Enthalpy of Neutralisation

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle from the chemistry lab. Mix dilute hydrochloric acid with sodium hydroxide, and measure the heat for every mole of water formed. Now repeat it with nitric acid and potassium hydroxide. The heat is the same, fifty seven point one kilojoules per mole of water. Is that just a coincidence? And why does a weak acid give less? This experiment comes up often in JEE practical chemistry, so let's find out.

0:34What is the enthalpy of neutralisation?

So, what is the enthalpy of neutralisation? It is the enthalpy change when one mole of H plus from an acid is neutralised by one mole of O H minus from a base. The raised plus and minus are the charges on the ions, and mol means mole. Okay? And this happens in dilute solution. H plus and O H minus give water, H two O, a liquid. The a q in brackets means aqueous, l means liquid, the small two is a subscript, and the arrow means gives. Right? The heat for this is delta H equals minus fifty seven point one kilojoules per mole; the raised minus one means per. Delta, the little triangle, means change in, and the minus sign means heat is given out. In round brackets is the same value, minus thirteen point seven kilocalories per mole. Clear?

1:27Every strong acid with every strong base

Now, back to our puzzle. Hydrochloric acid, H C l, with sodium hydroxide, N a O H, and nitric acid, HNO three, with potassium hydroxide, KOH; the plus sign means mixed with. Both give minus fifty seven point one, and so does every strong acid with every strong base. Got it? So let me ask you this. These are completely different chemicals, so why should the heat be the same every time? Here is the reason: strong acids and strong bases are fully ionised in dilute solution. Hydrochloric acid is already H plus and chloride ions, and sodium hydroxide is sodium ions and O H minus. With me? So when they mix, the only reaction is H plus with O H minus, giving water, and the other ions take no part. Same reaction, so the same heat, every time. It was never a coincidence. Okay?

2:21Weak acid or weak base: a smaller value

Now, what if the acid is weak, only partly ionised? For each mole of water, will more heat come out than fifty seven point one kilojoules, or less? The answer is less heat, that is correct. With a weak acid or a weak base, delta H is usually numerically smaller. Careful with that word: it means smaller in size, so delta H is less negative. Right? The reason is that the weak electrolyte is not fully ionised, and ionising it takes energy. So part of the heat is used to ionise it, and less is left to warm the solution. Hydrofluoric acid, H F, is a known exception, and it is explained on this page. Clear?

3:04Measuring it in a calorimeter

Now let us measure this heat. Equal volumes of the acid and the alkali, both at the same initial temperature, are mixed in a calorimeter. They start equal, so that the whole rise comes from the reaction. Okay? Small q, the heat given out, equals m times c, plus C cal, all in brackets, times delta T, the rise in temperature. With me? Small m is the mass of the solution, and small c is its specific heat, taken as that of water: four point one eight joules per gram per kelvin. Right? And capital C with the small cal below it is the calorimeter constant, the heat capacity of the vessel, which warms up too. Clear? Now think about the sign. The mixture warms up, so the reaction is giving out heat. Then should delta H be positive or negative? It is negative, exactly right. So delta H equals minus q over n, where small n, with H two O below it, is the moles of water formed. Always divide by the moles of water, not simply the moles of acid. Got it?

4:10Example 1: HCl and NaOH in a calorimeter, find delta H

Time for the first example. Fifty millilitres of one point zero molar hydrochloric acid and fifty millilitres of one point zero molar sodium hydroxide, both at twenty five point zero degrees Celsius, are mixed in a calorimeter of constant forty six point four joules per kelvin. The temperature rises to thirty one point two degrees Celsius. Find the enthalpy of neutralisation, taking the density as one gram per millilitre. Here m L is millilitres, capital M is moles per litre, and the small circle means degrees. Pause the video and try it first. Done? Okay, let's check. Fifty millilitres is zero point zero five zero litres, capital L, and the cross means times. Times one point zero molar gives zero point zero five zero moles each of H plus and O H minus, so zero point zero five zero moles of water. Right? Next comes the mass. Fifty millilitres of acid, at a density of one gram per millilitre, is fifty grams. So m is fifty grams, right? No, that is the trap, because both solutions are in the calorimeter, so the mass is fifty plus fifty, one hundred grams. And delta T is thirty one point two minus twenty five point zero, which is six point two kelvin; a rise of one degree Celsius equals a rise of one kelvin. Clear? Now let us find the heat. One hundred times four point one eight is four hundred and eighteen; adding forty six point four gives four hundred and sixty four point four. Times six point two, that gives about two thousand eight hundred and seventy nine joules. With me? Finally, delta H is minus two thousand eight hundred and seventy nine joules over zero point zero five zero moles, which is minus fifty seven point six kilojoules per mole. That is very close to minus fifty seven point one, as expected for a strong acid and a strong base. Okay?

6:07Example 2: heat from Ba(OH)2 and HCl, the limiting ion

Now let us try a trickier one. Two hundred millilitres of zero point two molar barium hydroxide is mixed with two hundred millilitres of zero point three molar hydrochloric acid. Find the heat evolved. In barium hydroxide, the small two after the round brackets means two O H groups. Pause the video and try it first. Done? Okay, let's check. Two hundred millilitres is zero point two litres. So barium hydroxide is zero point two times zero point two, which is zero point zero four moles. Right? And the acid gives zero point two times zero point three, which is zero point zero six moles of H plus. Clear? Now, zero point zero four is less than zero point zero six; the pointed sign on screen means less than. So the base is limiting, and the heat is zero point zero four times fifty seven point one, right? That is exactly the trap, because each barium hydroxide gives two O H minus ions. So O H minus is two times zero point zero four, which is zero point zero eight moles. Got it? Now compare again. Zero point zero six is less than zero point zero eight, so H plus is limiting, and zero point zero six moles of water form, with O H minus left over. With me? So the heat evolved is zero point zero six times fifty seven point one, which is three point four three kilojoules. Always count the water from the limiting ion. Okay?

7:37Quick revision: 57.1 kJ, weak acids, calorimeter equation

So, quick revision. Strong acid with strong base: minus fifty seven point one kilojoules per mole of water, every time. Weak acid or weak base: usually numerically smaller, as some heat is used to ionise it. And in the calorimeter, q equals m times c plus C cal, times delta T; then divide by the moles of water formed. Now go and try the practice problems on this page. All the best!