temperature

Examples of temperature in the following topics:

• Solid Solubility and Temperature

  • Solubility often depends on temperature; the solubility of many substances increases with increasing temperature.
  • The solubility of a given solute in a given solvent typically depends on temperature.
  • Many salts show a large increase in solubility with temperature.
  • Some solutes exhibit solubility that is fairly independent of temperature.
  • A few, such as cerium(III) sulfate, become less soluble in water as temperature increases.

• Gas Solubility and Temperature

  • Solubility of a gas in water tends to decrease with increasing temperature, and solubility of a gas in an organic solvent tends to increase with increasing temperature.
  • Several factors affect the solubility of gases: one of these factors is temperature.
  • In severe cases, temperature changes can result in large-scale fish kills.
  • The trend that gas solubility decreases with increasing temperature does not hold in all cases.
  • There are several molecular reasons for the change in solubility of gases with increasing temperature, which is why there is no one trend independent of gas and solvent for whether gases will become more or less soluble with increasing temperature.

• Changes in Temperature

  • Changes in temperature shift the equilibrium state of chemical reactions; these changes can be predicted using Le Chatelier's Principle.
  • Changes in temperature can affect the equilibrium state of a reversible chemical reaction.
  • This law can be applied to changes in pressure, volume, concentration, and temperature.
  • Applied to temperature, Le Chatelier's Principle predicts that the addition of heat to a system will cause an opposing reaction in the system to remove heat.
  • Evaluate the effect of temperature on the equilibrium state of a chemical reaction

• Temperature

  • Although there is an entire field of study devoted to measuring temperature (thermometry), the focus of this section is on the fundamental measurements of temperature.
  • Temperature-related phenomena were always being observed.
  • Therefore, the temperature must be "absolute 0."
  • Temperature can be measured and represented in many different ways.
  • A comparison of temperature scales table illustrates a variety of temperature scales, some of which are no longer used.

• Charles' and Gay-Lussac's Law: Temperature and Volume

  • Charles' Law describes the relationship between the volume and temperature of a gas.
  • This law states that at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature (in Kelvin); in other words, temperature and volume are directly proportional.
  • If a gas contracts by 1/273 of its volume for each degree of cooling, it should contract to zero volume at a temperature of –273°C; this is the lowest possible temperature in the universe, known as absolute zero.
  • Run the model and change the temperature.
  • Why does the barrier move when the temperature changes?

• Supercritical Fluids

  • This can be rationalized by thinking that at high enough temperatures (above the critical temperature) the kinetic energy of the molecules is high enough to overcome any intermolecular forces that would condense the sample into the liquid phase.
  • The relationship with temperature is a little more complicated.
  • At constant density, solubility will increase with temperature.
  • Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again.
  • Thus, above the critical temperature a gas cannot be liquified by pressure.

• The Third Law of Thermodynamics and Absolute Energy

  • The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches zero.
  • Specifically, the entropy of a pure crystalline substance at absolute zero temperature is zero.
  • At zero temperature the system must be in a state with the minimum thermal energy.
  • For such systems, the entropy at zero temperature is at least ln(2)kB, which is negligible on a macroscopic scale.
  • The entropy (S) of a substance (compound or element) as a function of temperature (T).

• Solubility

  • The solubility of a substance fundamentally depends on the solvent used, as well as temperature and pressure.
  • The solubility of a given solute in a given solvent typically depends on temperature.
  • For many solids dissolved in liquid water, solubility tends to correspond with increasing temperature.
  • The solubility of gases displays the opposite relationship with temperature; that is, as temperature increases, gas solubility tends to decrease.
  • In a chart of solubility vs. temperature, notice how solubility tends to increase with increasing temperature for the salts and decrease with increasing temperature for the gases.

• Pressure and Free Energy

  • Gibbs free energy measures the useful work obtainable from a thermodynamic system at a constant temperature and pressure.
  • Gibbs energy (also referred to as ∆G) is also the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature.
  • As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature.
  • Therefore, Gibbs free energy is most useful for thermochemical processes at constant temperature and pressure.

• Free Energy Changes in Chemical Reactions

  • Remember that ΔG is meaningful only for changes in which the temperature and pressure remain constant.
  • (it is appropriate to say "almost" because the values of ΔH and ΔS are themselves slightly temperature dependent; both gradually increase with temperature).
  • An exothermic reaction whose entropy increases will be spontaneous at all temperatures.
  • This means that there is a temperature defined by $T = \frac{\Delta H}{\Delta S}$ at which the reaction is at equilibrium; the reaction will only proceed spontaneously below this temperature.
  • Since the effect of the temperature is to "magnify" the influence of a positive ΔS, the process will be spontaneous at temperatures above $T = \frac{\Delta H}{\Delta S}$ .