8. Thermochemistry

Hess's Law

8. Thermochemistry

# Hess's Law - Video Tutorials & Practice Problems

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**Hess's Law** involves the use of partial reactions in the determination of the overall enthalpy of reaction.

Hess's Law

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concept

## Hess's Law

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**Multiplication** of the original reaction results in multiplication of ∆Hrxn.

**Division** results in the division of ∆Hrxn..

**Reversing** the original reaction results in reversing the sign of ∆Hrxn.

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example

## Hess's Law Example 1

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3

Problem

ProblemCalculate the ∆Hrxn for the following thermochemical equation:

When given the following:

A

-296.8 kJ

B

-148.4 kJ

C

148.4 kJ

D

296.8 kJ

4

concept

## Hess's Law

Video duration:

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Addition of ∆Hrxn values of multiple steps gives us the ∆Hrxn of the overall reaction.

5

example

## Hess's Law Example 2

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6

Problem

ProblemCalculate the ∆H_{rxn} for

Given the following set of reactions:

A

-494.4 kJ

B

-692.8 kJ

C

494.4 kJ

D

-346.4 kJ

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Problem

ProblemCalculate the ∆H_{rxn} for

Given the following reactions:

A

-177.85 kJ

B

-355.73 kJ

C

-258.36 kJ

D

-487.27 kJ

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PRACTICE PROBLEMS AND ACTIVITIES (48)

