Just a few questions. (1 Viewer)

[king_of_boredom]

hey all chem goers, just need help on a few easy questions (explainations would be very helpful for each answer):

1) The best indicator to distinguish between rainwater and 0.1 molL^-1 hydrochloric acid is

a) Bromothymol blue
b) Litmus
c) Phenolphtalein
d) Methyl orange

2) Phosgene is prepared from the reaction of carbon monoxide and chlorine in the presence of a catalyst:

CO(g) + Cl2(g) <-----> COCl2(g) H = -9.93kJ mol^-1

Which of the following sets of conditions would produce the most yield of phosgene?

a) high temperature, high pressure
b) low temperature, low pressure
c) low temperature, high pressure
d) high temperature, low pressure


thanks in adv, i might post up more questions later

 

[Dreamerish*~]

Those questions look awfully familiar.

1. d) Methyl Orange - because as rain water is slightly acidic, and HCl is a strong acid, it is a matter of distinguishing between weak and strong acids. Hence use MO, which has a pH range of 3.1-4.4.

2. c) Low temperature, high pressure. Low temperature, because as the reaction is exothermic, removing heat would shift the equilibrium to the right, adjusting the change by producing more heat, therefore more products. Low pressure, because the reactants and products are gaseous, and as the left hand side has 2 moles of gas, while the right hand side has only 1, decreasing the pressure will shift the equilibrium to the side with less moles of gas - the right hand side, hence producing more COCl₂.

You're welcome.

 

[Templar]

Low pressure, because the reactants and products are gaseous, and as the left hand side has 2 moles of gas, while the right hand side has only 1, decreasing the pressure will shift the equilibrium to the side with less moles of gas - the right hand side, hence producing more COCl₂.

Decrease pressure shifts equilibrium towards side with more moles of gas to restore pressure.

 

[gosh]

2. c) Low temperature, low pressure. Low temperature, because as the reaction is exothermic, removing heat would shift the equilibrium to the right, adjusting the change by producing more heat, therefore more products. Low pressure, because the reactants and products are gaseous, and as the left hand side has 2 moles of gas, while the right hand side has only 1, decreasing the pressure will shift the equilibrium to the side with less moles of gas - the right hand side, hence producing more COCl₂.

dreamerish, did you mean increase the pressure or decrease the pressure?
the answer i got was:
c) low temperature, high pressure
if you increase the pressure of the equilibrium, the reaction will shift to side with the less moles of gas. In this case, to the right which is what we want.
if we decrease the pressure, the reaction will shift in the opposite direction, the side with more moles. i.e. left

maybe you meant c) as well, but please corrct me if i am wrong

 

[Dreamerish*~]

Decrease pressure shifts equilibrium towards side with more moles of gas to restore pressure.

Oh sorry, I wrote c), but wrote low pressure instead of high. My bad.

Corrected.

 

[king_of_boredom]

thanks for the help chemists! i hope to be flooding this thread with more questions before my trial, for now i need more help:

(side note: for those of you playing at home, i'm currently working on the CSSA 2004 trial paper)

are the following reactions exothermic or endothermic?

- fermentation of glucose
- dehydration of ethanol
- cracking of petroleum
- reaction of a galvanic cell

thanks in adv again

 

[Dreamerish*~]

Fermentation of glucose: Exothermic - the temperature is kept at 37ºC, which is relatively low.
Dehydration of ethanol: Endothermic - when bonds are broken, heat is required.
Cracking of petroleum: Endothermic - heat is provided for the larger fractions to be "cracked" into smaller fractions.
Reaction of a Galvanic Cell: A Galvanic cell produces electrical energy, which is neither endothermic nor exothermic.

 

[velox]

What about the hydration of ethene, is that exothermic?

 

[king_of_boredom]

wow Dreamerish*~! you are good....chemistry has to be my weakest subject. some of the multiple choices are getting to my head! :S

The conjugate acid of HPO4^2- is:

a) H30^+

b) H2PO4^-

c) PO4^3-

d) H3PO4

 

[Dreamerish*~]

wow Dreamerish*~! you are good....chemistry has to be my weakest subject. some of the multiple choices are getting to my head! :S

The conjugate acid of HPO4^2- is:

a) H30^+

b) H2PO4^-

c) PO4^3-

d) H3PO4

b)H₂PO₄^-

The conjugate acid of HPO₄^2- differs by one H⁺. Since HPO₄^2- is a conjugate base, you add a H⁺ (bases accept protons) to find its conjugate acid.

What about the hydration of ethene, is that exothermic?

The hydration of ethene is:

C₂H₄ + H₂O dil. H₂SO₄ cat. > C₂H₅OH

When bonds are formed, heat is released. Hence it is exothermic.

 

[velox]

With conjugate bases/acids dont you just switch the numbers around?

 

[Libbster]

Isn't a galvanic cell a spontaneous exothermic reaction?

 

[Dreamerish*~]

Isn't a galvanic cell a spontaneous exothermic reaction?

A Galvanic Cell produces converts chemical energy to electrical energy. It's hard to define in terms of exo- and endothermic reactions. I always thought of it as being neither, but I asked my chemistry tutor recently, just to be sure. She said that it's hard to say, but if you're asked a multiple choice question in an exam, for example:

Which of the following is an exothermic/endothermic reaction?

... you definitely would not choose Galvanic Cells.

 

[Libbster]

Ok because our teacher told us it was

 

[Dreamerish*~]

Ok because our teacher told us it was

Your teacher lies!

Joking. Different teachers also have different views. That's just plain frustrating for us students.

A Galvanic Cell does produce a form of energy, I guess. I suppose if you think of a reaction being exothermic as giving off energy, not specifically heat, then yes, Galvanic Cells could be said to be exothermic.

 

[Libbster]

Yeah I went to clarify this issue today, and he sad basically what you said that it gives off a form of energy and thus is exothermic lol so confusing