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. 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 itStrong 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 |
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