Specific Q

[monokekie]

Heinn P467 Q24

An Experiment was carried out as follows to compare the effectiveness of two different methods of generating energy from liquid methanol (CH3OH) as a source of fuel.

a) The heat of combustion of methanol was determined in bomb calorimeter which had a calibration factor of 8.26kjc. When 2 g of methanol was completely burned in excess oxygen the temp rose from 20 to 25.5.

i) How much energy is released when 2 g of methanol is burned in excess oxygen. (no problems)
ii) How much energy is released when 1 mol of methanol is burned in excess oxygen? (no peoblems)


BUT, iii) What is the *trangle*H (enthalpy) for the reaction:
2CH3OH(l)+3O2(g)*arrow*CO2(g)+4H20(l)

i got similar to the answer but why isn't it negative???

plspls anyone??

[monokekie]

anddd...with regards to the answer of Q23(d) on p467 , what is the definition for "the sites of the oxidation and reduction half reactions"?

can't find the term "site" in the book index, and mayb i missed it somewhere in the book...

tkx in advance

[d0minicz]

are you sure page 364 ?

[monokekie]

hoops...


its page....467..... sry sry sry....

 Edited!

[monokekie]

and... also...
 
Q15 P399, The Yallourn power station in Victoria has a maximum capacity of 1450 MW. Th eaverage daily rate at which Melbourne receives solar energy is 1.0 KW/m

a) What area of solar cells, operating at 20% efficiency, would produce the same amount of power as the Yallourn station?

* i don't understand why the answer times 1 by 10% since question says 20%?



lol i ask many questions at once, because i will forget them after a whilez. Sry guys if it brings you inconvenience when reading them and replying..

[monokekie]

Q14 P 293

in pure water at 25C the conc of both H+ and OH- ions is 10^-7 M due to the reaction described by the equations:

2H2O*arrow*(H3O+) + (OH-)         *i know look so wiered, my apologies..

The forward reaction is endother mic. Suppose some pure water is heated

b) What happens to the conc of H3O+ and OH- ions? answer: both increase

c) What happens to the pH? answer: decreases. <<<<<<<<====== but WHY??since both conc increases as stated in B.......

[Mao]

both concentration increase in B. reaction is endothermic.

there are more H3O+ ions, hence lower pH

[monokekie]

both concentration increase in B. reaction is endothermic.

there are more H3O+ ions, hence lower pH

umm... but doesn't OH- increase at the same time....? sry but i am confuzed :s

[NE2000]

both concentration increase in B. reaction is endothermic.

there are more H3O+ ions, hence lower pH

umm... but doesn't OH- increase at the same time....? sry but i am confuzed :s

Your confusion is probably related to the definition of pH.

The reaction is endothermic, so [H3O+] and [OH-] will increase at the same time, as the forward reaction is favoured by the increase in temperature.

However, pH technically isn't a measure of how acidic the mixture will be. At 25 degrees, a pH of 7 is neutral, at other temperatures a pH of 7 may not be neutral. Fundamentally, pH is a logarithmic measure of the concentration of H3O+ in the mixture. So in this solution at the higher temperature, the concentration of hydronium ions still = the concentration of hydroxide ions. But these concentrations are higher than before. And since pH is measured simply by -log([H3O+]) the pH has decreased. In other words, the concentration of hydroxide ions has no influence on the pH. The pH will be decreased, i.e. it will be below 7. But that this particular temperature this new pH will represent the neutral pH as the solution will still be neutral. If you were to decrease the temperature, the pH would be above 7 despite [OH-] also decreasing, simply because [H3O+] has decreased.

[monokekie]

both concentration increase in B. reaction is endothermic.

there are more H3O+ ions, hence lower pH

umm... but doesn't OH- increase at the same time....? sry but i am confuzed :s

Your confusion is probably related to the definition of pH.

The reaction is endothermic, so [H3O+] and [OH-] will increase at the same time, as the forward reaction is favoured by the increase in temperature.

However, pH technically isn't a measure of how acidic the mixture will be. At 25 degrees, a pH of 7 is neutral, at other temperatures a pH of 7 may not be neutral. Fundamentally, pH is a logarithmic measure of the concentration of H3O+ in the mixture. So in this solution at the higher temperature, the concentration of hydronium ions still = the concentration of hydroxide ions. But these concentrations are higher than before. And since pH is measured simply by -log([H3O+]) the pH has decreased. In other words, the concentration of hydroxide ions has no influence on the pH. The pH will be decreased, i.e. it will be below 7. But that this particular temperature this new pH will represent the neutral pH as the solution will still be neutral. If you were to decrease the temperature, the pH would be above 7 despite [OH-] also decreasing, simply because [H3O+] has decreased.

ohh...... I see....!

thank you so much!

[monokekie]

and, with regards to the two equations in middle of page 339,

why are sulfuric acids (catalyst) in liquid form? ie, like "(l)"

tkx

[NE2000]

and, with regards to the two equations in middle of page 339,

why are sulfuric acids (catalyst) in liquid form? ie, like "(l)"

tkx

Because it's highly concentrated (and dangerous )

[monokekie]

alrite , thanks NE2000!