Lec.3

1. Lec.3 Chemical equilibrium

2. What is a Chemical Equilibrium?  A+B C+D • The concentrations of all reactants &products remain constant with time. • Any chemical reactions carried out in a closed vessel will reach Equilibrium.

3. Types of chemical reactions 1- Complete (irreversible) reactions: the reaction takes place instantaneously or at once, these reactions occurs in one direction i.e., the product can’t combine with each other again reforming the reactants. • Examples:NaCl +AgNO3 NaNO3 + AgCl (white ppt) • Mg+ H2 SO4 MgSO4+ H2 2-Incomplete (reversible) reactions: reactions occur in both directions, .forward and backward, so both reactant and products are always found in solution and can combine again. • Example: CH3COOH + C2H5OH CH3COOC2H5 + H2O Acetic acid ethyl alcohol ethyl methyl ester

4. Factors affecting the rate of chemical reaction: The rate of chemical reaction is defined as the amount of the reacting substances (in moles) transformed per second into resultants. Factors influence the rate of chemical reaction are: 1) Temperature 2) Catalyst 3) Concentration ( pressure in case of gases) • 1. Temperature : • Temperature is a measure of the average kinetic energy of the molecules and • their velocity, as temperature increases, the velocity of the molecules increase and • the probability of collision between the reacting molecules increase . • 2.Catalyst • Catalyst is a substance, which affects the velocity of the reaction without • itself being altered in composition or quantity.  • 3.Concentration • Increasing the concentration of substance; increases the chances of collision • of its molecules with those of other reacting substances and increases the • speed of reaction.

5. The law of Mass action :1. Irreversible Reaction A+B C+D • The velocity of a chemical reaction at constant temperature is proportional to the product of the active masses (concentration)of the reacting substance . • Where :active mass= moles/litre V α [A] * [B] V = K *[A] *[B] • Where :Square brackets " are used to denote molecular concentration. • K is the velocity constant for given temperature. If the reaction 2A+ B 2C V= K [A]2 *[B] at constant temp.

6. 2.Reversible reactions

7. The rate of chemical reaction is measured by: • Change in concentration of a reactant or product per unit time Note:Concentration unit:- mole/litre time unit:- minute or second. • During chemical reaction: • Irreversible reaction(complete),the concentration of reactants decrease and • the concentration of products increases. • Reversible reaction(incomplete), the concentration of reactants decreases • until equilibrium state is reached.

8. Effect of temperature on equilibrium reactions: • The effect of a change of temperature on a reaction will depend on whether the reaction is exothermic or endothermic. • • In exothermic reactions, increasing temperature will force the reaction to go • to the backward direction. • • In endothermic reactions, increasing temperature will force the reaction to • go to the forward direction. “At constant temperature & pressure for a certain reaction K is constant no matter how you change the concentration of the reactants”.

9. Note: Chemical equilibrium occurs only in reversible reaction when the rate of forward reaction is equal to the rate of backward where the concentration of reactants and products remains unchanged at the same temperature and pressure.

11. Example 2: The reaction for the formation of nitrosyl chloride: 2NO(g)+Cl2(g) 2NOCl(g) was studied at 25 ºC. The pressures at equilibrium were: PNOCl=1.2 atm. PNO=5*10-2 atm. PCl2 =3*10-1 atm. Calculate the value of Kp at 25 ºC. Answer: Kp= (PNOCl)2/ PCl2*(PNO)2 = (1.2)2/ 3*10-1 * ( 5*10-2)2 =1.9*103