Calorimetry & Heat Flow - Worked Chemistry Problems

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

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 q_water
• 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:

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

  q_water = 550 J

• ΔH = -(q_water) = - 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

• 550 J (be sure to have 2 significant figures)
• -550 J
• -55 kJ

Would you like an example for bomb calorimetry? Then continue on to the second worked calorimetry problem > Next Page

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