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Le Chatelier Principle

Le Chatelier Principle Facts in Chemistry

Le Chatelier principle predicts the effect on the chemical system at equilibrium when some of the factors such as temperature, pressure, and concentration change. Therefore, how the system behaves in chemical science if any of these parameters of the system be altered was predict by Le Chatelier in 1885 and Braun in 1886. Le Chatelier made the generalization that if a system in equilibrium, a change in any of the factors that determine the condition of equilibrium will cause the equilibrium to shift in such a way as to minimize the effect of this change.

Hence we use the Le Chatelier principle which is sometimes called the principle of mobile equilibrium to express the thermodynamics effect of pressure, temperature, and concentration on equilibrium in learning chemistry.

Le Chatelier Principle facts, effect and change of chemical Equilibrium position

Equilibrium Pressure Change by Le Chatelier

According to the Le Chatelier principle, If the pressure increases in the system at equilibrium, the volume would tend to diminish due to minimizing the effect of pressure. Therefore, Le Chatelier principle suggested that the synthesis of ammonia from hydrogen and nitrogen molecule, the volume would be decreased if the number of molecules decreases or more ammonia gas is formed.

Hence when the pressure increases, the shift of the chemical reaction in such direction where the sum of the stoichiometric coefficient of gas molecules lowered. In other words, an increase in pressure shifts the equilibrium to the low volume side to the high volume side where the decrease of pressure minimizes its effect according to Le Chatelier.

Le Chatelier Principle for Temperature Change

Van’t Hoff equation would numerically calculate the shift of equilibrium with temperature. But Le Chatelir principle quantitative analysis of equilibrium with the change of temperature. According to the Le Chatelier principle when the temperature increases, the equilibrium will shift in the endothermic direction. Therefore, the equilibrium shifted from low enthalpy to the high enthalpy side with the absorption or utilization of the specific heat. Let us illustrate this by elementary reaction for the formation of ammonia by bonding nitrogen and the hydrogen atom.

For the synthesis of ammonia,
N2 + 3H2 ⇆ 2NH3 ΔH = -300 joule
where ΔH = ΣHproduct – ΣHreactant

  1. In the above reaction, the enthalpy of the reactant side is greater than the product side. Therefore, with the increases in temperature backward reaction favors where the equilibrium shifted to the higher entropy side. Hence according to the Le Chatelier principle, the dissociation of ammonia occurs with the decreasing product.
  2. But with the decrease of temperature, the equilibrium will shift in the exothermic direction. Since the shift will be high enthalpy side to low enthalpy side. Therefore, according to the Le Chatelier principle with the decreases in temperature forward reaction favors and the production of ammonia increased.

The heat change or temperature change in chemical reactions also depends on the physical state like gaseous, solid, or liquid state of the reacting component. Therefore, the oxidation of liquid benzene in solution depends on the concentration of the solution. But for gaseous benzene in very dilute solutions heat of the reactions remains the constant value.

Effect of inert  gas on Equilibrium

According to Le Chatelier principle addition of inert gas or periodic table elements like helium, neon, argon, and krypton is done in two ways as constant temperature and constant volume of the ideal system.

  1. The addition of inert gas at constant volume can not affect the equilibrium. Since the concentration of the total reacting components remains unchanged according to Le Chatelier.
  2. When inert gas added to the system at constant pressure the volume of the reacting system increased. Hence the total concentration or density decreased. Therefore, according to the Le Chatelier principle, the system will move in the direction where the number of moles increases.

Catalyst effect on Equilibrium

Catalyst is the foreign substance in chemical kinetics which enhances the speed of the chemical reaction itself remaining unaltered in mass and chemical composition. Therefore, catalysts can speed up the rate reaction without effect on the equilibrium. For example, oxidizing carbon monoxide to dioxide in presence of nitric oxide speeds up the rate. Hence Le Chatelier principle suggested that the reaction proceeds by lowering the activation energy to reach equilibrium quickly.

Sometimes a small quantity of foreign substances inhabits the reaction rate. These are called catalyst poison. In some cases, the poisons may form definite chemical compounds with catalyst atoms. For example, the small quantity of arsenic destroys the catalytic activity of the platinum element by forming platinum arsenide in the sulfuric acid manufacture process.

Question: What happens to the vapour pressure of a liquid, when a nonvolatile solute dissolved in it according to the Le Chatelier principle?

Answer: The pure solution, the mole faction x1 = 1. When the non-volatile solute added to the solvent the mole fraction of the solvent changes 1 to ㄑ1. Therefore, to reduce these effects, according to the Le Chatelier principle the solvent is less vaporized. This leads to a lowering of the vapour pressure.

Conclusions of Le Chatelier’s principle

Although the equilibrium constant has no dimension and independent of the pressure of reaction for the ideal gas molecule. But Le Chatelier’s principle conclusions some special facts for reacting and product components

  • For example, if the volume of the nonreactive system decreased by a specific amount the pressure rises correspondingly. Therefore, according to the Le Chatelier principle, the equilibrium shifted to the low volume sides. Hence the pressure increases become less than that of the non-reactive system. This shift in the equilibrium position natural gases makes the reactive system higher compressibility than the non-reactive one.
  • Similarly, if the fixed quantity of the heat supplied to the non-reacting system temperature of corresponding increases. Therefore, the heat supplied does not increase the temperature so much. Since the equilibrium shift to the higher enthalpy or free energy side. This shift of equilibrium makes the heat much higher than the non-reactive system. Since the reacting system chosen as a heat storage medium by Le Chatelier principle.