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: Given the following reactionsH2O (l) →H2O (g)                    ΔH = 44.01 kJ2H2 (g) + O2 (g) → 2H2O (g)  ΔH = -483.64 kJthe enthalpy for the decomposition of liquid water into gaseous hydrogeand oxygen2H2O (l) → 2H2 (g) + O2 (g) is __________ kJ.A) -395.62B) -527.65C) 439.63D) 571.66E) 527.65

Problem

Given the following reactions

H2O (l) →H2O (g)                    ΔH = 44.01 kJ

2H2 (g) + O2 (g) → 2H2O (g)  ΔH = -483.64 kJ

the enthalpy for the decomposition of liquid water into gaseous hydrogeand oxygen

2H2O (l) → 2H2 (g) + O2 (g) 

is __________ kJ.

A) -395.62

B) -527.65

C) 439.63

D) 571.66

E) 527.65

Solution

We’re being asked to determine the enthalpy change (ΔHrxn) for the chemical reaction:


2 H2O (l) → 2 H2 (g) + O2 (g) 


We can use Hess’s Law to determine the enthalpy change of the overall reaction from the given reactions:


1. H2O (l) →H2O (g)                                                         ΔH1 = 44.01 kJ

2. 2H2 (g) + O2 (g) → 2H2O (g)                                        ΔH2 = - 483.64 kJ


We now need to find a combination of reactions that when added up, gives us the overall reaction.


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