6. Answer the following questions from the figure given below
Solid
Liquid
Gas
S
(1) Give the name of processes P, Q, R and S.
(2) Which processes are exothermic ?
(3) Which processes are endothermic ?
Answers
Answer:
Dissolution
The rate of dissolution is represented by the Noyes-Whitney equation: dm/dt = D*A*(Cs - C)/h
Where:
dm/dt represents the rate of dissolution
D represents the diffusion coefficient for the compound
A represents the surface area available for dissolution
Cs represents the solubility of the compound
C represents the solute concentration in bulk solution at time t
h represents the thickness of the dissolution layer
Solubility
Temperature
Effect of temperature on liquid and solid solutes
As temperature increases, the solubility of a solid or liquid can increase or decrease depending on whether the dissolution reaction is exothermic or endothermic.
Increasing solubility with increasing temperature
In endothermic reactions, the net energy from the bonds breaking and forming results in heat energy being absorbed when the solute dissolves in solution. When the temperature of the system increases this introduces heat into the system.
So according to Le Chatelier’s Principle, the system will adjust to this increase in the heat by promoting the dissolution reaction to absorb some of the heat energy. Hence increasing the temperature of the system increases the solubility of the solute.
An example of a solute that increases in solubility with increasing temperature is ammonium nitrate which can be used in first-aid cold packs. As ammonium nitrate dissolving in solution is an endothermic reaction, heat energy is absorbed from the environment. This causes the surrounding environment to feel cold.
Decreasing solubility with increasing temperature
In exothermic reactions, increasing temperature decreases the solubility of the solute. This is because heat energy is released when the solute dissolves in solution. Increasing temperature introduces more heat into the system. So according to Le Chatelier’s Principle, the system will adjust to this excess in heat energy by inhibiting the dissolution reaction.
An example of a solute that decreases in solubility with increasing temperature is calcium hydroxide which can be used in medical situations to treat chemical burns and as an antacid.
Effect of temperature on gas solutes
A gas becomes less soluble as temperate increases. This is because in general heat energy is released as gas dissolves in solution i.e. the process is exothermic.
Increasing temperature (increased heat energy) results in increased kinetic energy. This increase in kinetic energy allows greater movement of the gas particles resulting in the intermolecular bonds between the gas solute and solvent being broken.
Pressure: Henry’s Law
The solubility of gas is affected by changes in pressure on the system. A gas dissolves in liquids to form solutions. This results in equilibrium in the system where a proportion of gas molecules is dissolved in liquid while a proportion remains in the gas above the liquid.
Henry’s law states that: “At constant temperature, the amount of gas that dissolves in a volume of liquid is proportional to the partial pressure of the gas in equilibrium with the liquid.”
C = kP
Where:
C represents the solubility of the gas at a certain temperature in a specific solvent
K represents Henry’s Law constant
P represents the partial pressure of the gas i.e. the pressure the gas exerts on the system at a given volume and temperature.
Hence as the pressure of the gas above the liquid in the system increases, the gas molecules become more soluble in the solvent. Likewise, if the pressure of the gas in the system decreases, gas becomes less soluble in the solvent.
Dependence of Change of state on heat energy
Explanation:
The states of matter can be interchanged by changing the conditions of temperature and Pressure .
It is shown in the given figure that is attached below .
In it "P" is melting
In it "Q" is Vaporization .
In it "R" is Freezing
In it "S" is condensation .
The process of melting (change of state of solid to liquid ) and vaporization (change of liquid state to gaseous state ) is endothermic that is heat is absorbed by the system .
The process of freezing (change of liquid to solid state ) and Condensation (change of gaseous state to liquid state ) is exothermic that is heat is released by the system .
