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8. Thermochemistry

Hess's Law

8. Thermochemistry

Hess's Law

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Hess's Law Example 1

Hess's Law Example 1

Hess's Law Example 2

Hess's Law Example 2

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Relationships Involving ΔHrxn - Hess's Law

Relationships Involving ΔHrxn - Hess's Law

Ben's Chem Videos

ALEKS - Using Hess's Law to Calculate Net Reaction Enthalpy

ALEKS - Using Hess's Law to Calculate Net Reaction Enthalpy

Practice Problem: Hess's Law

Practice Problem: Hess's Law

Professor Dave Explains

Enthalpy of Reaction

Enthalpy of Reaction

Bozeman Science

Hess's Law Trick Question You Should Know

Hess's Law Trick Question You Should Know

Melissa Maribel

Hess's Law and Heats of Formation

Hess's Law and Heats of Formation

Professor Dave Explains

Hess's Law Common Test Question

Hess's Law Common Test Question

Melissa Maribel

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Practice this topic

Multiple Choice

Calculate the ∆Hrxn for the following thermochemical equation:

When given the following:

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Multiple Choice

Calculate the ∆H_rxn for

Given the following set of reactions:

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Multiple Choice

Calculate the ∆H_rxn for

Given the following reactions:

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Multiple Choice

Find ΔH_rxn for the following reaction:

N₂ + 2 O₂ → 2 NO₂
Based on the following data.
2 NO → N₂ + O₂ ΔH = −180 kJ
2 NO + O₂ → 2 NO₂ ΔH = −112 kJ

Textbook Question

Calculate the enthalpy change for the reaction C(s) + 2 H2(g) → CH4(g) ΔH = ? Given the enthalpy values for the following reactions CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH = −890.4 kJ C(s) + O2(g) → CO2(g) ΔH = −393.5 kJ H2(g) + 1/2 O2(g) → H2O (g) ΔH = −285.8 kJ (a) −1569.7 kJ (b) +211.1 kJ (c) −1855.5 kJ (d) −74.7 kJ

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Textbook Question

Consider the following equilibrium: 2 H21g2 + S21g2 Δ 2 H2S1g2 Kc = 1.08 * 107at 700 C (c) Calculate the value of Kc if you rewrote the equation H21g2 + 12S21g2 Δ H2S1g2.

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Textbook Question

Consider the following equilibrium, for which Kp = 0.0752 at 480 C: 2 Cl21g2 + 2 H2O1g2 Δ 4 HCl1g2 + O21g2 (a) What is the value of Kp for the reaction 4 HCl1g2 + O21g2 Δ 2 Cl21g2 + 2 H2O1g2?

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Textbook Question

This reaction has an equilibrium constant of Kp = 2.26 * 104 at 298 K. CO(g) + 2 H2(g) ⇌ CH3OH(g) Calculate Kp for each reaction and predict whether reactants or products will be favored at equilibrium. c. 2 CH3OH(g) ⇌ 2 CO(g) + 4 H2(g)

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Textbook Question

This reaction has an equilibrium constant of Kp = 2.26 * 104 at 298 K. CO(g) + 2 H2(g) ⇌ CH3OH(g) Calculate Kp for each reaction and predict whether reactants or products will be favored at equilibrium. b. 1/2 CO(g) + H2 (g) ⇌ 1/2 CH3OH(g)

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Textbook Question

This reaction has an equilibrium constant of Kp = 2.26 * 104 at 298 K. CO(g) + 2 H2(g) ⇌ CH3OH(g) Calculate Kp for each reaction and predict whether reactants or products will be favored at equilibrium. a. CH3OH(g) ⇌ CO(g) + 2 H2(g)

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Textbook Question

This reaction has an equilibrium constant of Kp = 2.2 * 106 at 298 K. 2 COF2(g) ⇌ CO2(g) + CF4(g) Calculate Kp for each reaction and predict whether reactants or products will be favored at equilibrium. c. 2 CO2(g) + 2 CF4(g) ⇌ 4 COF2(g)

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Textbook Question

This reaction has an equilibrium constant of Kp = 2.2 * 106 at 298 K. 2 COF2(g) ⇌ CO2(g) + CF4(g) Calculate Kp for each reaction and predict whether reactants or products will be favored at equilibrium. b. 6 COF2(g) ⇌ 3 CO2(g) + 3 CF4(g)

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Textbook Question

This reaction has an equilibrium constant of Kp = 2.2 * 106 at 298 K. 2 COF2(g) ⇌ CO2(g) + CF4(g) Calculate Kp for each reaction and predict whether reactants or products will be favored at equilibrium. a. COF2 (g) ⇌ 1/2 CO2(g) + 1/2 CF4(g)

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Textbook Question

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 kJ (3) A + 2B → D ΔH° = ? (b) What is the value of ΔH° for the overall reaction A + 2 B → D ΔH° = ?

