by laura.cp897117 » Fri Nov 27, 2015 7:24 pm
I have 2 questions:
[B]2) When the body’s intake of calcium is low, in order to maintain homeostasis, there is an increased renal absorption of calcium. Based on the information provided, which of the following changes in urine is most consistent with a decrease in calcium intake in a person with kidney stones?
A) Decreased oxalate ions
B) Increased solubility product
C) Decreased pH
D) Increased pH
Result: Your answer is wrong.
Your choice: A Correct choice: D
Explanation:
We learn from the question stem that a decrease in calcium intake is associated with increased renal absorption of calcium, which means there will be less calcium in the urine. Now we just apply Le Chatelier’s principle: Beginning with reaction (3), if calcium is low (i.e., being removed), the reaction shifts to the left, producing more oxalate ions; reaction (2) shifts to the left; and then reaction (1) shifts to the left. The net result of the latter two reactions is the net removal of hydrogen ions, which means that the pH increases. [B/]
Doesn't the question say that there is a DECREASE in calcium intake, therefore the [Ca+] in urine
would be higher?
[B] 7) The equilibrium constant of a reaction could be affected by which of the following?
A. the addition of a catalyst
B. an increase in temperature
C. a change in the initial concentrations of reactants
D. the reactants being converted mostly to product
Result: Your answer is incorrect.
Your choice: C Correct choice: B
Explanation: Answer choice B is correct as this is part of Le Chatelier’s principal. Moreover, changes in concentrations (answer choice C) or volume and/or pressure (answer D) can shift the position of an equilibrium constant without changing the equilibrium constant itself.
Adding a catalyst will decrease the activation energy; however, catalysts do not affect the position of an equilibrium as a catalyst will affect both the forward as well as the reverse reactions equally. A catalyst will only bring a system to equilibrium sooner and NOT affect the constant.
As for the temperature, changing the temperature of a system at equilibrium (i.e., an increase in temperature as for answer choice B), causes the equilibrium to shift because it changes the constant K. The enthalpy change of a reaction is the critical factor i.e., if it is exothermic, the heat is part of the products and is released and if it is endothermic, the heat is added and is part of the reactant side. So changing a temperature will affect the constant as the temperature will be part of the equation at equilibrium and added either to the reactant side (if endothermic) or product side (if exothermic). [B/]
Could you explain again how a change in temp causes the constant K to change? And also how this differs from a change in concentration does not cause the Keq to change? I assumed since Keq = [product]/[reactant], that the change in concentration will cause the Keq to change?