Chemical Equilibrium: Strong Acids and Bases, Study notes of Chemistry

Download Chemical Equilibrium: Strong Acids and Bases and more Study notes Chemistry in PDF only on Docsity! -61- STRONG ACIDS AND BASES • Before we look at the behaviour of strong acids and bases we must briefly discuss THE CONCEPT OF CHEMICAL EQUILIBRIUM.... • A chemical reaction is at equilibrium when it is proceeding forward and backward at the same rate. • The concentrations of all the reactants remain constant. For the reaction: a A + b B º c C + d D • At equilibrium: Keq = • In this expression, [A] represents the concentration (usually a Molarity, or mol L—1 ) of reactant A and Keq is the Equilibrium Constant for the reaction at that temperature. • The magnitude (how large is the numerical value?) of Keq indicates the “position” of the equilibrium; if Keq is large, the equilibrium lies far to the right; there are more products than reactants. • If Keq is small, the equilibrium lies to the left; there are more reactants than products. • Solids do not appear in Keq expressions; their concentrations (/densities) are constant. -62- WATER AS A SOLVENT • As most of the solution chemistry we study is carried out in water, or in aqueous solution, we need to consider the behaviour of water............. • Water undergoes auto-ionization to a very small extent, written as the equilibrium: 2 H2O º H3O + + OH — Keq = • What is the position of this equilibrium ? • In aqueous solutions, the concentration of water, [H2O], is roughly constant ; • So we do not include the [H2O]2 term in the Keq expression giving: • This is called the Ion Product of Water. • Its numerical value at 25EC is 1.00 x 10 —14 ........... so; • As [H3O+] = [OH —] in pure water; -65- Memorize these six strong bases !! LiOH, NaOH, KOH are all monobasic. Ca(OH)2 Sr(OH)2 and Ba(OH)2 are all dibasic. • Recall that bases are substances which either: i) increase OH — ion concentration in water ii) decrease H3O+ ion concentration in water EXAMPLES: i) Soluble hydroxides of the Groups I and II metals: Sodium hydroxide, NaOH: Barium hydroxide, Ba(OH)2: • All six are considered strong bases, 100% ionized in solution. For a summary of the common strong acids and bases, see Table 4.1 in MH5, or the Tutorial Problems Manual. • Another contrast, ammonia, NH3 is a weak base that increases OH — concentration in water: NH3 + H2O º NH4 +(aq) + OH— (aq) ammonia ammonium ion -66- ii) Many metal oxides and hydroxides are insoluble in water, but react with acids: • By decreasing [H+], Mg(OH)2 is acting as a base. • Zinc oxide, ZnO will react similarly: • Barium oxide, BaO and cadmium oxide, CdO are two more metal oxides that will act as strong bases. • So, what happens if we mix an acid with a base? • IMPORTANT: If you mix any acid with any base, the result will be a neutralization reaction. • The stoichiometry is: • The reaction will proceed until all of the Limiting Reagent (either the H+ from the acid or the OH — from the base) has been used up. • If moles H+ = moles OH —............ -67- ACIDITY: The pH Scale • The quantity pH (power of hydrogen) is defined as: pH = ! log10 [H +] • It is a convenient scale for expressing the concentration of H+ (or hydronium, H3O+) ions...... • In pure water; [H+] = 1.0 x 10!7 M pH = ! log (1.0 x 10!7) = 7.00 • So this is a neutral solution with pH = 7.00 • Because of the minus sign, pH decreases as [H+] in solution increases............... pH < 7.00 [H+] > 1.0 x 10!7 M; solution is acidic pH > 7.00 [H+] < 1.0 x 10!7 M; solution is basic • We can use pOH to express [OH —] concentration: pOH = ! log10 [OH —] • And, since [H+] [OH —] = Kw , taking logs we get: log [H+] + log[OH—] = logKw or pH + pOH = pKw = 14.00 • pH ranges from 0 (acidic) to 14 (basic), pOH ranges from 14 to 0 -70- 2) A 2.50 g sample of Ba(OH)2 is dissolved in water. This solution requires 41.50 mL of HNO3 for neutralization. What is the molarity of the HNO3 ? -71- 3) An organic acid is known to be diprotic. When 0.125 g is dissolved in water, it requires 17.5 mL of 0.0893 M KOH for neutralization. What is the molar mass of the acid ? -72- DILUTION OF SOLUTIONS • When a solution is diluted (either by adding water or another solution), the concentration of that solution is decreased. • The number of moles of the solute do not change, but the volume does.... • If two solutions are mixed together, the volume of the resultant solution is always the total volume of the two original solutions. EXAMPLE: A 50.0 mL sample of a 0.150 M solution of HNO3 is diluted by adding 75.0 mL of water. Calculate the pH of the resultant solution.