- Calculate the enthalpy change for the reaction C(s) + 2 H2(g) → CH4(g) ΔH = ? Given the enthalpy values for th...
- Consider the following equilibrium: 2 H21g2 + S21g2 Δ 2 H2S1g2 Kc = 1.08 * 107at 700 C (c) Calculate the value...
- Consider the following equilibrium, for which Kp = 0.0752 at 480 C: 2 Cl21g2 + 2 H2O1g2 Δ 4 HCl1g2 + O21g2 (a)...
- This reaction has an equilibrium constant of Kp = 2.26 * 104 at 298 K. CO(g) + 2 H2(g) ⇌ CH3OH(g) Calculate Kp...
- This reaction has an equilibrium constant of Kp = 2.26 * 104 at 298 K. CO(g) + 2 H2(g) ⇌ CH3OH(g) Calculate Kp...
- This reaction has an equilibrium constant of Kp = 2.26 * 104 at 298 K. CO(g) + 2 H2(g) ⇌ CH3OH(g) Calculate Kp...
- This reaction has an equilibrium constant of Kp = 2.2 * 106 at 298 K. 2 COF2(g) ⇌ CO2(g) + CF4(g) Calculate Kp...
- This reaction has an equilibrium constant of Kp = 2.2 * 106 at 298 K. 2 COF2(g) ⇌ CO2(g) + CF4(g) Calculate Kp...
- This reaction has an equilibrium constant of Kp = 2.2 * 106 at 298 K. 2 COF2(g) ⇌ CO2(g) + CF4(g) Calculate Kp...
- The reaction of A with B to give D proceeds in two steps: (1) A + B → C ΔH° = -20 kJ (2) C + B → D ΔH° = +50 k...
- The reaction of A with B to give D proceeds in two steps: (1) A + B → C ΔH° = -20 kJ (2) C + B → D ΔH° = +50 k...
- Acetylene, C2H2, reacts with H2 in two steps to yield ethane, CH3CH3: (1) HC?CH + H2 → H2C?CH2 ΔH° = -175.1 kJ...
- Consider the combustion of liquid methanol, CH3OH1l2: CH3OH1l2 + 32O21g2¡ CO21g2 + 2 H2O1l2 H = -726.5 kJ (b)...
- Consider the combustion of liquid methanol, CH3OH1l2: CH3OH1l2 + 32O21g2¡ CO21g2 + 2 H2O1l2 H = -726.5 kJ (a) ...
- Consider the decomposition of liquid benzene, C6H61l2, to gaseous acetylene, C2H21g2: C6H61l2 ¡ 3 C2H21g2 H = ...
- The reaction 2 AsH31g2 ∆ As21g2 + 3 H21g2 has Kp = 7.2 * 107 at 1073 K. At the same temperature, what is Kp fo...
- Consider the following hypothetical reactions: A ¡B HI = +60 kJ B ¡ C HII = -90 kJ (b) Construct an enthalpy d...
- Calculate the enthalpy change for the reaction P4O61s2 + 2 O21g2 ¡ P4O101s2 given the following enthalpies of ...
- Show how Equations 18.7 and 18.9 can be added to give Equation 18.10.
- From the enthalpies of reaction 2 C1s2 + O21g2¡ 2 CO1g2 H = -221.0 kJ 2 C1s2 + O21g2 + 4 H21g2¡ 2 CH3OH1g2 H =...
- From the enthalpies of reaction H21g2 + F21g2¡ 2 HF1g2 H = -537 kJ C1s2 + 2 F21g2¡ CF41g2 H = -680 kJ 2 C1s2 +...
- Given the data N21g2 + O21g2¡ 2 NO1g2 H = +180.7 kJ 2 NO1g2 + O21g2¡ 2 NO21g2 H = -113.1 kJ 2 N2O1g2¡ 2 N21g2 ...
- For each generic reaction, determine the value of ΔH2 in terms of ΔH1. a. A + B¡2 C ΔH1 2 C¡A + B ΔH2 = ?
- For each generic reaction, determine the value of ΔH2 in terms of ΔH1. c. A¡B + 2 C ΔH1 12 B + C¡12 A ΔH2...
- For each generic reaction, determine the value of ΔH2 in terms of ΔH1. b. A + 12 B¡C ΔH1 2 A + B¡2 C ΔH2 = ?
- Consider the generic reaction: A + 2 B¡C + 3 D ΔH = 155 kJ Determine the value of ΔH for each related reaction...
- Calculate ΔHrxn for the reaction: Fe2O3(s) + 3 CO( g)¡2 Fe(s) + 3 CO2( g) Use the following reactions and give...
- Calculate ΔHrxn for the reaction: CaO(s) + CO2( g)¡CaCO3(s) Use the following reactions and given ΔH's: Ca(...
- Calculate ΔHrxn for the reaction: 5 C(s) + 6 H2( g)¡C5H12(l ) Use the following reactions and given ΔH's: C...
- Calculate ΔHrxn for the reaction: CH4( g) + 4 Cl2( g)¡CCl4( g) + 4 HCl( g) Use the following reactions and g...
- What is Hess's law, and why does it 'work'?
- The following steps occur in the reaction of ethyl alcohol (CH3CH2OH) wiht oxygen to yield acetic acid (CH3CO2...
- The industrial degreasing solvent methylene chloride, CH2Cl2, is prepared from methane by reaction with chlori...
- Hess's law can be used to calculate reaction enthalpies for hypothetical processes that can't be carried out i...
- Find ∆H° in kilojoules for the reaciton of nitric oxide with oxygen, 2 NO(g) + O2(g) → N2O4(g), given the foll...
- Set up a Hess's law cycle, and use the following information to calculate ΔH°f for aqueous nitiric acid, HNO3(...
- Sulfuric acid (H2SO4), the most widely produced chemical in the world, is amde yb a two-step oxidaiton of sulf...
- The standard enthalpy change for the reaciton of SO3(g) with H2O(l) to yield H2SO4(aq) is ΔH° = -227.8 kJ. Use...
- We can use Hess's law to calculate enthalpy changes that cannot be measured. One such reaction is the conversi...
- The ΔH for the oxidation of sulfur in the gas phase to SO3 is -204 kJ/mol and for the oxidation of SO2 to SO3 ...
- The ΔH °f of TiI3(s) is -328 kJ>mol and the ΔH ° for the reaction 2 Ti(s) + 3 I2( g)¡2 TiI3(s) is -839 kJ. ...
- 9.149 Consider the reaction: 4 CO1g2 2 NO21g2 4 CO21g2 N21g2. Using the following information, determine ΔH° f...
- Hydrazine, a component of rocket fuel, undergoes combus- tion to yield N2 and H2O: N2H41l2 + O21g2 S N21g2 + 2...
- Reaction of gaseous fluorine with compound X yields a sin- gle product Y, whose mass percent composition is 61...
- Calculate the enthalpy of the reaction 4B(s) + 3O2(g) → 2B2O3(s)
- Consider the combustion of liquid methanol ch3oh(l)
- Calculate the enthalpy of the reaction 4B(s) + 3O2(g) → 2B2O3(s).
- Consider the reaction H2O(l) → H2O(g), ΔH =44.0 kJ what will ΔH be for the reaction if it is reversed?