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Offline Vileblood

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Thermodynamics and thermochemistry related questions
« on: February 03, 2013, 01:41:21 PM »
Here are a few questions from a worksheet of 25, that i cannot figure out. I have looked in my textbook and online for help but did not come across anything that explained any of this. I have provided the answers i've chosen, but i'm not sure if they are correct. Can anyone please help me understand these problems and what are the correct answers. Thank you.

1) The molar heat of fusion, ΔHfus, for water is 6.01 kJ mol^-1. Which expression gives the molar entropy of fusion, ΔSfus, in kJ K^-1 mol^-1 for ice at its normal melting point?
a)6.01/273
b)6.01/298
c)6.01 x 4.18
d)6.01 x 273
e)6.01 x 298

2)The molar heat of fusion, ΔHfus, for water is 6.01 kJ mol^-1. The specific heat capacity for water, Cp, is 75 J mol^-1 degrees C^-1. Which expression gives the quantity of energy needed to change 1.0 mol ice at 0 degrees C to liquid water at 25 degrees C?
a)6010/(75x25)
b)6.01 + 75
c)6010 + (75 x 25)
d)6010/298 + (75x25)
e) 6010 + (75x25)/(298)

3) The heat of neutralization for a strong acid in dilute water solution is about 60 kJ mol^-1. what quantity of heat in kJ is produced when 100. ml of 1.0 M h2so4 is mixe with 100. ml of 1.0 M KOH?
a)0.10
b)0.30
c)0.40
d)6.0
e)18

4) for which reaction is ΔH most nearly equal to ΔE?
a)H2(g) + 1/2O2(g) > H2O(g)
b)H2(g) + Cl2(g) > 2HCl(g)
c)C2H5OH(l)>C2H5OH(g)
d)N2O4(g) > 2NO2(g)
e)BaO2(s)>BaO(s) + 1/2O2(g)

My answers:
1)b
2)d
3)d
4)b

Offline Yggdrasil

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Re: Thermodynamics and thermochemistry related questions
« Reply #1 on: February 03, 2013, 05:11:34 PM »
1) The molar heat of fusion, ΔHfus, for water is 6.01 kJ mol^-1. Which expression gives the molar entropy of fusion, ΔSfus, in kJ K^-1 mol^-1 for ice at its normal melting point?
a)6.01/273
b)6.01/298
c)6.01 x 4.18
d)6.01 x 273
e)6.01 x 298

At ice's melting point, liquid water is in equilibrium with the ice.  What does this tell you about the free energy change (ΔG) of converting ice into water?

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2)The molar heat of fusion, ΔHfus, for water is 6.01 kJ mol^-1. The specific heat capacity for water, Cp, is 75 J mol^-1 degrees C^-1. Which expression gives the quantity of energy needed to change 1.0 mol ice at 0 degrees C to liquid water at 25 degrees C?
a)6010/(75x25)
b)6.01 + 75
c)6010 + (75 x 25)
d)6010/298 + (75x25)
e) 6010 + (75x25)/(298)

I always find it easier to split this problem into two parts.  First consider melting 1 mol of ice (i.e. transforming 1 mol of ice at 273K into 1 mol of liquid water at 273K).  How much heat is required to melt 1 mol of ice?  Next, how much heat is required to heat 1 mol of liquid water at 273K to 298K?

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3) The heat of neutralization for a strong acid in dilute water solution is about 60 kJ mol^-1. what quantity of heat in kJ is produced when 100. ml of 1.0 M h2so4 is mixe with 100. ml of 1.0 M KOH?
a)0.10
b)0.30
c)0.40
d)6.0
e)18

How many moles of H2SO4 do you have?

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4) for which reaction is ΔH most nearly equal to ΔE?
a)H2(g) + 1/2O2(g) > H2O(g)
b)H2(g) + Cl2(g) > 2HCl(g)
c)C2H5OH(l)>C2H5OH(g)
d)N2O4(g) > 2NO2(g)
e)BaO2(s)>BaO(s) + 1/2O2(g)

Your answer is correct.  ΔH = ΔE + Δ(PV), so for reactions that do not change volume, ΔH = ΔE.  Under most conditions, gasses take up much more volume than solids or liquids, so reactions that change the number of gas molecules will create a change in volume.  Because B has the same number of gas molecules on the product and reactant side, it is the correct answer.

Offline Vileblood

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Re: Thermodynamics and thermochemistry related questions
« Reply #2 on: February 03, 2013, 05:27:48 PM »
1) ΔG will be negative and spontaneous
2) should i use this equation ΔH = mCpΔT and convert moles to g?
3) .1 mole of H2SO4 and .1 mole of KOH, but i dont know what i can do with these values
4) thank you, that was my reasoning for the answer

Offline Yggdrasil

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Re: Thermodynamics and thermochemistry related questions
« Reply #3 on: February 03, 2013, 05:53:21 PM »
1) ΔG will be negative and spontaneous
For a process at equilibrium, the reverse reaction will be just as favorable as the forward reaction.

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2) should i use this equation ΔH = mCpΔT and convert moles to g?
Here Cp is given in units of J mol^-1 oC^-1, so you just need to multiply by the number of moles and the change in temperature to calculate the amount of heat.

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3) .1 mole of H2SO4 and .1 mole of KOH, but i dont know what i can do with these values
Oops, I just realized that your original answer was correct.  Since the heat of neutralization is 60 kJ mol^-1 and you are neutralizing 0.1 mol of acid, you just multiply the two to get the correct answer (which you had originally done).

Offline Vileblood

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Re: Thermodynamics and thermochemistry related questions
« Reply #4 on: February 03, 2013, 06:18:38 PM »
1) the answer has to be a or d, i'm going to say a, would you agree with that  (6.01/273)
2) im going to say the answer is c 6010 + (75x25)

Offline Yggdrasil

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Re: Thermodynamics and thermochemistry related questions
« Reply #5 on: February 04, 2013, 08:39:44 AM »
1) the answer has to be a or d, i'm going to say a, would you agree with that  (6.01/273)

At the melting point, the solid and liquid phases are at equilibrium, so ΔG for melting is zero.  Do you know of an equation that relates ΔG, ΔH, and ΔS?  Once you get this, it should be clear which answer is correct.

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2) im going to say the answer is c 6010 + (75x25)
Yes, that is correct.

Offline Vileblood

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Re: Thermodynamics and thermochemistry related questions
« Reply #6 on: February 04, 2013, 10:35:08 PM »
Yup i got it, thank you very much for your help

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