Exam Preparation and Hess's Law Examples

Hess's Law - Key Principles

  • Rules to Remember: These rules are fundamental for this section of the course.

    • Changing/reversing an equation changes the sign of enthalpy.
    • Enthalpy is extensive; changing the number of moles requires multiplying the enthalpy by that amount.
    • You can use numerous individual reactions to achieve a desired overall reaction in Hess's Law.

  • Time Management: Exam questions are typically allocated one minute per mark. Plan accordingly.

Importance of Practice

  • Practice is crucial. Do not attempt Hess's Law questions for the first time during the exam.

  • Utilize available resources such as office hours, emails, and consultations with instructors.

Example 1: Ethanol Production from Ethene

  • Problem: Calculate the enthalpy change (delta H) for the reaction of ethene with water to produce ethanol.

  • Given: Thermochemical equations for the combustion of ethene and ethanol.

  • Step 1: Understand the Question

    • The question asks for the delta H of the reaction: ethene + water -> ethanol

  • Step 2: Write the Balanced Chemical Equation

    • C2H4(g) + H2O(l) \rightarrow C2H_5OH(l)
    • Verify that the equation is balanced (number of atoms of each element is the same on both sides).
    • Note the states of matter (gas, liquid) as they influence delta H values.

  • Step 3: Manipulate the Given Equations

    • Given equations:
      • C2H4(g) + 3O2(g) \rightarrow 2CO2(g) + 2H_2O(l) \Delta H = -1411 \frac{kJ}{mol}
      • C2H5OH(l) + 3O2(g) \rightarrow 2CO2(g) + 3H_2O(l) \Delta H = -1367 \frac{kJ}{mol}
    • Reverse the second equation to have ethanol as a product.
      • 2CO2(g) + 3H2O(l) \rightarrow C2H5OH(l) + 3O_2(g) \Delta H = +1367 \frac{kJ}{mol}

  • Step 4: Add the Equations

    • Add the modified equations together, canceling out species that appear on opposite sides.
      • C2H4(g) + 3O2(g) + 2CO2(g) + 3H2O(l) \rightarrow 2CO2(g) + 2H2O(l) + C2H5OH(l) + 3O2(g)
    • Net equation:
      • C2H4(g) + H2O(l) \rightarrow C2H_5OH(l)

  • Step 5: Calculate the Enthalpy Change

    • Sum the enthalpy changes of the modified equations.
      • \Delta H = -1411 \frac{kJ}{mol} + 1367 \frac{kJ}{mol} = -44 \frac{kJ}{mol}

Example 2: Combustion of Carbon

  • Problem: Determine the energy cost for carbon to seal (likely a typo, should be oxidation state or similar context).

  • Given: Enthalpies of combustion for:

    • Carbon to carbon dioxide
    • Carbon monoxide to carbon dioxide

  • Relevance of Hess's Law: The energy change from the same starting material to the same product must be the same, regardless of the pathway.

    • Direct pathway: C(s) + O2(g) \rightarrow CO2(g)
    • Two-step pathway: C(s) + \frac{1}{2}O2(g) \rightarrow CO(g) followed by CO(g) + \frac{1}{2}O2(g) \rightarrow CO_2(g)

Example 3: Formation of Ethylene

  • Problem: Calculate the delta H for the formation of ethylene (C2H2) from solid carbon.

  • Given: Three thermochemical equations involving acetylene, carbon dioxide, and water.

  • Key Information: Knowing the chemical formula of ethylene (C2H2) is essential.

  • Target Equation: 2C(s) + H2(g) \rightarrow C2H_2(g)

  • Strategy: Match the given equations to the starting materials in the target equation.

    • The bottom two equations resemble the starting materials (carbon and hydrogen).
    • The top equation relates to the product (ethylene) but needs to be reversed.

  • Balancing Equations: Ensure the equations are balanced, not only internally but also in terms of the number of atoms needed to match the target equation.

    • If generating C2H2, you need two carbon atoms.
    • Multiply the carbon equation by two, adjusting the oxygen, carbon dioxide, and delta H values accordingly.

2 [C(s) + O2(g) \rightarrow CO2(g)] \implies 2C(s) + 2O2(g) \rightarrow 2CO2(g)

  • Cancellation: Cancel out species on opposite sides of the equations.

  • Enthalpy Calculation: Add up the energy values, taking into account any necessary multiplications (e.g., multiplying by two for two moles of carbon).

Standard Enthalpies of Formation

  • Data Tables: Extensive tables provide data for calculating reaction energetics.

  • Reference State: An element in its natural state has a delta H of zero.

  • State Consideration: If an element or compound exists in multiple forms, its state (solid, liquid, gas) must be considered.

Practical Implications

  • Real-World Relevance:

    • While a practicing chemist might not always calculate delta H from scratch, understanding energetics is crucial for controlling reactions.
    • Chemists often refer to existing literature for similar reactions and conditions.

  • Combustion of Propane Example:

    • Hess's law can be applied to calculate the energy released during propane combustion.

  • Calculation Approaches: Two methods for calculating delta H:

    • Stepwise: Breaking down the reaction into individual steps.
    • Formulaic: Using the equation \Delta H = \sum \Delta H{products} - \sum \Delta H{reactants}

Exam Information

  • Format: Expect a similar format to previous years' exams.

  • Resources: Past papers with answers are typically provided.

  • Restricted Open Book Exam: Allowed one A4 sheet of notes.

  • Provided Information: Key equations, constants, and the periodic table will be supplied.

  • Calculator: Non-programmable calculators are permitted.

  • Multiple Choice Questions: No penalty for incorrect answers; attempt every question.

  • Content Areas: Expect questions on chemical bonds, ionic compounds, naming columns in the periodic table, and calculations involving moles.

  • Problem-Solving Tips: Use the periodic table to deduce charges of ions. Consider the units and prefixes (milli, nano, pico).

  • Open Door Policy: Instructors are available for help and clarification.