(1) (12 points) Consider the weak bases NH3(aq) (Kb = 1.8 105) and NaF(aq) (Ka(H
ID: 917613 • Letter: #
Question
(1) (12 points) Consider the weak bases NH3(aq) (Kb = 1.8 105) and NaF(aq) (Ka(HF) = 6.8 104) in water.
(a) (2 pt) Which is the stronger base, NH3(aq) or F(aq)? Explain your choice.
(b) (2 pts) Write the base hydrolysis equilibria for NH3(aq) and F(aq).
(c) (6 pts) Using the thermodynamic data provided, calculate H (kJ), S (J/K), and G (kJ) at 298.15 K for the chemical equilibria in part (b).
(d) (2 pts) Calculate the base hydrolysis constants, Kb, for NH3(aq) and F(aq) using your results in (b) and (c).
(2) (9 points) One important industrial process to obtain hydrogen gas is by steam reforming of natural gas at temperatures 700-1100C. An important chemical equilibrium in this process is the equilibrium between methane and water vapor with carbon monoxide and hydrogen gas.
(a) (2 pts) Write the chemical equilibrium between methane and water vapor as reactants and carbon monoxide and hydrogen gas as products, using integer coefficients.
(b) (2 pts) Evaluate H (kJ), S (J/K) at 298.15 K for the equilibrium in part (a).
(c) (2pts)AssumingthatHandSdonotchangesignificantlywithtemperature,forwhattemperatures(inC) is the equilibrium in part (a) spontaneous from left-to-right?
(d) (2 pts) Using the values of H and S from part (a), estimate the values of the equilibrium constant for the equilibrium in part (a) at 400 C and 900 C.
(e) (1 pt) How does your result in part (d) demonstrate LeChátelier’s principle?
Explanation / Answer
1.
a. A higher Kb means a stronger base.so, as HF has a higher Kb of 6.8E-4, F-(aq) is a stronger base.
b. Base hydrolysis reaction: Base is reacted with water to give conjuagte acid and hydroxide ions.
NH3 (aq) + H2O (aq) ====> NH4+ (aq) + OH-
F- (aq) + H2O(aq) ====> HF (aq) + OH-
Need thermodynamic data for C part.
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