Practice Problems: Heat and Phase Changes

1. How much heat is required to raise the temperature of 789 g of ethanol from 25oC to 70oC?

2. How much heat is released when 432 g of water cools from 71oC to 18oC?

3. How many joules of heat are given off when 5.9 g of water cool from 75oC to 25oC?

4. How many joules does it take to melt 35 g of ice at 0oC and then heat it to 10oC?

5. How much heat is released when 85 g of steam condense to liquid water and cooled to 50oC?

6. How much heat is necessary to raise the temperature of 25 g of water from 10 oC to 60 oC?

7. How much heat is given off when 50 g of water at 0oC freezes and then cools to -15oC?

8. How many kilojoules of heat are required to vaporize 50.0g of ethanol? The boiling point of ethanol is 78.5oC. Its molar heat of vaporization is 43.5 kJ/mol.

  1. Heat changes can also occur when a solute dissolves in a solvent. The heat change caused by dissolution of one mole of substance is the ______. (ΔHsoln)
  1. When one mole of sodium hydroxide dissolves in water, sodium ions and hydroxide ions separate and interact with the water. The temperature of the solution increases, releasing 445.1 kJ of heat. Write the thermochemical equation.
  1. The heat of solution for the dissolving process of calcium chloride is -82.8 kJ/mol. Write this thermochemical equation.
  1. The heat of solution for the dissolving process of ammonium nitrate is +25.7 kJ/mol. Write this thermochemical equation.

Sample Problems:

1. How much heat (in kJ) is released when 2.5 mol of NaOH is dissolved in water? (Use the thermochemical equation from above)

2. How much heat (in kJ) is released when 100 grams of sodium hydroxide are dissolved in

water?

  1. How many grams of ammonium nitrate must be dissolved in water so that 88 kJ of heat is absorbed?