Word Finder - calorimeter

definition

An apparatus for measuring the heat generated or absorbed by either a chemical reaction, change of phase or some other physical change.

A calorimeter is any piece of apparatus in which heat is measured.

For ordinary combustions compressed oxygen is used, so that the combustible substance burns almost instantaneously, the action being induced by means of some electrical device which can be controlled from without the calorimeter.

The accuracy of heats of combustion determined in the closed calorimeter is in favourable cases about one-half per cent.

This test was applied by Joule in the well-known experiment in which he allowed a gas to expand from one vessel to another in a calorimeter without doing external work.

Under this condition the increase of intrinsic energy would be equal to the heat absorbed, and would be indicated by fall of temperature of the calorimeter.

But owing to the large thermal capacity of his calorimeter, the test, though sufficient for his immediate purpose, was not delicate enough to detect and measure the small deviations which actually exist.

To test this two vessels similar to that used in the last experiment were placed in the same calorimeter and connected by a tube with a stop-cock.

On repeating the experiment when the two vessels were placed in different calorimeters, it was found that heat was absorbed by the vessel containing the compressed air, while an equal quantity of heat was produced in the calorimeter containing the exhausted vessel.

When the solutions employed are dilute, no water is placed in the calorimeter, the temperature-change of the solutions themselves being used to estimate the thermal effect brought about by mixing them.

It is of course in such a case necessary to know the specific heat of the liquid in the calorimeter.

These tubes are generally in the form of worms immersed in the water of the calorimeter.

The steel combustion chamber is of about 250 c.c. capacity, and is wholly immersed in the calorimeter.

If the heat of solution be measured in a calorimeter, no work is done, so that, if we call this calorimetric heat of solution L, the two quantities are connected by the relation L = X+P(v - v).

If l is the heat of dilution per unit change of volume in a calorimeter where all the energy goes to heat, the change in internal energy U is measured by ldv.

The gradient near the entrance to the calorimeter was deduced from observations with five thermometers at suitable intervals along the bar.

The heat-flow through the central column amounted to about 7.5 calories in 54 seconds, and was measured by continuing the tube through the iron plate into the bulb of a Bunsen ice calorimeter, and observing with a chronometer to a fifth of a second the time taken by the mercury to contract through a given number of divisions.

The chief uncertainty of this method is the area from which the heat is collected, which probably exceeds that of the central column, owing to the disturbance of the linear flow by the projecting bulb of the calorimeter.

The two carried out some of the earliest thermochemical investigations, devised apparatus for measuring linear and cubical expansions, and employed a modification of Joseph Black's ice calorimeter in a series of determinations of specific heats.

The Method of Mixture consists in imparting the quantity of heat to be measured to a known mass of water, or some other standard substance, contained in a vessel or calorimeter of known thermal capacity, and in observing the rise of temperature produced, from which data the quantity of heat may be found as explained in all elementary text-books.

Some heat is generally lost in transferring the heated body to the calorimeter; this loss may be minimized by performing the transference rapidly, but it cannot be accurately calculated or eliminated.

Some heat is lost when the calorimeter is raised above the temperature of its enclosure, and before the final temperature is reached.

The coefficient of heating of a calorimeter when it is below the temperature of its surroundings is seldom, if ever, the same as the coefficient of cooling at the higher temperature, since the convection currents, which do most of the heating or cooling, are rarely symmetrical in the two cases, and moreover, the duration of the two stages is seldom the same.

In any case, it is desirable to diminish the loss of heat as much as possible by polishing the exterior of the calorimeter to diminish radiation, and by suspending it by non-conducting supports, inside a polished case, to protect it from draughts.

The method of lagging the calorimeter with cotton-wool or other non-conductors, which is often recommended, diminishes the loss of heat considerably, but renders it very uncertain and variable, and should never be used in work of precision.

The bad conductors take so long to reach a steady state that the rate of loss of heat at any moment depends on the past history more than on the temperature of the calorimeter at the moment.

