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

  • $C<em>2H</em>4(g) + H<em>2O(l) \rightarrow C</em>2H_5OH(l)$
  • 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:
    • $C<em>2H</em>4(g) + 3O<em>2(g) \rightarrow 2CO</em>2(g) + 2H_2O(l) \Delta H = -1411 \frac{kJ}{mol}$
    • $C<em>2H</em>5OH(l) + 3O<em>2(g) \rightarrow 2CO</em>2(g) + 3H_2O(l) \Delta H = -1367 \frac{kJ}{mol}$
  • Reverse the second equation to have ethanol as a product.
    • $2CO<em>2(g) + 3H</em>2O(l) \rightarrow C<em>2H</em>5OH(l) + 3O_2(g) \Delta H = +1367 \frac{kJ}{mol}$

• Step 4: Add the Equations

  • Add the modified equations together, canceling out species that appear on opposite sides.
    • $C<em>2H</em>4(g) + 3O<em>2(g) + 2CO</em>2(g) + 3H<em>2O(l) \rightarrow 2CO</em>2(g) + 2H<em>2O(l) + C</em>2H<em>5OH(l) + 3O</em>2(g)$
  • Net equation:
    • $C<em>2H</em>4(g) + H<em>2O(l) \rightarrow C</em>2H_5OH(l)$

• 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: $C(s) + O<em>2(g) \rightarrow CO</em>2(g)$
  • Two-step pathway: $C(s) + \frac{1}{2}O<em>2(g) \rightarrow CO(g)$ followed by $CO(g) + \frac{1}{2}O</em>2(g) \rightarrow CO_2(g)$

Example 3: Formation of Ethylene

• Problem: Calculate the delta H for the formation of ethylene ($C<em>2H</em>2$) from solid carbon.

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

• Key Information: Knowing the chemical formula of ethylene ($C<em>2H</em>2$) is essential.

• Target Equation: $2C(s) + H<em>2(g) \rightarrow C</em>2H_2(g)$

• 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 $C<em>2H</em>2$, you need two carbon atoms.
  • Multiply the carbon equation by two, adjusting the oxygen, carbon dioxide, and delta H values accordingly.

$2 [C(s) + O<em>2(g) \rightarrow CO</em>2(g)] \implies 2C(s) + 2O<em>2(g) \rightarrow 2CO</em>2(g)$

• 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 $\Delta H = \sum \Delta H<em>{products} - \sum \Delta H</em>{reactants}$

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.