For the same substance, the transferred heat also depends on the phase (gas, liquid, or solid).The quantitative relationship between heat transfer and temperature change contains all three factors:where Q is the symbol for heat transfer, m is the mass of the substance, and ΔT is the change in temperature.
The calibration is accomplished using a reaction with a known q, such as a measured quantity of benzoic acid ignited by a spark from a nickel fuse wire that is weighed before and after the reaction. This quantity is known as the specific heat capacity (or simply, the specific heat), which is the heat capacity per unit mass of a material. The inner cup holds a known amount of a solute, usually water, that absorbs the heat from the reaction. The temperature change produced by the known reaction is used to determine the heat capacity of the calorimeter. [latex]\text{dH}=\delta \text{Q}+\text{VdP}[/latex],[latex](\frac{\partial \text{H}}{\partial \text{T}})_{\text{P}}=(\frac{\partial \text{Q}}{\partial \text{T}})_{\text{P}}=\text{C}_{\text{P}}[/latex].The specific heat is an intensive property that describes how much heat must be added to a particular substance to raise its temperature.Summarize the quantitative relationship between heat transfer and temperature changeThe heat capacity is an extensive property that describes how much heat energy it takes to raise the temperature of a given system. Experiments show that the transferred heat depends on three factors: (1) The change in temperature, (2) the mass of the system, and (3) the substance and phase of the substance. For example, when an exothermic reaction occurs in solution in a calorimeter, the heat produced by the reaction is absorbed by the solution, which increases its temperature. An example is a coffee-cup calorimeter, which is constructed from two nested Styrofoam cups and a lid with two holes, allowing insertion of a thermometer and a stirring rod.
The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C Use these data to determine the specific heat of the metal. The transferred heat also depends on the substance so that, for example, the heat necessary to raise the temperature is less for alcohol than for water. Recalling the first law of thermodynamics,[latex]\text{dU}=\delta \text{Q}-\delta \text{W}[/latex].For work as a result of an increase of the system volume we may write,[latex]\text{dU}=\delta \text{Q}-\text{PdV}[/latex].If the heat is added at constant volume, then the second term of this relation vanishes and one readily obtains[latex]\displaystyle{\left( \frac{\partial \text{U}}{\partial \text{T}}\right) _{\text{V}}=\left( \frac{\partial \text{Q}}{\partial \text{T}}\right) _{\text{V}}=\text{C}_{\text{V}}}[/latex]. For constant-volume calorimetry:[latex]\delta \text{Q}=\text{C}_{\text{V}}\Delta \text{T}=\text{mc}_{\text{V}}\Delta \text{T}[/latex]where δQ is the increment of heat gained by the sample, CTo find the enthalpy change per mass (or per mole) of a substance A in a reaction between two substances A and B, the substances are added to a calorimeter and the initial and final temperatures (before the reaction started and after it has finished) are noted. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. They range from simple coffee cup calorimeters used by introductory chemistry students to sophisticated bomb calorimeters used to determine the energy content of food.Calorimetry is used to measure amounts of heat transferred to or from a substance. The measurement of heat transfer using this approach requires the definition of a system (the substance or substances undergoing the chemical or physical change) and its surroundings (the other components of the measurement apparatus that serve to either provide heat to the system or absorb heat from the system).
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