Question

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

Answer 1

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=nxxDeltaH`

where:
`q` = heat
`n` = moles
`DeltaH` = 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

`35 g xx ((1 mol)/(18.02g)) = 1.94 mols`

Step 2: Multiply by enthalpy of fusion

`q=nxxDeltaH= 1.94xx6.02= 11.678 kJ`

Step 3: Convert kJ to J

`11.678 kJxx((1000 J)/(1 kJ)) = 11,678 J`

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.