Electrochemistry, Entropy & Equilibrium Practicals (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about Required Practical 8, making simple cells, the entropy of vaporisation and the determination of Kc.

All four are physical chemistry practicals that set a lab measurement against a number the theory gives you. A cell's EMF against standard electrode potentials, the entropy of vaporisation out of the Gibbs equation, and Kc out of the equilibrium composition.

Required Practical 8 is measuring the EMF of an electrochemical cell. Making simple cells uses the same apparatus to find out what changes that EMF. The kettle experiment finds the entropy change when water boils, and the esterification experiment finds the equilibrium constant for the reaction between ethanol and ethanoic acid. What joins them is that in each one, the reading taken off a voltmeter, a balance or a burette is turned into, or checked against, a value the theory already predicts.

We'll start with Required Practical 8 and how a cell's EMF is measured. Then making simple cells, and what happens to that EMF when the electrodes or the concentrations change. After that the entropy of vaporisation, and last the determination of Kc.

Required Practical 8 is measuring the EMF of an electrochemical cell. You need two small beakers, strips of metals such as copper, zinc, iron and silver, solutions of their ions, and a high-resistance voltmeter. The two half-cells are joined by a salt bridge, a strip of filter paper soaked in saturated potassium nitrate, placed so that both ends are well immersed in the solutions.

The metal strips are cleaned first to remove any oxide coatings, then connected to the voltmeter, and you wait for a steady reading before recording it. It is unlikely that those readings will closely match the theoretical values, because those are defined under standard conditions that are difficult to achieve in a school laboratory. What should hold is the relative order: a higher EMF means a greater difference in reactivity between the metals, so electrons are transferred more readily from one to the other.

Making simple cells uses that same set-up to investigate what changes the EMF. The first thing to vary is the electrode combination. The metal foils are cleaned with sandpaper and rinsed under a cold tap, each is immersed in a solution of its own ions, and the EMF measured is compared with the voltage expected from standard electrode potentials. Measured results in the lab are generally lower than the calculated results from data tables, and that is usually because non-standard conditions have been used.

The second thing to vary is concentration. One of the solutions is diluted by a factor of ten, the EMF is measured again, and the process is repeated until five dilutions have been carried out. Decreasing the concentration reduces the cell EMF, because there are fewer particles to react.

The entropy change when water boils is measured with a kettle and a top pan balance. At the boiling point, liquid water and water vapour exist in equilibrium, so the free energy change is zero. Rearranging the Gibbs equation then gives the entropy change from the enthalpy change: delta S equals delta H over T.

A fixed volume of water goes into an electric kettle whose power rating is known. The kettle is boiled, switched off and weighed, then re-boiled for a hundred seconds with the automatic cut-off switch held down, and reweighed to find how much water has been lost. The power rating and the time give the energy supplied, the mass lost gives the moles of water that evaporated, and together they give the enthalpy of vaporisation. Dividing that by the boiling temperature in kelvin gives the entropy change. The entropy is never measured directly here; it comes out of the Gibbs equation once the enthalpy change is known.

The determination of Kc uses the esterification of ethanol and ethanoic acid in the presence of an acid catalyst. The two reactants and a drop of concentrated sulfuric acid go into a conical flask, and a second flask is made up as a blank, containing a drop of sulfuric acid and distilled water. The reaction is slow, so the mixture is left to reach equilibrium over the course of a week.

Both flasks are then titrated against one mol per decimetre cubed sodium hydroxide, using phenolphthalein as the indicator, and the difference between the two titres gives the amount of ethanoic acid present at equilibrium. Once that is known, the equilibrium concentrations of the other substances follow and Kc can be determined. Several flasks are made up and analysed so that an average value can be found. Kc is not measured directly either; it is calculated from the equilibrium composition that the titration reveals.

An electrode always contains a metal in contact with a solution of its own ions. By convention, EMF values are positive and the more negative electrode is written on the left, and when you calculate the EMF you subtract the right-hand electrode potential from the left-hand one.

The equilibrium constant expression uses concentrations rather than moles. In the esterification example the volume is constant and the concentration terms are the same, so the volumes cancel out, but always check that, because it depends on the equation and will not always be true.

Required Practical 8 is measuring the EMF of an electrochemical cell, and making simple cells extends it by changing the electrode pair and the concentrations.

The entropy of vaporisation comes from a kettle, a balance and the Gibbs equation, with delta S equal to delta H over T.

Kc comes from letting an esterification reach equilibrium and titrating the acid that is left.

In every one of them, a reading taken in the lab is set against the value the theory predicts.