Can someone please explain when should we use the following two equations?
delta S= q / T
delta S = nCln (Tf / Ti)
I know you should use the second one if temperature changes and the first if temp is constant. However, in hw question 9.5 Even though the temperature changed the first equ is still used, but for 9. 43, the 2nd equation is used. Why can't we used the same approach for 9.43? (calculating q first then delta S at separate temp??)
9.5 What is the total entropy change of a process in which 40.0 kJ of energy is transferred as heat from a large reservoir at 800. K to one at 200. K?
9.43 Suppose that 50.0 g of H2O(l) at 20.0 $C is mixed with 65.0 g of H2O(l) at 50.0C at constant atmospheric pressure in a thermally insulated vessel.
Thank you :)
Entropy change when Temperature changes [ENDORSED]
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Re: Entropy change when Temperature changes
In question 9.5, you have 2 separate baths at certain temperatures. The first is at 800K and the second is at 200K. When you transfer heat between the two, the temperature of a given bath does not change. In 9.43, you are fully combining 2 solutions are different temperatures. They will reach the same final temperature and thus you use the delta S= nCln(T2/T1) equation.
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Re: Entropy change when Temperature changes
Can someone explain how you solve number 9.5? Like what do you divide the heat with?
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Re: Entropy change when Temperature changes
For 9.5, we use to calculate for the 800K reservoir losing 40kJ of energy (negative value) plus for the 200K reservoir gaining the 40kJ of energy (positive value).
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Re: Entropy change when Temperature changes [ENDORSED]
9.43: Suppose that 50.0 g of H2O(l) at 20.0 C is mixed with 65.0 g of H2O(l) at 50.0 C at constant atmospheric pressure in a thermally insulated vessel.
Steps to solve:
1. Use q(cold water) = -q(cold water) to solve for the final temperture.
2. Calculate the decrease in entropy for the hot water from 50. 0 C to final temperature. Use Delta S = g Csp ln T2/T1
3. Calculate the increase in entropy for the cold water from 20. 0 C to final temperature. Use Delta S = g Csp ln T2/T1
Add Step 2 and Step 3 to calculate the total entropy.
Steps to solve:
1. Use q(cold water) = -q(cold water) to solve for the final temperture.
2. Calculate the decrease in entropy for the hot water from 50. 0 C to final temperature. Use Delta S = g Csp ln T2/T1
3. Calculate the increase in entropy for the cold water from 20. 0 C to final temperature. Use Delta S = g Csp ln T2/T1
Add Step 2 and Step 3 to calculate the total entropy.
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Re: Entropy change when Temperature changes
What is the number used for the Csp in the equation that you put down?
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