Salt Hydrolysis

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle first. Sodium acetate is the salt of acetic acid and sodium hydroxide, made by neutralisation. So its solution should be neutral, isn't it? But a zero point one molar solution has a p H of almost nine! How can that be? Today we find out. This is salt hydrolysis, and it comes up often in JEE, so let's begin.

0:28What is hydrolysis?

So, what is hydrolysis? When a salt dissolves, the ion from a weak acid, or from a weak base, reacts with water. Take capital A minus, the anion of a weak acid; the small minus up top is a negative charge. Its parent acid is weak, and a weak acid holds on to its proton tightly. So, what will A minus take from water? Yes, exactly! An H plus. So, A minus, plus H two O, gives H A, plus O H minus; the small two is a subscript. The two half arrows mean equilibrium: the reaction goes both ways. That O H minus makes the solution basic. With me? Now the other side. Here B is a weak base, and B H plus is its cation, with a positive charge. B H plus, plus H two O, gives B, plus H three O plus, the hydronium ion. So, acidic. Clear? Now tell me. C l minus, the chloride ion, comes from H C l, a strong acid. Will chloride take an H plus back from water? No! H C l is a strong acid; it gives its proton away completely, so chloride does not take it back. The same for nitrate, N O three minus, and sulphate, S O four two minus; and for N a plus and K plus, from strong bases. Got it?

1:53Which salts are acidic, basic or neutral?

Now, which salts are acidic, basic or neutral? Strong acid plus strong base, like N a C l and KNO three: no hydrolysis, so neutral. p H seven, at two hundred ninety eight kelvin. Okay? Weak acid plus strong base, like C H three COON a, sodium acetate, and N a C N, sodium cyanide. The anion makes O H minus, so the solution is basic. Right? Strong acid plus weak base, like N H four C l, ammonium chloride. The ammonium ion hydrolyses, making H three O plus. So, acidic. Clear? Now a trap. F e C l three, ferric chloride. Chloride does not hydrolyse. So the solution is neutral, right? No, no, careful! The iron ion in it is small and highly charged, and it is hydrated in water; such metal ions make the solution acidic. The same goes for C u S O four, copper sulphate. Got it?

3:02Weak acid + strong base: the formulas

Now the formulas, for weak acid plus strong base. The hydrolysis constant, K h, with a small h below, equals K w, the ionic product of water, over K a, the acid constant. Okay? So tell me. If the acid is weaker, its K a is smaller. Then does the salt hydrolyse more, or less? More, correct! Smaller K a at the bottom, bigger K h. Now, h is the degree of hydrolysis, the fraction of ions that react, and c is the salt concentration. h equals the square root of K h over c. With me? Next, p H equals seven, plus half of, p K a plus log c. p K a means minus log of K a, with the log to the base ten. Clear? But note the conditions. This seven needs K w equal to ten to the minus fourteen, true at two hundred ninety eight kelvin, or twenty five degrees Celsius. And the shortcut holds only while h is below about five percent; the wavy sign means less than, or about equal to. Right?

4:13The other salts

Now, strong acid plus weak base, like N H four C l: the mirror image. K h equals K w over K b, the base constant. p H equals seven, minus half of, p K b plus log c; see, minus now, since it is acidic. Okay? Next, weak acid plus weak base, like ammonium acetate, C H three COONH four. Both ions hydrolyse: K h equals K w over, K a times K b, and h is approximately root K h; the wavy equals sign means approximately. Got it? Look at this h. If I dilute ammonium acetate with water, what happens to h? Nothing! There is no c in it. Both h and the p H are independent of concentration: p H equals seven, plus half of, p K a minus p K b. So if p K a equals p K b, the p H is just seven. Clear? One special case: N a HCO three, sodium hydrogencarbonate. Its HCO three minus ion can give and take a proton; it is amphiprotic. p H equals half of, p K a one plus p K a two; the subscripts one and two mean the parent acid's first and second dissociation constants. Same for N a H two P O four. Right?

5:47Example 1: pH of 0.10 M sodium acetate

Now, Example one, our opening puzzle. Find the p H of zero point one zero molar sodium acetate; the capital M means molar, moles per litre. p K a is four point seven four. Pause the video, and try it first. Done? Okay, let's check. Acetic acid is weak, sodium hydroxide is strong: weak acid plus strong base. So, p H equals seven, plus half of, p K a plus log c. Okay? Now, c is zero point one zero. So, seven plus half of, four point seven four plus one, right? Oh no, careful! Log of zero point one zero is minus one, not plus one. Any concentration below one molar has a negative log. Clear? So, four point seven four minus one is three point seven four; half of that is one point eight seven. Seven plus one point eight seven: eight point eight seven. Basic, almost nine. Puzzle solved! Acetate took H plus from water, leaving O H minus behind.

6:58Example 2: pH of 0.010 M NaCN using Kh = Kw/Ka

Now a harder one. Find the p H of zero point zero one zero molar N a C N, sodium cyanide. K a of HCN is four point nine times ten to the minus ten. Pause and try it. Hint: find K h first. Done? Okay, let's check. HCN is a weak acid, so the solution is basic. K h equals K w over K a: ten to the minus fourteen, over four point nine times ten to the minus ten, which is two point zero times ten to the minus five. Okay? Each cyanide ion that reacts gives one O H minus; square brackets mean concentration. So O H minus equals the square root of K h times c. K h times c is two point zero times ten to the minus seven, and its square root is four point five times ten to the minus four molar. With me? Minus log of this O H minus is three point three five. But careful, that is the p O H, not the p H! So, p H equals fourteen minus three point three five: ten point six five. Got it? One last check. h is O H minus over c: four point five times ten to the minus four, over zero point zero one zero, which is four point five percent. Under five percent, so just within the approximation. That's it!

8:30Quick revision: which salts hydrolyse, pH formulas

So, quick revision. Only the ion of a weak acid or a weak base hydrolyses. Weak acid plus strong base: basic; strong acid plus weak base: acidic. For weak acid plus strong base, p H equals seven, plus half of, p K a plus log c, at two hundred ninety eight kelvin; for weak plus weak, c drops out. Now try the practice problems on this page. All the best!