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Textbook Question

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 kJ (3) A + 2B → D ΔH° = ? (a) Which Hess's law diagram represents the reaction steps and the overall reaction? Diagram 1 Diagram 2

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Textbook Question

Acetylene, C2H2, reacts with H2 in two steps to yield ethane, CH3CH3: (1) HC?CH + H2 → H2C?CH2 ΔH° = -175.1 kJ (2) H2C?CH2 + H2 → CH3CH3 ΔH° = -136.3 kJ Which arrow (a–c) in the Hess's law diagram corresponds to which step, an2d which a2rrow cor2responds to the net reaction? Where are the reactants located on the diagram, and where are the products located?

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Textbook Question

Consider the combustion of liquid methanol, CH3OH1l2: CH3OH1l2 + 32O21g2¡ CO21g2 + 2 H2O1l2 H = -726.5 kJ (b) Balance the forward reaction with whole-number coefficients. What is H for the reaction represented by this equation?

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Textbook Question

Consider the combustion of liquid methanol, CH3OH1l2: CH3OH1l2 + 32O21g2¡ CO21g2 + 2 H2O1l2 H = -726.5 kJ (a) What is the enthalpy change for the reverse reaction?

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Textbook Question

Consider the decomposition of liquid benzene, C6H61l2, to gaseous acetylene, C2H21g2: C6H61l2 ¡ 3 C2H21g2 H = +630 kJ (a) What is the enthalpy change for the reverse reaction?

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Textbook Question

The reaction 2 AsH31g2 ∆ As21g2 + 3 H21g2 has Kp = 7.2 * 107 at 1073 K. At the same temperature, what is Kp for each of the following reactions? (a) As21g2 + 3 H21g2 ∆ 2 AsH31g2

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Textbook Question

Consider the following hypothetical reactions: A ¡B HI = +60 kJ B ¡ C HII = -90 kJ (b) Construct an enthalpy diagram for substances A, B, and C, and show how Hess's law applies.

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Textbook Question

Calculate the enthalpy change for the reaction P4O61s2 + 2 O21g2 ¡ P4O101s2 given the following enthalpies of reaction: P41s2 + 3 O21g2 ¡P4O61s2 H = -1640.1 kJ P41s2 + 5 O21g2 ¡P4O101s2 H = -2940.1 kJ

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Textbook Question

Show how Equations 18.7 and 18.9 can be added to give Equation 18.10.

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Textbook Question

From the enthalpies of reaction 2 C1s2 + O21g2¡ 2 CO1g2 H = -221.0 kJ 2 C1s2 + O21g2 + 4 H21g2¡ 2 CH3OH1g2 H = -402.4 kJ calculate H for the reaction CO1g2 + 2 H21g2¡CH3OH1g2

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Textbook Question

From the enthalpies of reaction H21g2 + F21g2¡ 2 HF1g2 H = -537 kJ C1s2 + 2 F21g2¡ CF41g2 H = -680 kJ 2 C1s2 + 2 H21g2¡ C2H41g2 H = +52.3 kJ calculate H for the reaction of ethylene with F2: C2H41g2 + 6 F21g2¡ 2 CF41g2 + 4 HF1g2

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Textbook Question

Given the data N21g2 + O21g2¡ 2 NO1g2 H = +180.7 kJ 2 NO1g2 + O21g2¡ 2 NO21g2 H = -113.1 kJ 2 N2O1g2¡ 2 N21g2 + O21g2 H = -163.2 kJ use Hess's law to calculate H for the reaction N2O1g2 + NO21g2¡ 3 NO1g2

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Textbook Question

For each generic reaction, determine the value of ΔH2 in terms of ΔH1. a. A + B¡2 C ΔH1 2 C¡A + B ΔH2 = ?

