Calorimetry & Heat Flow - Worked Chemistry Problems

Coffee Cup and Bomb Calorimetry

It's called coffee cup calorimetry because a styrofoam coffee cup is used.
It's called coffee cup calorimetry because a styrofoam coffee cup makes a perfect easy-to-find container for the reaction. Rusty Hill, Getty Images

Calorimetry Review

Calorimetry is the study of heat transfer and changes of state resulting from chemical reactions, phase transitions, or physical changes. The tool used to measure heat change is the calorimeter. Two popular types of calorimeters are the coffee cup calorimeter and bomb calorimeter. These problems demonstrate how to calculate heat transfer and enthalpy change using calorimeter data.

While working these problems, you may wish to review the section on coffee cup and bomb calorimetry and the laws of thermochemistry.

Coffee Cup Calorimetry Problem

The following acid-base reaction is performed in a coffee cup calorimeter:

H+(aq) + OH-(aq) → H2O(l)

The temperature of 110 g of water rises from 25.0°C to 26.2°C when 0.10 mol of H+ is reacted with 0.10 mol of OH-.

  • Calculate qwater
  • Calculate ΔH for the reaction
  • Calculate ΔH if 1.00 mol OH- reacts with 1.00 mol H+

Solution

  • Use this equation:

    q = (specific heat) x m x Δt

    where q is heat flow, m is mass in grams, and Δt is the temperature change. Plugging in the values given in the problem:

    qwater = 4.18 (J / g·°C;) x 110 g x (26.6°C - 25.0°C)

    qwater = 550 J

  • ΔH = -(qwater) = - 550 J
  • We know that when 0.010 mol of H+ or OH- reacts, ΔH is - 550 J:

    0.010 mol H+ ~ -550 J

    Therefore, for 1.00 mol of H+ (or OH-):

    ΔH = 1.00 mol H+ x (-550 J / 0.010 mol H+)

    ΔH = -5.5 x 104 J

    ΔH = -55 kJ

    Answer

    Bomb Calorimetry Problem

    When a 1.000 g sample of the rocket fuel hydrazine, N2H4, is burned in a bomb calorimeter which contains 1200 g of water, the temperature rises from 24.62°C to 28.16°C. If the C for the bomb is 840 J/°C, calculate

    • qreaction for combustion of a one-gram sample
    • qreaction for combustion of one mole of hydrazine in the bomb calorimeter

    Solution

    • For a bomb calorimeter, use this equation:

      qreaction = -(qwater + qbomb)

      qreaction = -(4.18 J / g·°C x mwater x Δt + C x Δt)

      qreaction = -(4.18 J / g·°C x mwater + C)Δt

      where q is heat flow, m is mass in grams, and Δt is the temperature change. Plugging in the values given in the problem:

      qreaction = -(4.18 J / g·°C x 1200 g + 840 J/°C)(3.54°C)

      qreaction = -20,700 J or -20.7 kJ

    • We now know that 20.7 kJ of heat is evolved for every gram of hydrazine that is burned. Using the periodic table to get atomic weights, we can calculate that one mole of hydrazine, N2H4, weight 32.0 g. Therefore, for the combustion of one mole of hydrazine:

      qreaction = 32.0 x -20.7 kJ/g

      qreaction = -662 kJ

    Answers

    • -20.7 kJ
    • -662 kJ