The Physical Chemistry - Essay Example

1. Calculate the value of K at 25oC for the following reaction: 4NH3(g) + 7O2(g)  4NO2(g) + 6H2O(g) R is 0.08206Latm/Kmol and Kp is 9 x 103. Equilibrium constant K is written in terms of partial pressure equilibrium constant Kp as (Lewis): Kp = 1.9×103 atm-1 R = 0.08206Latm/Kmol T = (273+25)K = 298K n = number of product molecules - reactant molecules = (4+6)-(4+7) =- 1 Therefore, K = (1.9×103 atm-1)/( 0.08206Latm/Kmol×298K)-1 = 46.46×103 mol/L. 2. These questions are concerned with pH, molarity, and neutralization of an acid with a base. a. What is the molarity of H+ ions in a solution with a pH of 1.5 (the pH of stomach acid). Concentration of hydrogen ions in an aqueous solution is expressed as (Lewis): [H+(aq)]=1×10-pH = ( 1×10-1.5 )mol/L = 31.62×10-3 mol/L In terms of molarity, which refers to amount of a species in unit volume of solution, 1 mol/L = 1M Molarity of H+(aq) ions with pH of 1.5 is 31.62×10-3 M or 31.62 mM. b. How many moles of H+ ions are there in 0.067 L of the solution described in part (a)? Number of moles of a species in a solution can be calculated as (Lewis): n = cV, where c is the concentration and V the volume. Here, for H+ ions, c = 31.62×10-3 mol/L V=0.067L n = (31.62×10-3 mol/L)( 0.067L)= 2.12×10-3 mol = 2.12 mmol Therefore, 0.067 L of the solution contains 2.12 mmol of H+ ions. c. How many grams of sodium hydroxide would be required to neutralize the acid in part (b)? In aqueous solution, solid sodium hydroxide immediately dissociates to sodium ions and hydroxide ions Neutralization of acid with sodium hydroxide involves reaction between hydroxonium ions and the hydroxide ions: H3O+(aq) + OH- (aq)  2H2O(l) The overall reaction is: NaOH (s) + H3O+ (aq)  Na+ (aq) + 2H2O (l) Neutralization involves change in solution pH from 1.5 to 7.0. pH = 5.5 pOH = 14-5.5 = 8.5 (Lewis). Change in hydroxide ion concentration = 1×10-8.5 = 3.162×10-9 mol/L Number of moles of hydroxide ions required to neutralize 0.067L of acid or n is: n= (3.162×10-9 mol/L)(0.076L)=2.12×10-7 mol As hydroxide ions are formed by the dissociation of sodium hydroxide, 2.12×10-7 mol of sodium hydroxide is required for the neutralization. Molar mass of sodium hydroxide (Mmol) is 40g/mol. Mass of sodium hydroxide required to neutralize 0.067L of acid or M is calculated as: M = n×Mmol = (40g/mol)( 2.12×10-7 mol) = 8.4g Therefore, 8.4g of sodium hydroxide is required to neutralize 0.067L of acid with pH1.5. 3. Short answer questions. a. Elements with atomic numbers of 110 and beyond have not yet been discovered. Which of the elements with atomic numbers in the range of 110 - 118 would have chemical properties most like those of thallium (Tl)? Explain why it is possible to make this kind of prediction. On the basis of periodicity of physical and chemical properties of elements in the periodic table, the element with atomic number 113 would have properties similar to thallium. The two elements belong to the same group in the periodic table and have same valence shell electronic configuration ("Visual Elements: Patterns in the Periodic Table"). Periodicity can be found in ionization energies and electronic affinities along the groups in the periodic table (Atkins). This prediction can be made because of arrangement of atoms on the basis of valence shell electrons in the periodic table. b. Explain why some molecules are polar and others are nonpolar. (A picture is worth 103 words. i.e., include a drawing with your explanation.) Use the back of this page for more space. In heteronuclear molecules, some atoms are more electronegative than others. The electron pair in a bond in such a molecule is shared unequally. The more electronegative atom attracts the shared electron pair, causing a polarity in the bond. The more electronegative atom has a partial negative charge, while the other atom has a partial positive charge. Molecules where the difference in electronegativities is low (0) are nonpolar, while those with greater differences are polar (see Figure 1). In molecules with very high electronegativity differences, the bonds are ionic as the electrons are with the electronegative atom (Atkins; Lewis). 4. The questions below refer to the following system: A 2-liter flask initially contains 1.2 mol of gas A, 0.60 mol of gas B, 0.60 mol of gas C and 0.50 mol of gas D. The equilibrium constant is 6.0 at 500K. Note the following reaction: A + B ↔ C + D a. Determine the initial concentrations of A, B, C and D. Concentration (c) can be calculated from number of moles (n) and volume (V): Initial concentration of A or [A]0 = 1.2mol/2L = 0.6 mol/L Initial concentration of B or [B]0 = 0.6mol/2L = 0.3 mol/L Initial concentration of C or [C]0 = 0.6mol/2L = 0.3 mol/L Initial concentration of D or [D]0 = 0.5 mol/2L = 0.25 mol/L b. Calculate the reaction quotient. The reaction quotient Q is calculated as (Gilbert): Here, at time t=0, c. Determine the direction of shift in the equilibrium. Right or Left? Here, Q Read More