Lesson Worksheet: Hess’s Law Chemistry

In this worksheet, we will practice defining and using Hess’s law to calculate enthalpy changes.


Consider the enthalpy changes of the following reactions: Fe()+Cl()FeCl()kJFeCl()+Cl()FeCl()kJsgssgs22223Δ𝐻=βˆ’341.812Δ𝐻=βˆ’57.7 Calculate the enthalpy of formation Ξ”π»βˆ˜οŒΏ of FeCl3 for the given reaction: Fe()+Cl()FeCl()sgs3223

  • Aβˆ’284.1 kJ/mol
  • Bβˆ’399.5 kJ/mol
  • C+399.5 kJ/mol
  • D+284.1 kJ/mol


Consider the 3 reactions shown.


Determine the value of Δ𝐻 for the following reaction. CH()CH()+H()26242ggg

  • Aβˆ’3256.8 kJ/mol
  • Bβˆ’136.8 kJ/mol
  • C+3256.8 kJ/mol
  • D+136.8/kJmol


The algebraic formula for Hess’s law is .

  • AΔ𝐻=Δ𝐻×Δ𝐻×Δ𝐻×⋯Δ𝐻+Δ𝐻+Δ𝐻+β‹―οŠ§οŠ¨οŠ©οŠ§οŠ¨οŠ©
  • BΔ𝐻=Δ𝐻+Δ𝐻+Δ𝐻+β‹―οŠ§οŠ¨οŠ©
  • CΔ𝐻=Ξ”π»βˆ’Ξ”π»βˆ’Ξ”π»βˆ’β‹―οŠ§οŠ¨οŠ©
  • DΔ𝐻=Ξ”π»Γ—Ξ”π»Γ—Ξ”π»Γ—β‹―οŠ§οŠ¨οŠ©


The equation shows the formation of one mole of ethanol, CHOH25, from its constituent elements under standard conditions and with standard states. 2C()+3H()+O()CHCHOH()+3O()sgglg22322312

ChemicalCarbon, C()sHydrogen, H2Ethanol, CHOH()25l
Standard Enthalpy of Combustion (kJβ‹…molβˆ’1)βˆ’394βˆ’286βˆ’1,371

Given the combustion values in the table, calculate the value for the standard enthalpy of formation.


Consider the reactions shown: CH()+2O()CO()+2HO()kJmolHO()HO()kJmol422222gggglgΔ𝐻=βˆ’900/Δ𝐻=+44/

What would the value of Δ𝐻 of the combustion of methane be if HO()2l formed instead of HO()2g?

  • Aβˆ’856 kJ/mol
  • Bβˆ’944 kJ/mol
  • Cβˆ’812 kJ/mol
  • Dβˆ’988 kJ/mol


Use the reactions C+HOCO+HkJmolCO+HHO+COkJmol22222Δ𝐻=+131/Δ𝐻=+41/∘∘

to calculate Ξ”π»βˆ˜ for the following reaction: 2COC+CO2

  • A+90 kJ/mol
  • B+172 kJ/mol
  • Cβˆ’90 kJ/mol
  • Dβˆ’172 kJ/mol


By using the reactions 1212Δ𝐻=+90.29/12Δ𝐻=+33.2/N()+O()NO()kJmolN()+O()NO()kJmol22222gggggg

Calculate Ξ”π»οŠ© which refers to the heat of combustion of nitric oxide (NO) for the following reaction: NO()+O()NO()kJmolggg12Δ𝐻=/22

  • Aβˆ’123.49
  • B+57.09
  • C+123.49
  • Dβˆ’57.09


Which of the following values is equivalent to Ξ”π»οŠ©?

  • Aβˆ’Ξ”π»+Δ𝐻+Ξ”π»οŠ§οŠ¨οŠͺ
  • Bβˆ’Ξ”π»βˆ’Ξ”π»βˆ’Ξ”π»οŠ§οŠ¨οŠͺ
  • CΞ”π»βˆ’Ξ”π»βˆ’Ξ”π»οŠ§οŠ¨οŠͺ
  • DΔ𝐻+Ξ”π»βˆ’Ξ”π»οŠ§οŠ¨οŠͺ
  • EΔ𝐻+Δ𝐻+Ξ”π»οŠ§οŠ¨οŠͺ


Using Hess’s law and the standard enthalpies of combustion in the given table, what is the standard enthalpy of formation of ethane, CH()26g?

ChemicalCarbon, C()sHydrogen, H2Ethane, CH()26g
Standard Enthalpy of Combustion, (kJβ‹…molβˆ’1)βˆ’394βˆ’286βˆ’1,560


Which of the following are necessary to have when calculating the average bond enthalpy of CCl in a molecule of tetrachloromethane, CCl4, using Hess’s law?

  1. Standard enthalpy change of atomization (C)
  2. Standard enthalpy change of atomization (Cl)
  3. Standard enthalpy change of formation (Cl2)
  4. Standard enthalpy change of formation (CCl4)
  5. Standard enthalpy change of combustion (C)
  6. Standard enthalpy change of combustion (Cl2)
  7. Standard enthalpy change of combustion (CCl4)
  • A1, 2, and 4
  • B4, 5, and 6
  • C1, 2, and 7
  • D5, 6, and 7
  • E2, 3, and 6

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