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Textbook Question

For each generic reaction, determine the value of ΔH2 in terms of ΔH1. c. A¡B + 2 C ΔH1 12 B + C¡12 A ΔH2 = ?

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Textbook Question

For each generic reaction, determine the value of ΔH2 in terms of ΔH1. b. A + 12 B¡C ΔH1 2 A + B¡2 C ΔH2 = ?

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Textbook Question

Consider the generic reaction: A + 2 B¡C + 3 D ΔH = 155 kJ Determine the value of ΔH for each related reaction. a. 3 A + 6 B¡3 C + 9 D b. C + 3 D¡A + 2 B c. 12 C + 32 D¡12 A + B

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Textbook Question

Calculate ΔHrxn for the reaction: Fe2O3(s) + 3 CO( g)¡2 Fe(s) + 3 CO2( g) Use the following reactions and given ΔH's: 2 Fe(s) + 32 O2( g)¡Fe2O3(s) ΔH = -824.2 kJ CO( g) + 12 O2( g)¡CO2( g) ΔH = -282.7 kJ

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Textbook Question

Calculate ΔHrxn for the reaction: CaO(s) + CO2( g)¡CaCO3(s) Use the following reactions and given ΔH's: Ca(s) + CO2( g) + 12 O2( g)¡CaCO3(s) ΔH = -812.8 kJ 2 Ca(s) + O2( g)¡2 CaO(s) ΔH = -1269.8 kJ

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Textbook Question

Calculate ΔHrxn for the reaction: 5 C(s) + 6 H2( g)¡C5H12(l ) Use the following reactions and given ΔH's: C5H12(l ) + 8 O2( g)¡5 CO2( g) + 6 H2O( g) ΔH = -3244.8 kJ C(s) + O2( g)¡CO2( g) ΔH = -393.5 kJ 2 H2( g) + O2( g)¡2 H2O( g) ΔH = -483.5 kJ

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Textbook Question

Calculate ΔHrxn for the reaction: CH4( g) + 4 Cl2( g)¡CCl4( g) + 4 HCl( g) Use the following reactions and given ΔH's: C(s) + 2 H2( g)¡CH4( g) ΔH = -74.6 kJ C(s) + 2 Cl2( g)¡CCl4( g) ΔH = -95.7 kJ H2( g) + Cl2( g)¡2 HCl( g) ΔH = -92.3 kJ

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Textbook Question

What is Hess's law, and why does it 'work'?

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Textbook Question

The following steps occur in the reaction of ethyl alcohol (CH3CH2OH) wiht oxygen to yield acetic acid (CH3CO2H). Show that equations 1 and 2 sum to give the net equation and calculate ΔH° for the net equation. (1) CH3CH2OH(l) + 1/2 O2(g) → CH3CHO (g) + H2O(l) ΔH° = -174.2 kJ (2) CH3CHO(g) + 1/2 O2(g) → CH3CO2H(l) ΔH° = -318.4 kJ (Net) CH3CH2OH(l) + O2(g) → CH3CO2H(l) + H2O(l) ΔH° = ?

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Textbook Question

The industrial degreasing solvent methylene chloride, CH2Cl2, is prepared from methane by reaction with chlorine: CH4(g) + 2 Cl2(g) → CH2Cl2(g) + 2 HCl(g) Use the following data to calcualte ΔH° in kilojoules for the reaction: CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) ΔH° = -98.3 kJ CH3Cl(g) + Cl2(g) → CH2Cl2(g) + HCl(g) ΔH° = -104 kJ

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Textbook Question

Hess's law can be used to calculate reaction enthalpies for hypothetical processes that can't be carried out in the labo- ratory. Set up a Hess's law cycle that will let you calculate ∆H° for the conversion of methane to ethylene: 2 CH4(g) → C2H4(g) + 2 H2(g) You can use the following information: 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l) ∆H° = -3120.8 kJ CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ∆H° = -890.3 kJ C2H4(g) + H2(g) → C2H6(g) ∆H° = -136.3 kJ H2O(l) ∆H°f = -285.8 kJ/mol

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Textbook Question

Find ∆H° in kilojoules for the reaciton of nitric oxide with oxygen, 2 NO(g) + O2(g) → N2O4(g), given the following data: N2O4(g) → 2 NO2(g) ∆H° = 55.3 kJ NO(g) + 1/2 O2(g) → NO2(g) ∆H° = -58.1 kJ

