How many joules does it take to melt 35 g of ice at 0° C?

1 Answer
Apr 29, 2016

It takes 12,000 Joules of energy to melt 35 grams of ice at 0 °C

Explanation:

The enthalpy change for melting ice is called the entlaphy of fusion. Its value is 6.02 kj/mol. This means for every mole of ice we melt we must apply 6.02 kj of heat. We can calculate the heat needed with the following equation:

q=n×ΔH

where:
q = heat
n = moles
ΔH = enthalpy

In this problem we would like to calculate the heat needed to melt 35 grams of ice at 0 °C. This problem can be broken into three steps:
1. Calculate moles of water
2. multiply by the enthalpy of fusion
3. Convert kJ to J

Step 1: Calculating moles of water

35g×(1mol18.02g)=1.94mols

Step 2: Multiply by enthalpy of fusion

q=n×ΔH=1.94×6.02=11.678kJ

Step 3: Convert kJ to J

11.678kJ×(1000J1kJ)=11,678J

Finally rounding to 2 sig figs (since 34°C has two sig figs) we get

q=12,000J

For more examples on phase changes and enthalpy, see the video below:

One last note, if the temperature were not 0 °C then the ice would have to be heated in addition to melted. This would be a phase change problem combined with a heat capacity problem.