Lecture 6 - General Inorganic Chemistry PDF

Pharmaceutical Analytical Chemistry I (PC 111) Level I Students (Pharm D Clinical) General Inorganic Chemistry [Lecture 6] Dr. Galal Magdy, PhD 1  N.B. Metathesis (double replacement) reaction: It involves two...

Pharmaceutical Analytical Chemistry I (PC 111) Level I Students (Pharm D Clinical) General Inorganic Chemistry [Lecture 6] Dr. Galal Magdy, PhD 1  N.B. Metathesis (double replacement) reaction: It involves two compounds in solution and an exchange of cations between the two anions (also called double replacement reaction). (Q. Give name?) EX: AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) 2  Chemical Equilibrium  Assuming the following chemical reaction: A+B↔C+D Formation direction (Forward reaction) Decomposition direction (Backward reaction)  Products of a given reaction interact to reform the initial substances; in other words they proceed in two opposite directions at the same time.  As the reaction proceeds: products Concentration  The concentration of the reactants decrease, and so also the rate at which they react decreases.  The concentration of the products increase, reactants and so also the rate at which they react increases. Time 3  A state of equilibrium is reached when the rate of the forward reaction (between the reactants A and B) is equal to the rate of the backward reaction (between the products C and D)  Dynamic equilibrium.  Equilibrium: is a state where the concentrations of the reactants and products remain constant until the equilibrium is disturbed. Q. Complete: The relative concentration of products and reactants at equilibrium is………... 4  Reactants Products  First, system reaches equilibrium.  Then, system continually exchanges products and reactants, while maintaining equilibrium distribution. Q.: At equilibrium the amount of reactants and products are constant, but not necessarily equal. (T or F) 5  Reaction rate Definition: The change in concentration of a reactant or product per unit time. (Q. Give name?) Some reactions take place slowly while others proceed very rapidly. Generally, reactions involving the simple combination of ions occur very rapidly, but covalently bonded molecules or groups often react slowly. (T or F) Examples:  Very rapid reaction: Ag+(aq) + Cl-(aq)  AgCl (s)  Very slow reaction: iron to iron oxide (rust) 6  Factors affecting reaction rate: (Q. Enumerate?)  The nature of the reactants.  Concentration of the reactants.  Pressure in gaseous system.  Temperature.  Presence of catalysts.  Physical nature of medium of the reaction. 7  Effect of concentration on reaction rate: Collision between ions or molecules leads to product formation. As number of mols increases  Collision chance increases  reaction rate increases The law describes the effect of concentration on reaction rate is called: Law of Mass action (Law of Chemical Equilibrium)  Def. The rate of a chemical reaction is directly proportional to the active masses (molar concentrations) of the reacting substances present at the time. Ag+ Cl- AgCl Slow Rapid Lower conc. Higher conc. 8  Def. The rate of chemical reaction is directly proportional to the product of molar concentration of the reacting substances, each raised to a power equal to the number of ions or molecules appearing in the balanced equation for the reaction. mA + nB ↔ C + D Rate  [A]m[B]n Rate (r) = K [A]m[B]n Where: k is the rate constant; m and n are the stoichiometric coefficient with respect to compounds A and B, respectively. If [A] and [B] each = 1 M (1 mol/L), then, r = K. Therefore the rate constant can be defined as the rate of a reaction in which the concentration of each of the reacting substances is 1 M. (Q. Give name?) 9  Equilibrium constant (Keq) aA + bB ↔ cC + dD The rate of the forward reaction (r1) is proportional to the molar concentration of A and B (r1  [A]a.[B]b). r1= k1.[A]a. [B]b Similarly for the rate of the reverse reaction (r2): r2= k2.[C]c. [D]d At equilibrium, the two opposing rates will be equal(r1 = r2) K1. [A]a.[B]b = K2.[C]c.[D]d products Concentration K1/k2 = [C]c [D]d [A]a [B]b K1/k2 is constant = keq reactants Equilibrium constant (Keq) = [C]c [D]d [A]a [B]b Time 10  N.B: If Keq > 1, (K1 > K2) and the rate of the forward reaction is higher, the reaction is product- favored; product predominates at equilibrium. If Keq < 1, (K1 < K2) and the rate of the backward reaction is higher, the reaction is reactant- favored; reactant predominates at equilibrium. Keq is not affected by catalyst as it always affects both forward and backward reaction rates to the same extent. (T or F) Keq is not affected by changing the concentration; however, the equilibrium point will be shifted. (T or F) Keq is affected only by temperature and pressure and depends on the nature of the reactants. (T or F) 11  Le Chatelier's Principle Definition: “If a stress is applied to a system in equilibrium, the equilibrium tends to shift in a direction to neutralize the stress”. Factors affecting the Reaction equilibrium (direction):  Temperature.  Pressure.  Concentration.  12 1. Temperature: Exothermic reaction: produces heat (heat is a product)  Adding energy shifts the equilibrium to the left (away from the heat side). Endothermic reaction: absorbs energy (heat is a reactant)  Adding energy shifts the equilibrium to the right (away from the heat side). EX: 2 SO2 + O2  2 SO3 + 47,00 cal  If energy is added, the equilibrium will be shifted to the left. (T or F) 13  2. Pressure: Pressure influences equilibrium only if one or more of the substances involved is a gas. According to Le Chatelier’s principle, increasing the pressure shifts the equilibrium in the direction that decreases the number of moles in the gas phase and thus decreases the pressure.  EX1:In ammonia synthesis N2(g) + 3 H2 (g)  2 NH3 (g) 4 moles of gas  2 moles of gas  Increasing the pressure favors the forward reaction because 4 moles of gas are converted into 2 moles of gas.  EX2: H2 + I2  2 HI 2 moles of gas  2 moles of gas  No shift occurs upon increasing pressure since no change in the total no of moles occurs. 14  3. Concentration: concentration A + B C + D concentration A + B C + D  If the concentration of A or B is increased the equilibrium is shifted to right and vice versa (common ion effect).  If conc. changes → no change in Keq → Only position of equilibrium changes. 15  Common Ion Effect  If a strong electrolyte with an ion in common is added to a solution of weak electrolyte, the equilibrium will shift in a direction to decrease the concentration of that ion.  Common ion suppresses the ionization of weak acids or weak bases. EX1: Ionization of a solution of acetic acid will be suppressed by the addition of HCl. (Q. Why?) CH3COOH  CH3COO- + H+ HCl Cl- + H+  Acetic acid is ionized to acetate ions and hydrogen ions.  Once the strength of H+ [common ion effect] is increased by addition of HCl, then the reaction direction (the equilibrium) will be shifted to the left.  Consequently, the strength of acetic acid and its degree of ionization will decrease. 16  EX2: AgCl Ag+ + Cl- AgNO3 Ag+ + NO3- Adding AgNO3 to a saturated solution of AgCl will cause an increase in [Ag+] concentration leading to decrease the solubility of AgCl. (Q. Why?) Q. AgCl is more insoluble in AgNO3 than in H2O. (T or F) AgCl Ag+ + Cl- NaCl Na+ + Cl- Adding NaCl to a saturated solution of AgCl will cause an increase in [Cl-] concentration leading to decrease the solubility of AgCl. (Q. Why?) 17 Thank You