Learn about Senior Chemistry, Acids and Bases 8, in this comprehensive video by bannanaiscool.
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Rob Lederer: Here is the technique for writing Brönsted-Lowry equations, when you've given a mixture of acids and bases together. So, here is the question. Nitric acid, which is HNO3, reacts with sodium hydroxide, which is NAOH. Now, a simple double replacement reaction would be HNO3 plus NAOH, that will make HOH and NANO3, but you know what, we don't like to write that, because it's not a net ionic equation. Remember the redox unit, we like net ionic equations now. So, Brönsted-Lowry will help us to do that, when we take all the chemicals that we have and make a list of them and try to find from that list the strongest acid and the strongest base to react together. Now, here is something that looks kind of funny. Nitric acid when you dissociate, that forms an H positive and NO3 negative, but you know what? We know that H positive goes into water. It attaches itself to the polar water molecule to become H3O positive. So, when every one of strong acid in your list of chemicals, you know that strong acids make hydronium 100% of the time, so we just write hydronium. Strong acids, we'd write as hydronium, and then the negative ion, which is NO3 negative, we also write in the list of chemicals; strong acids hydronium. Then NaOH dissociates into Na positive and OH negative, and there is always water present in the reaction. It maybe one of the strong chemicals, you have to write it in there, and then determine. Now, you would then use an acid base data chart, if you have one, to be able to determine the strongest acid, which is usually the highest one on the left hand side where the acids are, because acids usually go up in strength on these charts from low Kas to high Ka values. So, the higher you are on the left, reacts with the lowest one on the right because the conjugate base ion of the strong acid is a very weak base, but the conjugate base ion to a very weak acid is quite a strong type of weak base, if that make sense. So, here is the thing. The highest on the left that you are ever going to get is hydronium, because hydronium is the strongest you can have. So that's your strongest acid in the list. What's your strongest base in this list? The strongest base is hydroxide, because it has no base stronger. Actually, O2 negative in solution is stronger when you get it, because when this oxide ion gets water, water donates a proton to this, and guess what you get? Can you see it? You get hydroxide ion from here, and then this leaves a hydroxide ion and makes 2OH negatives. So actually oxides are the strongest, but guess what? That's just between you and me for now; hydroxide is the strongest. So, what do we do? We would write an equation with hydronium and hydroxide in it. So the strongest acid hydronium reacts with the strongest base, that's hydroxide, both of those are aqueous ions in solution. Here is where I put many 'aqs' and make sure that all the subscripts are in your final equation. Then transfer proton from the acid to the base and you get H2O twice, water in liquid. So that's the net Brönsted-Lowry acid base reaction for this very important reaction; it's the most popular one. Strongest acid there is in base hydronium and hydroxide makes 2H2O. Now that strong acid, nitric acid is going to react with potassium bicarbonate instead, but potassium bicarbonate KHCO3, that's potassium ions, and bicarbonate or hydrogen carbonate; bi in chemistry means hydrogen, that mean two means hydrogen. So, when you break that down to ions as well as a nitric acid, which was the strong acid before and hydronium is still got to be the strong acid, isn't it? Then you are going to get potassium ions, bicarbonate ions and water still in your list. That is going to be the strongest base. When you look in this list of chemicals for the strongest base, which is the lowest one on the right side, nitric ion will be up here. Nitric ion; remember, it's the conjugate base to a strong acid, so it's neutral anyway. Bicarbonate is down in the middle h