QUESTIONS AND PROBLEMS

1. Find the value of the rate constant for the reaction A + B AB if at concentration of substances A and B equal to 0.05 M and 0.01 M respectively, the rate of the chemical reaction is 5 x 10 M/min.

2. How many times will the rate of the reaction 2A + B A₂B change if the concentration of A is doubled, and that of B is halved ?

3. What is the temperature coefficient of the reaction rate if the rate grows 15.6 times when the temperature is increased by 30 Kelvins?

4. Find the equilibrium constant of the reaction N₂O₄ 2NO₂ if the initial concentration of the N₂O₄ was 0.08 M, and by the moment when equilibrium was established 50% of N₂O₄ was dissociated.

5. In which direction will the following equilibria shift :

a) 2CO + O₂ 2CO₂

b) N₂ + O₂ 2NO

( 1 ) when the temperature is lowered; ( 2 ) when the pressure is increased; ( 3 ) when the concentration of oxygen is increased ?

Experiment 1

IONIC EQUILIBRIUM

Some substances while being dissolved interact with molecules of a solvent. As a result they dissociate and form ions. The process of dissociation can be written as:

A_nB_m nA^m+ + mB

Na₂CO₃ 2Na⁺ + CO

Ca(OH)₂ Ca²⁺ + 20H

The process of dissociation can be quantitatively characterized by a degree of dissociation (a) and a dissociation constant (K_dis).

a =

n - number of molecules (moles)

C - concentration

If a > 0.3, the electrolyte is called strong. Strong electrolytes are salts, strong acids (HCl, H₂SO₄, HNO₃, HClO₄ and some others), hydroxides of alkaline and alkaline-earth metals.

If a < 0.03, the electrolyte is called weak. The dissociation of a weak electrolyte is a reversible and a stepwise process.

K_dis is determined as an equilibrium constant of a reaction of dissociation of an electrolyte.

H₃PO₄ H⁺ + H₂PO

H₂PO₄^- H⁺ + HPO

HPO H⁺ + PO

K_1,K₂,K₃ - stepwise dissociation constants.

For the overall process of the dissociation of phosphoric acid

The dissociation constant of an electrolyte doesn't depend on concentration but increases with the evaluation of temperature.

For an electrolyte AB dissociating into ions A⁺ and B the dissociation constant and the degree of dissociation are related by the equation (Ostwald's dilution law):

For very weak electrolytes and very diluted solutions the equation may be simplified:

K = a²C

If an ion common to one of the ions formed in the dissociation process of a weak electrolyte is introduced into the solution, equilibrium of dissociation is violated and shifts towards the direction of formation of undissociated molecules so that the degree of dissociation of the electrolyte diminishes. For instance, the addition of ammonium ions (for example ammonium chloride) to a solution of ammonium hydroxide leads to an increase in concentration of NH₄⁺ ions and, in accordance with Le Chatelier's principle, the equilibrium of the dissociation NH₄OH NH₄⁺ + OH shifts to the left.

Ionic concentrations in solutions of strong electrolytes are quite great so that the forces of interionic interaction manifest themselves appreciably even at low concentration of an electrolyte. As a result, the ions are not completely free. This is why the state of ions in a solution is described, in addition to their concentration, by their activity, i.e. the conditional (effective) concentration of ions in accordance with which they act in chemical reactions.

a = f ^.C

where a is activity; f is activity coefficient

The activity coefficients depend on the composition and concentration of the solution, and on the charge and nature of the ion, and on other conditions. It can be considered approximately that in diluted solutions the activity coefficient of an ion depends only on the charge of the ion and ionic strength of the solution:

log f = - 0.5 Z² I^1/2

I = 0.5 (C₁ z₁² + C₂ z₂² + C₃ z₃² + ... + C_i z_i²)

where z - charge of the ion; I - ionic strength of the solution; C - concentrations of each ion in the solution

Equilibria in solutions of amphotheric electrolytes

Such hydroxides as Al(OH)₃, Cr(OH)₃, Zn(OH)₂, Pb(OH)₂, Sn(OH)₂ and some other can form both H+ and OH ions while dissociating in saturated solutions:

2H₂O + Zn²⁺ + 2OH Zn(OH)₂ 2H₂O H₂[Zn(OH)₄] 2H⁺ + Zn(OH)₄]²

3H₂O + Al³⁺ + 30H Al(OH)₃ 3H₂O H₃[Al(OH)₆] 3H⁺ + Al(OH)₆]

The addition of strong acids and bases shifts the equilibria of the dissociation process so that only one reaction becomes possible.

Date: 2015-01-12; view: 1256