Ch.6 - Thermochemistry WorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Solution: Enthalpy diagram illustrating Hesss law. The net reaction is the same as in Figure 5.21 in the textbook, but here we imagine different reactions in our two-step version. As long as we can write a seri

Problem

The overall reaction is CH4 (gas) plus 2 O2 (gas) goes to CO2 (gas) plus 2 H2O (liquid).  The enthalpy change (delta-H1) from the reactants to the products is negative 890 kilojoules. The diagram also shows the enthalpy changes in two steps.  The enthalpy change (delta-H2) from the reactants to the intermediate products CO2 (gas) plus 2 H2O (liquid) plus one half O2 (gas) is negative 607 kilojoules, and the enthalpy change (delta-H3) from these to the final products is negative 283 kilojoules.


Enthalpy diagram illustrating Hesss law. The net reaction is the same as in Figure 5.21 in the textbook, but here we imagine different reactions in our two-step version. As long as we can write a series of equations that add up to the equation we need, and as long as we know a value for H for all intermediate reactions, we can calculate the overall H.

Suppose the overall reaction were modified to produce 2H2O(g) rather than 2H2O(l). Would any of the values of H in the diagram stay the same?
 

Solution

We're asked to determine whether any of the values of H in the diagram would stay the same if the overall reaction were modified to produce 2H2O(g) rather than 2H2O(l).


We're given the enthalpy diagram for the overall reaction with the products: CO2 (g) and H2O (l).


Recall that by using Hess's law, we can determine the enthalpy change of an overall reaction by taking the sum of the enthalpies of individual steps.                        


The individual steps from the enthalpy diagram are as follows: (tip of arrow points to products)


  1. CH4 (g) + 2O(g) → CO2 (g) + 2H2O (l)                       ΔH1 = -890 kJ
  2. CH4 (g) + 2O2 (g) → CO (g) + 2H2O (l) + ½ O(g)       ΔH2 = -607 kJ
  3. CO (g) + 2H2O (l) + ½ O(g) → CO2 (g) + 2H2O (l)     ΔH3 = -283 kJ
Solution BlurView Complete Written Solution