Posted by Rachel | Posted in Coffee and Accessories | Posted on 09-03-2009
Tags: large coffee cup warmer
Heat of fusion of ice (enthalpy of fusion)?
We did this chemistry lab where we had to find the enthalpy of fusion of water by melting ice with warm water in a coffee-cup calorimeter. I’ve done my calculations, but the value comes out too large. What am I doing wrong?
I have:
Mass H2O = 94.32g
Mass Ice = 6.07g
Moles Ice (n) = 0.337mol
Initial Temp. H2O = 56.8429C
Initial Temp. ice = 0.00C (assumed)
Final Temp. mixture = 40.6894C
ΔH fusion= -(m c ΔT [ice] + m c ΔT [water]) / n
Plugging in the values:
-((6.07)(4.18)(40.6894) + (94.32)(4.18)(-16.1532)) / (0.337)
= 15.8 kJ/mol
The accepted value is 6.01 kJ/mol , so 15.8 is too large a value.
Is there something wrong with my calculations or is there something wrong with the experimental data? Help!
_____dHfus of water = 79.72 kcal/g or 333.55 J/g
was T measurements really to 4 decimal places?! WOW!
heat gained = heat lost
treat ice and wate separately
heat gained by ice = 6.07 g [?? J/g at 0C + 4.18 J/gC (Teq-0)C]
heat released = 94.32 g [4.18 J/gC * (56.8429 - Teq)] = ??
Plug Teq and SOLVE for ?? J/g (multiply by 18.02 g/mole for J/mole)
Basic mathematics is a prerequisite to chemistry – I just try to help you with the methodology of solving the problem.
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yep, the value is NOT the lit value. so some numbers must be wrong
my first thought is that the water T was measured before it was poured into the styrofoam cup and the effect of the cup on cooling was not included in the calculation; if left too long before measuring the final t, the T would be too low – both would make the final temp too low; even if the ice were -20C, it would not have much effect.
My guess, however, is that the amount of ice was NOT 6.07, but 12.65 g!!! give 6.01 on the snozz!
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