Electrolytes and the Theories of Acids and Bases

Transcript

0:00Introduction

Hello everyone! Okay, a quick question to start. Water: is it an acid, or a base? Most of you are thinking, neither, right? Water is neutral. But today you will see one reaction where water gives away a proton, just like an acid, and another reaction where water takes a proton, just like a base. So which one is it? By the end of this video, you will answer that in one line. On the way, we build three definitions of acids and bases, step by step, and questions on them come up often in JEE, so let's begin.

0:37Strong and weak electrolytes

So, first, electrolytes: substances that give ions in water. Take sodium chloride, hydrochloric acid and sodium hydroxide. In water, they are almost completely ionised. These are strong electrolytes. Okay? Now, acetic acid and ammonia. In water, they are only partly ionised. These are weak electrolytes. Right? So think. If only part of the acetic acid ionises, what is actually sitting in that solution? Both, yes, exactly! Ions, and un-ionised molecules, side by side. And between them there is an equilibrium. The two half arrows on screen mean equilibrium, going both ways. With me? This is called ionic equilibrium. And look, nothing new here: it is the equilibrium of Chapter one, applied to ions. So everything you learnt there works here too. Clear?

1:32Arrhenius: acids and bases in water

Now, the first definition of acids and bases, from Arrhenius. An acid gives H plus in water; the small plus sign up top is a positive charge. And in the brackets: in water, this H plus is held as H three O plus, the hydronium ion. The small three below is a subscript, three hydrogen atoms. Got it? And a base gives O H minus in water, the hydroxide ion. The small minus sign up top is a negative charge. Simple, right? But look, both lines end with the same two words: in water. So, what is the weak point of this definition? Correct! It works only in water. Arrhenius is limited to aqueous solution, which means a solution in water. So, we need a wider definition. Let's go.

2:17Brønsted–Lowry: proton donor and acceptor

Next, Bronsted and Lowry. An acid is a proton donor, and a base is a proton acceptor. The proton is simply H plus. And notice, the word water is gone. Okay? Here is the general reaction. H A gives its proton to B, the base. So H A becomes A minus, and B becomes H B plus. Again, the two half arrows mean equilibrium. Now read the same reaction from right to left. What does A minus do in the reverse reaction? It takes a proton back, so A minus is a base! An acid, and the base formed from it by losing H plus, are called a conjugate pair: H A and A minus. The same way, H B plus and B are a conjugate pair. Clear? Now, a common trap. If H A is a very strong acid, then A minus must be a strong base too, right? No, no, careful! It is the opposite. The stronger the acid, the weaker its conjugate base. The double arrow on screen is read as implies. Got it?

3:24Lewis: acids and bases by electron pairs

Now the widest definition, from Lewis. Here, an acid accepts an electron pair. Examples: boron trifluoride, B F three; aluminium chloride; the iron ion, F e, with a charge of three plus; and the silver ion, A g plus. Right? And a Lewis base donates an electron pair: ammonia, water, O H minus, C N minus, the cyanide ion, and F minus, the fluoride ion. Okay? Now, a question for you. B F three has no hydrogen at all. Is it an acid in the Arrhenius sense, or in the Bronsted sense? Neither! It has no hydrogen to give as H plus. But boron has an empty orbital, and that empty orbital accepts an electron pair. So B F three is an acid only in the Lewis sense. Clear? For example, with ammonia. Nitrogen gives its electron pair to boron, and we write F three B, arrow, N H three. The arrow shows the pair going from N to B, from the donor to the acceptor. With me?

4:26Amphiprotic: give and take a proton

One last word: amphiprotic. An amphiprotic species can both give and take a proton. The common ones: water, HCO three minus, HSO four minus, H two P O four minus, and H S minus. Right? Now tell me. N H four plus, the ammonium ion: is it amphiprotic? No! It can only give a proton, not take one. And C O three two minus, the carbonate ion with a charge of minus two, can only take one. Watch for these two in such lists. Got it?

4:59Example 1: conjugate pairs in NH3 + H2O and HCl + H2O

Now, Example one. Identify the conjugate pairs in two reactions. First: N H three plus water gives N H four plus and O H minus, at equilibrium. Second: H C l plus water gives H three O plus and C l minus, with a single arrow, because H C l ionises almost completely. Pause the video, and try it first. Done? Okay, let's check. In the first reaction, who gives the proton, ammonia or water? Water, yes! Water gives H plus and becomes O H minus, so water is the acid, and O H minus is its conjugate base. Ammonia takes the proton and becomes N H four plus: base N H three, conjugate acid N H four plus. Clear? Now the second one. H C l gives its proton, so H C l is the acid, and C l minus is its conjugate base. Water takes the proton and becomes H three O plus: base water, conjugate acid H three O plus. Okay? And now, look at water. An acid in the first reaction, and a base in the second! That is our opening puzzle solved: water can do both, because water is amphiprotic. Nice, right?

6:17Example 2: basic strength order of Cl-, CH3COO- and OH-

Now a slightly harder one. Arrange three bases in order of increasing basic strength: C l minus, the chloride ion; C H three COO minus, the acetate ion; and O H minus. Hint: find each conjugate acid first. Pause the video, and try it. Done? Okay, let's check. Add H plus to each one. Chloride gives H C l, a strong acid. Acetate gives C H three COOH, acetic acid, a weak acid. And hydroxide gives water, a very weak acid. With me? Now, H C l is the strongest acid here. So C l minus must be the strongest base, right? No, no, careful! Remember our rule. The weaker the acid, the stronger its conjugate base. So the order of the bases is the reverse of the order of the acids. Okay? Water is the weakest acid, so O H minus is the strongest base. H C l is the strongest acid, so C l minus is the weakest base. So: C l minus, less than acetate, less than O H minus. The less than signs on screen mean less basic than. Done in three lines!

7:30Quick revision: Arrhenius, Bronsted-Lowry and Lewis

So, quick revision. Strong electrolytes ionise almost completely; weak ones set up an equilibrium. Arrhenius: H plus or O H minus, in water. Bronsted Lowry: proton donor or proton acceptor. Lewis: electron pair acceptor or donor. And remember, the stronger the acid, the weaker its conjugate base. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!