Concept Check

Concept Check

Hess’s Law and Enthalpy
Supplemental Instruction
Iowa State University / Leader: / Kia Birnbaum
Course: / CHEM177
Instructor: / Anand
Date: / March 3rd, 2017

Concept Check

  1. Fill in the blank
  2. A _____ function means that the value is independent of the pathway.
  1. A calorimeter at constant pressure is known as a ______calorimeter.
  1. A calorimeter at constant volume is known as a ______calorimeter.
  1. Breaking bonds ______energy.
  1. Standard Enthalpy of Formation is the heat evolved from the formation of ______of product from its constitute elements in their ______form.

Practice Problems

  1. Determine the enthalpy of reaction for

CH3COCH3(l) + 2 O2(g) CH3COOH(l) + CO2(g) + H2O(g)

Using Hess’s law of constant heat summation and given:

CH3COCH3(l) + 4 O2(g)  3 CO2(g) + 3 H2O(l) ΔH= -1787.0 kJ

H2O(l)  H2O(g) ΔH= -44.1 kJ

CH3COOH(l) +2 O2(g) 2 CO2(g) + 2 H2O(l) ΔH= -835 kJ

  1. Calculate the standard enthalpy of formation of ozone [ΔH°f(O3)] from the following information: For the reaction, Pb(s) + C(s) + O3(g) PbCO3(s) the enthalpy of reaction (ΔHrxn) is -841.0 kJ and the standard enthalpy of formation for PbCO3(s) is -699.0 kJ/mol.
  1. Meals-ready-to-eat (MREs) are military meals that can be heated on a flameless heater. The heat is produced by the following reaction:

Mg(s) + 2 H2O(l)  Mg(OH)2(s) + 2 H2(g)

  1. Calculate the standard enthalpy change for this reaction. (ΔH°f(H2O(l))= -285.83 kJ/mol, ΔH°f(Mg(OH)2(s))= -924.7 kJ)
  1. Calculate the number of grams of Mg (mm=24.21 g) needed for this reaction to release enough energy to increase the temperature of 75 mL of water from 21 to 79°C.
  1. 5.00 kg of a hot metal at 200.0 deg C is added to 25.0 kg of water at 30.0 deg C. What is the final temperature of the metal? The specific heat of the metal is 0.800 J/g*deg C and for water it is 4.184 J/g*deg C.

Challenge Question

  1. Ammonia (NH3) boils at -33°C; at this temperature it has a density of 0.81 g/cm3. The enthalpy of formation of NH3(g) is -46.2 kJ/mol, and the enthalpy of vaporization of NH3(l) is 23.2 kJ/mol. Calculate the enthalpy change when 1 L of liquid NH3 is burned in air to give N2(g) and H2O(g).
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