is where the buffer zone is. Another thing to note is that when the weak acid has been half neutralised, pH = pKa.

There are of course different combinations of strong and weak acids and bases, and you should be able to see which is which from the pH curve. Whichever is weak will have a less steep gradient, and the equivalence point will be further from it. For example, a weak base will have the equivalence point lower than 7 (further away from base pH) and the gradient will be steeper on the base side of the equivalence point. I'm not very good at explaining so it's diagram time.

Chemical Indicators (HL)

Here's a video I made last year explaining how they work. It's just a very simple equilibrium!

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Calculations:

I'm going to make a video on how to do various calculations in this topic, and I'll put it here for you.

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And I'm pretty sure that's it!

I hope this is useful, let me know in the comments if I've missed anything or anything is unclear. Sorry this is so late!