heat of solution

Examples of heat of solution in the following topics:

• Heat of Solution

  • Heat of solution refers to the change in enthalpy when a solute is dissolved into a solvent.
  • The heat of solution, also referred to the enthalpy of solution or enthalpy of dissolution, is the enthalpy change associated with the dissolution of a solute in a solvent at constant pressure, resulting in infinite dilution.
  • The heat of solution, like all enthalpy changes, is expressed in kJ/mol for a reaction taking place at standard conditions (298.15 K and 1 bar).
  • The heat of solution can be regarded as the sum of the enthalpy changes of three intermediate steps:
  • The value of the overall heat of solution, $\Delta H^\circ_{sol}$, is the sum of these individual steps.

• Solutions and Heats of Hydration

  • When ions dissolve in water, the stabilizing interactions that result release energy called the "heat of hydration."
  • M^+ (g) + X^-(g) \to M^+ (aq) + X^-(aq)$ [heat of hydration]
  • The heat (enthalpy) of solution (Hsolution) is the sum of the lattice and hydration energies ( Hsolution = Hhydration + Hlattice energy).
  • In case you were wondering where we got the term "heat of hydration," it has to do with the fact that some solutions are highly exothermic when formed.
  • A hot solution results when the heat of hydration is much greater than the lattice energy of the solute.

• Constant-Pressure Calorimetry

  • The heat transferred to/from the solution in order for the reaction to occur is equal to the change in enthalpy ($\Delta H = q_P$), and a constant-pressure calorimeter thus measures this heat of reaction.
  • To determine the standard enthalpy of the reaction H+(aq) + OH–(aq) → H2O(l), equal volumes of 0.1 M solutions of HCl and of NaOH can be combined initially at 25°C.
  • This process is exothermic and as a result, a certain amount of heat qP will be released into the solution.
  • The number of joules of heat released into each gram of the solution is calculated from the product of the rise in temperature and the specific heat capacity of water (assuming that the solution is dilute enough so that its specific heat capacity is the same as that of pure water's).
  • The total quantity of transferred heat can then be calculated by multiplying the result with the mass of the solution.

• Solid Solubility and Temperature

  • The solubility of a given solute in a given solvent typically depends on temperature.
  • Some solutes exhibit solubility that is fairly independent of temperature.
  • As the temperature of a solution is increased, the average kinetic energy of the molecules that make up the solution also increases.
  • The average kinetic energy of the solute molecules also increases with temperature, and it destabilizes the solid state.
  • During recrystallization, an impure substance is taken up in a volume of solvent at a temperature at which it is insoluble, which is then heated until it becomes soluble.

• Heating Curve for Water

  • A constant rate of heating is assumed, so that one can also think of the x-axis as the amount of time that goes by as a substance is heated.
  • Instead, use the heat of fusion ($\Delta H_{fusion}$ ) to calculate how much heat was involved in that process: $q=m\cdot \Delta H_{fusion}$, where m is the mass of the sample of water.
  • Note that the specific heat capacity of liquid water is different than that of ice.
  • The liquid will begin to boil when enough heat has been absorbed by the solution that the temperature reaches the boiling point, where again, the temperature remains constant until all of the liquid has become gaseous water.
  • Use the heat of vaporization ($\Delta H_{vap}$ ) to calculate how much heat was absorbed in this process: $q=m\cdot C_{H_2O(g)}\cdot \Delta T$, where m is the mass of the sample of water.

• Solubility

  • Solubility is the relative ability of a solute (solid, liquid, or gas) to dissolve into a solvent and form a solution.
  • The solubility of a substance in a particular solvent is measured by the concentration of the saturated solution.
  • A solution is considered saturated when adding additional solute no longer increases the concentration of the solution.
  • As water molecules heat up, they vibrate more quickly and are better able to interact with and break apart the solute.
  • Pressure has a negligible effect on the solubility of solid and liquid solutes, but it has a strong effect on solutions with gaseous solutes.

• Specific Heat and Heat Capacity

  • Heat capacity is a measure of the amount of heat energy required to change the temperature of a pure substance by a given amount.
  • the molar heat capacity, which is the heat capacity per mole of a pure substance.
  • How much heat is required to raise the temperature of 36 grams of water from 300 to 310 K?
  • Specific heat capacity is the measure of the heat energy required to raise the temperature of a given quantity of a substance by one kelvin.
  • Latent heat of melting describes tœhe amount of heat required to melt a solid.

• Heat and Work

  • When energy is exchanged between thermodynamic systems by thermal interaction, the transfer of energy is called heat.
  • The units of heat are therefore the units of energy, or joules (J).
  • For example, when heat transfers from the hot water at the bottom of the pot to the cooler water at the top of the pot.
  • Work is the transfer of energy by any process other than heat.
  • This means that the total energy within a system is affected by the sum of two possible energy transfers: heat and work.

• Addition

  • The product is cyclohexane and the heat of reaction provides evidence of benzene's thermodynamic stability.
  • We have already noted that benzene does not react with chlorine or bromine in the absence of a catalyst and heat.
  • It is worth noting that these same conditions effect radical substitution of cyclohexane, the key factors in this change of behavior are the pi-bonds array in benzene, which permit addition, and the weaker C-H bonds in cyclohexane.
  • The addition of chlorine is shown below; two of the seven meso-stereoisomers appear in the second diagram below.
  • Another way of adding hydrogen to the benzene ring is by treatment with the electron rich solution of alkali metals, usually lithium or sodium, in liquid ammonia.

• Le Chatelier's Principle

  • Even if a desired product is not thermodynamically favored, the end-product can be obtained if it is continuously removed from the solution.
  • The effect of temperature on equilibrium has to do with the heat of reaction.
  • Recall that for an endothermic reaction, heat is absorbed in the reaction, and the value of $\Delta H$ is positive.
  • Heat is released in the reaction, so heat is a product, and the value of $\Delta H$ is negative:
  • Our heat of reaction is positive, so this reaction is endothermic.