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Textbook Question

Set up a Hess's law cycle, and use the following information to calculate ΔH°f for aqueous nitiric acid, HNO3(aq). You will need to use fractional coefficients for some equations. 3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g) ΔH° = -137.3 kJ 2 NO(g) + O2(g) → 2 NO2(g) ΔH° = -116.2 kJ 4 NH3(g) + 5 O2(g) → 4 NO (g) + 6 H2O(l) ΔH° = -1165.2 kJ NH3(g) ΔH°f = -46.1 kJ/mol H2O(l) ΔH°f = -285.8 kJ/mol

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Textbook Question

Sulfuric acid (H2SO4), the most widely produced chemical in the world, is amde yb a two-step oxidaiton of sulfur to sulfur trioxide, SO3, followed by reaciton with water. Calculate ΔH°f for SO3 in kJ/mol, given the following data: S(s) + O2(g) → SO2(g) ΔH° = -296.8 kJ SO2(g) + 1/2 O2(g) → SO3(g) ΔH° = -98.9 kJ

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Textbook Question

The standard enthalpy change for the reaciton of SO3(g) with H2O(l) to yield H2SO4(aq) is ΔH° = -227.8 kJ. Use the information in Problem 9.104 to calculate ΔH°f for H2SO4(aq) in kJ/mol. [For H2O(l), ΔH°f = -285.88 kJ/mol.]

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Textbook Question

We can use Hess's law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethane: 2 CH41g2 ¡C2H61g2 + H21g2 Calculate the H° for this reaction using the following thermochemical data: CH41g2 + 2 O21g2¡CO21g2 + 2 H2O1l2 H° = -890.3 kJ 2 H21g2 + O21g2¡2 H2O1l2 H ° = -571.6 kJ 2 C2H61g2 + 7 O21g2¡4 CO21g2 + 6 H2O1l2 H° = -3120.8 kJ

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Textbook Question

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 is 89.5 kJ/mol. Find the enthalpy of formation of SO2 under these conditions.

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Textbook Question

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. Calculate the ΔH of sublimation of I2(s), which is a solid at 25 °C.

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Textbook Question

9.149 Consider the reaction: 4 CO1g2 2 NO21g2 4 CO21g2 N21g2. Using the following information, determine ΔH° for the reaction at 25 °C. NO1g2 ΔH°f = + 91.3 kJ>mol CO21g2 ΔH°f = - 393.5 kJ>mol 2 NO1g2 + O21g2 S 2 NO21g2 ΔH° = - 116.2 kJ 2 CO1g2 + O21g2 S 2 CO21g2 ΔH° = - 566.0 kJ

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Textbook Question

Hydrazine, a component of rocket fuel, undergoes combus- tion to yield N2 and H2O: N2H41l2 + O21g2 S N21g2 + 2 H2O1l2 (b) Use the following information to set up a Hess's law cycle, and then calculate ΔH° for the combustion reac- tion. You will need to use fractional coefficients for some equations. 2 NH31g2 + 3 N2O1g2 S 4 N21g2 + 3 H2O1l2 ΔH° = - 1011.2 kJ N2O1g2 + 3 H21g2 S N2H41l2 + H2O1l2 ΔH° = - 317.2 kJ 4 NH31g2 + O21g2 S 2 N2H41l2 + 2 H2O1l2 ΔH° = - 286.0 kJ H2O1l2 ΔH°f = - 285.8 kJ>mol

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Textbook Question

Reaction of gaseous fluorine with compound X yields a sin- gle product Y, whose mass percent composition is 61.7% F and 38.3% Cl. (c) Calculate ΔH° for the synthesis of Y using the following information: 2 CIF1g2 + O21g2 S Cl2O1g2 + OF21g2 ΔH° = + 205.4 kJ 2 CIF31l2 + 2 O21g2 S Cl2O1g2 + 3 OF21g2 ΔH° = + 532.8 kJ OF21g2 ΔH°f = + 24.5 kJ>mol

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Calculate the enthalpy of the reaction 4B(s) + 3O₂(g) → 2B₂O₃(s)

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Consider the combustion of liquid methanol ch3oh(l)

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Calculate the enthalpy of the reaction 4B(s) + 3O₂(g) → 2B₂O₃(s).

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Consider the reaction H2O(l) → H2O(g), ΔH =44.0 kJ what will ΔH be for the reaction if it is reversed?

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