Calorimeter Guide, Meaning , Facts, Information and Description
A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes. The word calorimeter is derived from calorie, a unit of heat.
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2 Types
2.1 Constant-volume
3 See also2.2 Constant-pressure 2.3 Differential scanning calorimeter 2.4 Isothermal titration calorimeter 4 Weblinks |
Example
A simple calorimeter may just consist of a thermometer attached to an insulated container. To find the enthalpy change per mole of a substance X in a reaction between two liquids X and Y, they are added to the calorimeter and the initial and final (after the reaction has finished) temperatures are noted. Multiplying the temperature change by the mass and specific heat capacities of the liquids gives a value for the energy given off during the reaction (assuming the reaction was exothermic.) Dividing the energy change by how many moles of X were present gives its enthalpy change of reaction.
Types
Constant-volume
A constant-volume calorimeter is a calorimeter in which the chemical reaction occurs within a rigid vessel whose volume cannot change. Bomb calorimeters are constant-volume calorimeters capable of withstanding the large pressure and force of explosive reactions.
Constant-pressure
A constant-pressure calorimeter measures the change in enthalpy of a reaction occurring in solution during which the atmospheric pressure remains constant.
An example is a coffee-cup calorimeter, which is constructed from two nested Styrofoam cups and holes through which a thermometer and a stirring rod can be inserted. The inner cup holds the solution in which the reaction occurs, and the outer cup provides insulation.
Differential scanning calorimeter
In a differential scanning calorimeter (DSC), heat flow into a sample—usually contained in a small aluminum capsule or 'pan'—is measured differentially, i.e. by comparing it to the flow into an empty reference pan.
Both pans sit on a small slab of material with a known (calibrated) heat resistance K. The temperature of the calorimeter is raised linearly with time (scanned), i.e. the heating rate dT/dt = β is kept constant. This time linearity requires good design and good (computerized) temperature control.
Heat flows into the two pans by conduction. The flow of heat into the sample is larger because of its heat capacity Cp. The difference in flow dq/dt induces a small temperature difference ΔT across the slab. This temperature difference is measured using a thermocouple. The heat capacity can in principle be determined from this signal:
Note that this formula (equivalent to Newton's law of heat flow) is analogous to, and much older than, Ohm's law of electric flow: ΔV = R dQ/dt = R I.
When a sudden change in the heat capacity occurs (e.g. when the sample melts), the signal will respond and exhibit a peak. From the integral of this peak the enthalpy of melting can be determined, and from its onset the melting temperature.
Differential scanning calorimetry is a workhorse technique in many fields, particularly in polymer characterization.
A modulating differential scanning calorimeter is a type of DSC in which a small oscillation is imposed upon the otherwise linear heating rate. This has a number of advantages. It increases sensitivity, allowing slow scans. It facilitates the direct measurement of the heat capacity in one scan. It also permits the simultaneous measurement of heat effects that are reversible and not reversible at the timescale of the oscillation. (reversing and non-reversing heat flow resp.)
Isothermal titration calorimeter
In an isothermal titration calorimeter, the heat of reaction is used to follow a titration experiment. This permits determination of the end point (stoichiometry) of a reaction as well as its enthalpy.
The technique is gaining in importance particularly in the field of biochemistry, because it facilitates determination of substrate binding to enzymes.
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