The least trace of damp in the lagging, or of moisture condensed on the surface of the calorimeter, may produce serious loss of heat by evaporation.

This is another objection to Rumford's method of cooling the calorimeter below the surrounding temperature before starting.

It is best to make this correction as small as possible by using a large calorimeter, so that the mass of water is large in proportion to that of metal.

The calorimeter is surrounded by an air-jacket connected to a petroleum gauge which indicates any small change of temperature in the calorimeter, and enables the manipulator to adjust the supply of cold water to compensate it.

H is an electric heater for raising the body to a suitable temperature, which can swing into place directly over the calorimeter.

A common example of this method is the determination of the specific heat of a liquid by filling a small calorimeter with the liquid, raising it to a convenient temperature, and then setting it to cool in an enclosure at a steady temperature, and observing the time taken to fall through a given range when the conditions have become fairly steady.

The same calorimeter is afterwards filled with a known liquid, such as water, and the time of cooling is observed through the same range of temperature, in the same enclosure, under the same conditions.

The ratio of the times of cooling is equal to the ratio of the thermal capacities of the calorimeter and its contents in the two cases.

As the method is usually practised, the calorimeter is made very small, and the surface is highly polished to diminish radiation.

For accurate work it is essential that the liquid in the calorimeter should be continuously stirred, and also in the enclosure, the lid of which must be waterjacketed, and kept at the same steady temperature as the sides.

This difficulty was overcome by the invention of the Bunsen calorimeter, in which the quantity of ice melted is measured by observing the diminution of volume, but the successful employment of this instrument requires considerable skill in manipulation.

One of the chief difficulties in the practical use of the Bunsen calorimeter is the continued and often irregular movement of the mercury column due to slight differences of temperature, or pressure between the ice in the calorimeter and the ice bath in which it is immersed.

If the inner bulb is filled with mercury instead of water and ice, the same arrangement answers admirably as a Favre and Silbermann calorimeter, for measuring small quantities of heat by the expansion of FIG.

If such variations of density exist, they may introduce some uncertainty in the absolute values of results obtained with the ice calorimeter, and may account for some of the discrepancies above enumerated.

Joly in the construction of his steam calorimeter, a full description of which will be found in text-books.

The Calorimeter Was Suspended By A Steel Wire, The Torsion Of Which Made The Equilibrium Stable.

The Power Was Transmitted To The Paddles By Bevel Wheels F, G, Rotating A Spindle Passing Through A Stuffing Box In The Bottom Of The Calorimeter.

The Water Equivalent Of The Calorimeter Was About 85 Grammes, And Was Determined By Varying The Quantity Of Water From 140 To 260 Or 280 Grammes, So That The Final Results Depended On A Difference In The Weight Of Water Of 120 To 140 Grammes.

The Water Equivalent Of The Calorimeter Is Immaterial, Since There Is No Appreciable Change Of Temperature.

The Direct Determination Of The Specific Heat At Constant Volume Is Extremely Difficult, But Has Been Successfully Attempted By Joly With His Steam Calorimeter, In The Case Of Air And C02.

The method commonly adopted in measuring the latent heat of a vapour is to condense the vapour at saturation-pressure in a calorimeter.

The quantity of heat so measured is the total heat of the vapour reckoned from the final temperature of the calorimeter, and the heat of the liquid h must be subtracted from the total heat measured to find the latent heat of the vapour at the given temperature.

Another method, which is suitable for volatile liquids or low temperatures, is to allow the liquid to evaporate in a calorimeter, and to measure the quantity of heat required for the evaporation of the liquid at the temperature of the calorimeter and at saturation-pressure.

The ideal method of determining by direct experiment the relation between the total heat and the specific heat of a vapour is that of Joule and Thomson, which is more commonly known in connexion with steam as the method of the throttling calorimeter.

The simplest method of measuring the specific heat appears to be that of supplying heat electrically to a steady current of vapour in a vacuum-jacket calorimeter, and observing the rise of temperature produced.