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Topic: equilibrium test questions  (Read 19389 times)

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777888

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Re:equilibrium test questions
« Reply #15 on: January 08, 2005, 12:44:10 AM »
7. For the rxn: A + B + 200kJ <-> C + D, predict the effects after drecreasing [D].
a)release energy
b)increase [C]
c)both a and b

[b is definitely right, but I'm not quite sure about a...the temperature would decrease, but the 200kJ is in the left side of the rxn...]

Offline Donaldson Tan

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Re:equilibrium test questions
« Reply #16 on: January 08, 2005, 08:51:05 AM »
haha.. the tabulation method i taught u is now called ICE. Cute name..
Normally, exam-wise, if you arent given the Ksp of Ca(OH)2, you can safely assume 100% dissociation.

what is dH for question(7) ?
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:equilibrium test questions
« Reply #17 on: January 08, 2005, 11:19:17 AM »
haha.. the tabulation method i taught u is now called ICE. Cute name..
Normally, exam-wise, if you arent given the Ksp of Ca(OH)2, you can safely assume 100% dissociation.

what is dH for question(7) ?
what is dH ???

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Re:equilibrium test questions
« Reply #18 on: January 08, 2005, 11:59:20 AM »
7. For the rxn: A + B + 200kJ <-> C + D, predict the effects after drecreasing [D].
a)release energy
b)increase [C]
c)both a and b

what is deltaH for the reaction in question 7?
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:equilibrium test questions
« Reply #19 on: January 08, 2005, 12:20:58 PM »
what is deltaH for the reaction in question 7?
+200kJ, but according to Le Chatelier's principle, the 200kJ will decrease...

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Re:equilibrium test questions
« Reply #20 on: January 08, 2005, 12:44:29 PM »
deltaH of a reaction is different from applying Q of 200kJ to the reaction system.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:equilibrium test questions
« Reply #21 on: January 08, 2005, 01:18:31 PM »
deltaH of a reaction is different from applying Q of 200kJ to the reaction system.
But after the stress and the equilibrium shift, the temperature of the SYSTEM will decrease(because heat energy decreased), right? (releasing energy?!)
« Last Edit: January 08, 2005, 02:29:52 PM by 777888 »

777888

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Re:equilibrium test questions
« Reply #22 on: January 08, 2005, 01:45:21 PM »
Let me make an example that is similar.
5.20mL of 0.20M HCN is titrated with 0.35M NaOH. Find
a)pH initially and
b)at midpoint(halfway to equivalence).


I calculated this way:
a)HCN+NaOH->H2O+NaCN
HCN+H2O<->H3O+ + CN- (hydrolysis)
Ka=6.2x10^-10=x^2 / 0.2 (assumption made)
pH=4.95

b)HCN +   NaOH->H2O+NaCN
n=0.004    n=0.002
V=0.02      c=0.35
V of NaOH required to reach midpoint=0.002/0.35=0.00571

After rxn:
HCN +   NaOH->H2O+NaCN
n=0.002  n=0            n=0.002
c=n/V(total)
c=0.1556                   c=n/V0.07779

HCN+H2O<->H3O+ +CN-(hydrolysis)
6.2x10^-10=0.07779x/0.1556 (assumption made)
pH=9.21
The results are surprising to me...the pH is so high at midpoint...all other weak acid/strong base quesitons I did before have midpoint at about 5 point something only. So did I calculated them right?
« Last Edit: January 08, 2005, 05:39:15 PM by 777888 »

777888

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Re:equilibrium test questions
« Reply #23 on: January 08, 2005, 02:20:08 PM »
8.Which of the following is in a state of equilibrium
a)A container with 20mL of water
b)A container with an ice cube and water
c)both a and b
[would a container with 20mL of water be in a state of equilibrium]

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Re:equilibrium test questions
« Reply #24 on: January 08, 2005, 05:18:45 PM »
(7) I'm not sure of your notation. I assume "..+200kJ" means u add 200kJ of heat, and it's not deltaH of the reaction. Adding heat to the system favours the endothermic reaction.

(8) for (a). H2O(l) <-> H2O (g), but for (b), there will be heat transfer from water to the ice that cause it to melt, hence no equilibrium.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:equilibrium test questions
« Reply #25 on: January 08, 2005, 05:30:36 PM »
(7) I'm not sure of your notation. I assume "..+200kJ" means u add 200kJ of heat, and it's not deltaH of the reaction. Adding heat to the system favours the endothermic reaction.

(8) for (a). H2O(l) <-> H2O (g), but for (b), there will be heat transfer from water to the ice that cause it to melt, hence no equilibrium.
7)Yes the reaction is endothermic:
A + B + heat <-> C + D
and then [D] is decreased, so shift right and [A], and heat all decreased. But heat decreased means temperature is lowed and is releasing energy, right? (heat energy is consumed...)

8b)H2O(l) <-> H2O(s) why not equilibrium? I thought that there will be equilibrium of melting and solidfying processes occuring at the same rate...
« Last Edit: January 08, 2005, 07:03:14 PM by 777888 »

Offline Donaldson Tan

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Re:equilibrium test questions
« Reply #26 on: January 09, 2005, 09:27:16 AM »
(8) Common sense lah, u put ice in water, what do u see? however, u find liquid water exist without any significant change in its mass..
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

777888

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Re:equilibrium test questions
« Reply #27 on: January 09, 2005, 10:23:55 PM »
Let me make an example that is similar.
5.20mL of 0.20M HCN is titrated with 0.35M NaOH. Find
a)pH initially and
b)at midpoint(halfway to equivalence).


I calculated this way:
a)HCN+NaOH->H2O+NaCN
HCN+H2O<->H3O+ + CN- (hydrolysis)
Ka=6.2x10^-10=x^2 / 0.2 (assumption made)
pH=4.95

b)HCN +   NaOH->H2O+NaCN
n=0.004    n=0.002
V=0.02      c=0.35
V of NaOH required to reach midpoint=0.002/0.35=0.00571

After rxn:
HCN +   NaOH->H2O+NaCN
n=0.002  n=0            n=0.002
c=n/V(total)
c=0.1556                   c=n/V0.07779

HCN+H2O<->H3O+ +CN-(hydrolysis)
6.2x10^-10=0.07779x/0.1556 (assumption made)
pH=9.21
The results are surprising to me...the pH is so high at midpoint...all other weak acid/strong base quesitons I did before have midpoint at about 5 point something only. So did I calculated them right?
OK! Thanks!
How about the titration calculations? Did I do it right?(espically the hydrolysis part...) :)

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