Degree conversion

[joonkimdds]

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I was solving problem number 46 on page 671 of Kaplan blue book.
it has delta H = 131 kj/mol and delta S = 134J/mol*K
the question asks what temperature will the reaction become spontaneous.

It's spontaneous when delta G is negative.

delta G = delta H - (T * delta S)

I know that I should convert 131 kJ/mol into 131,000 J/mol.

but I don't know how to change 134J/mol*K into ?J/mol*C 😕

I know that K = C + 273.15 but I don't know how to use this in railroad method to cancel out K and convert it into C.

and when it says J/mol*C, is it (J/mol)*C or J/(mol*C)? I am so confused.

oh ! and one more thing, this book states (p.256) that temperature is always positive so depends on the sign of delta H and delta S, we can find out whether it's spontaneous or not. Why can't we make the temperature go below 0 ?

 

[tinman831]

K = 273.15 + C
C = K - 273.15

plug that into the number 134J/mol*C

so

134J/ mol*C = 134J/mol*(K-273.15)

 

[joonkimdds]

K = 273.15 + C
C = K - 273.15

plug that into the number 134J/mol*C

so

134J/ mol*C = 134J/mol*(K-273.15)

but the question doesn't tell me what K is.
So if unit becomes from J/(mol*C) to J/(mol*K),
what does 134 become?
Do i need to subtract 273.14 from 134?

 

[tinman831]

but the question doesn't tell me what K is.

What question?

So if unit becomes from J/(mol*C) to J/(mol*K),
what does 134 become?
Do i need to subtract 273.14 from 134?

You don't do anything to 134. Just plug in the numbers and work it all out.

 

[allstardentist]

just solve the problem in terms of K and convert it to C after

 

[keibee82]

is the answer 977K?
since spontaneous means negative delta G, set delta G equal to
zero and solve for T.

 

[keibee82]

I was solving problem number 46 on page 671 of Kaplan blue book.
it has delta H = 131 kj/mol and delta S = 134J/mol*K
the question asks what temperature will the reaction become spontaneous.

It's spontaneous when delta G is negative.

delta G = delta H - (T * delta S)

I know that I should convert 131 kJ/mol into 131,000 J/mol.

but I don't know how to change 134J/mol*K into ?J/mol*C 😕

I know that K = C + 273.15 but I don't know how to use this in railroad method to cancel out K and convert it into C.

and when it says J/mol*C, is it (J/mol)*C or J/(mol*C)? I am so confused.

oh ! and one more thing, this book states (p.256) that temperature is always positive so depends on the sign of delta H and delta S, we can find out whether it's spontaneous or not. Why can't we make the temperature go below 0 ?

Temperature can't never go under zero on Kelvin scale. In that equation,
you must use Kelvin.

 

[tinman831]

I was solving problem number 46 on page 671 of Kaplan blue book.
it has delta H = 131 kj/mol and delta S = 134J/mol*K
the question asks what temperature will the reaction become spontaneous.

It's spontaneous when delta G is negative.

delta G = delta H - (T * delta S)

I know that I should convert 131 kJ/mol into 131,000 J/mol.

but I don't know how to change 134J/mol*K into ?J/mol*C 😕

I know that K = C + 273.15 but I don't know how to use this in railroad method to cancel out K and convert it into C.

and when it says J/mol*C, is it (J/mol)*C or J/(mol*C)? I am so confused.

oh ! and one more thing, this book states (p.256) that temperature is always positive so depends on the sign of delta H and delta S, we can find out whether it's spontaneous or not. Why can't we make the temperature go below 0 ?

delta G = delta H - (T * delta S)
it has delta H = 131 kj/mol and delta S = 134J/mol*K
so delta G = 131,000J/mol - (T*134J/mol*K)
set delta G = 0
0 = 131,000J/mol - (T*134J/mol*K)
T*134J/mol*K = 131,000J/mol
T= 131,000/ (134J/K) = 977K

Joon, you don't have to convert that to C unless the question asks for the temperature in terms of C.

 

[joonkimdds]

delta G = delta H - (T * delta S)
it has delta H = 131 kj/mol and delta S = 134J/mol*K
so delta G = 131,000J/mol - (T*134J/mol*K)
set delta G = 0
0 = 131,000J/mol - (T*134J/mol*K)
T*134J/mol*K = 131,000J/mol
T= 131,000/ (134J/K) = 977K

Joon, you don't have to convert that to C unless the question asks for the temperature in terms of C.

the answer is 977 kelvin but the answer choices are in celcius.
749C is the answer.

but! according to the explanation, it says u get 977K and subtract around 300 and the closest answer is 749.

R we supposed to get the closest answer on DAT?
977-273.15 = 704 and the difference from the answer is 45C.
That's a huge!!! huge!!! gap.
0 celcius makes water freeze, and 45 celcius is a really really hot summer temperature.

can we really say that we have to choose the closest answer or should i guess that i did something wrong and do it again?