Thermochemistry

(c) A solution of copper sulphate will react with zinc, dissolving it and depositing copper. The heat evolved by the solution of an "equivalent" of zinc can be measured. In the Daniell cell, which consists of a zinc electrode dipping into zinc sulphate solution in a porous pot, surrounded by another container where a copper elec trode dips into copper sulphate solution, this chemical reaction takes place in such a way that it can yield an electric current. We can obtain work from such a cell, e.g., by using it to drive a small electromotor. The work can be measured accurately. If E is the electromotive force (volts) and C the current (amperes), then work done is EC units per second. While the current is passing we can measure the heat changes in the cell itself. These are found to be small, but not zero. If we express in heat units the electrical work done by the solution of unit mass, and add (or subtract) the simultaneous evolution (or absorption) of heat in the cell, we obtain a result which is the same as that obtained when the simple reaction was allowed to take place without the production of elec trical energy. Alternatively, we could join the two electrodes with a wire and measure the heat produced in the wire by the passage of the electric current and the heat produced in the cell itself at the same time. The sum of these two would be the same as the heat produced by dissolving the same weight of zinc in a similar solution of copper sulphate, but by arranging the experiment in this way, most of the heat appears outside the solution instead of inside it.

It will be observed that the applications of the principle of con servation of energy depend on the assumption that a unit of one kind of energy always bears a constant relation to a unit of any other kind of energy. This is a necessary deduction from the law. If a mass of m grams is held h centimetres above the earth's surface, its potential energy is measured by the product mgh, where g is the constant of gravity and mg the force (in dynes) with which the earth attracts it. If it is allowed to fall freely, and we neglect the resistance of the air, it will have a velocity of v centimetres per second just before hitting the earth. Its potential energy has then been entirely converted into kinetic energy which is measured by the product As no energy is lost, mgh and v = V 2gh. When it hits the earth its kinetic energy is entirely converted into heat, and the amount of heat produced is exactly proportional to or to mg/i. The potential energy is so called because, if the restraining force is removed, the mass acquires kinetic energy if left to itself. The energy is originally latent and only becomes apparent when the restraining force is removed; we can speak similarly of latent chemical energy.

Every element and every chemical compound has a definite con tent of energy which varies with the temperature. We know it

varies with the temperature, because if we want to raise the tem perature of anything we have to put heat into it. We do not know, however, what is the total energy content of any chemical sub stance, nor how it is divided up within the molecules ; what we can determine is the change in total energy content which takes place during chemical reactions, and we can express the law of conserva tion of energy, in its application to chemistry, in the form U=A—Q, where U is the diminution in total energy content accompanying a chemical reaction, A is the work done during or by means of the reaction, and Q is the actual measured absorption of heat during the reaction. In applying this equation it is necessary to express all quantities in the same units. A, the work done by a chemical reac tion, is usually either mechanical work, as in the internal combus tion engine, or electrical work, as in the accumulator. The unit of mechanical work in the C.G.S. system is the erg, the unit of heat is the calorie, which is the amount of heat necessary to raise the temperature of i gram of water from 15° to i6° C. The expendi ture of 4.18X ergs, or 4.18 joules will produce i calorie of heat.

Determination of Total Energy Changes.

The realization that the total energy content of a chemical substance was a prop erty of the substance as important as any other property, and that the changes in energy accompanying chemical reactions were closely connected with the nature of the reaction, led to a very large number of experimental determinations of heats of reaction by J. Thomsen and M. Berthelot. Heats of reaction are deter mined in principle by causing the reaction to take place rapidly under such conditions that its heat is transferred to a large and well stirred volume of water, which is thereby raised a few degrees in temperature. As the specific heat of water is by definition unity at 15° C, the heat evolved can be calculated if the mass of water and its rise in temperature are accurately known. Allowance must be made for the actual heating of the containing vessels and the reacting substances, and also for the loss of heat during the time the reaction takes place. This loss of heat is kept down by keeping the rise in temperature small, which necessitates the use of very sensitive thermometers. Accurate calorimetry depends essentially on making proper allowances for this loss of heat, or alternatively on methods employed to counterbalance it. In general the data given in chemical literature cannot be relied upon to within 1%, and in many cases the error of determination is much greater. An experimental error of this order may be serious in certain cases, as we shall see later.