Exam Review and Calorimetry Lab Notes

Quiz Question Discussion

• Questions 8 and 9 involve substances not on the table, making them unsolvable without additional information.
• If the substances and chemical reaction are provided, you can solve for \Delta H using the equation: \Delta G = \Delta H - T\Delta S.
• This involves plugging in known values for Gibbs free energy, temperature, and entropy to solve for enthalpy, or vice versa.
• These are considered simpler "plug and chug" problems.

Clarification on Problem Number Four

• A student calculated the energy using two methods and obtained slightly different results. One calculation yielded a result around -514.8, while another using H and S gave -514.4.
• The value -514.4 is considered the correct one.
• Another student obtained 2878.2 using all given values and -3967410.7 initially, then corrected to 3367 for another calculation. Not bad.

Temperature Considerations

• If the temperature is not explicitly given, assume the data was gathered at 298.15 K. This is the standard temperature to use unless specified otherwise.

Determining Spontaneity and Temperature

• For problems where you need to find the temperature at which a reaction becomes spontaneous, set \Delta G to zero.
• \Delta G = 0 represents the tipping point of spontaneity.
• Any temperature above the calculated temperature will result in a spontaneous reaction.
• For example, if 200 K is the calculated temperature, then 200.00000001 K will be spontaneous
• Calculate using the definition of \Delta G using \Delta H and \Delta S. This calculation is described as literally super duper simple.

Thermodynamic Definitions and Calculations

• \Delta H represents the heat of reaction, which can be calculated using heats of formation (products minus reactants).
• \Delta S represents the change in entropy, also calculated as products minus reactants, using values from a table.
• The Gibbs free energy of reaction is distinct from the Gibbs free energy of formation unless you are forming a single product from its elements.
• If glucose is the only product, then the Gibbs free energy of formation and the heat of reaction would be the same.

Missing Data and Resources

• If data is missing from the provided table (e.g., silver oxide in number seven), it may be necessary to search for the information online.

Stoichiometry and Calculations

• Example calculation: H_2S value is 33.6, so 3 ims 33 ilda 100; potassium chloride 2.96, so 4 ims 2.96 \tilda 1000.

Spontaneity and Thermodynamic Factors

• Spontaneous reactions always have a negative \Delta G.
• This occurs when \Delta H is negative (exothermic) and \Delta S is positive (increased entropy).
• Exothermic reactions that increase entropy are always spontaneous. This is a potential true/false question.

Calorimetry Lab Introduction

• The lab will focus on calorimetry and understanding how it works, emphasizing conceptual understanding over simply copying answers.
• Calorimeters will be used to measure the heat of reaction between hydrochloric acid and either sodium bicarbonate or sodium carbonate.
• Hess's law will be applied to determine the \Delta H of the reaction:
  2 NaHCO3 \rightarrow CO2 + H2O + Na2CO_3
• This reaction's \Delta H cannot be directly measured with the available equipment, so Hess's law is used.

Procedure and Calculations

• React sodium bicarbonate and sodium carbonate with hydrochloric acid separately.
• Flip one of the equations to allow for the cancellation of hydrochloric acid, adjusting the sign of \Delta H.
• Double the first equation, meaning the obtained energy value will have to be doubled.

Determining Moles and Heat of Reaction

• Weigh the amount of sodium carbonate used to convert mass to moles.
• The hydrochloric acid solution is treated as water for calculations.
• Hydrochloric acid is mostly water, so the mass of the hydrochloric acid solution is used as the mass of water in the calculations.
• Weigh the solution before and after to determine the mass of the water.
• Use 4.18 J/g^{\circ}C as the specific heat of water for the hydrochloric acid solution.
• Calculate q for the water using: q = mc\Delta T, where:
  • m is the mass of the water (hydrochloric acid solution).
  • c is the specific heat of water ( ilda 4.18).
  • \Delta T is the change in temperature.
• If the water gains energy (positive q), the reaction is endothermic, and the heat of reaction will be the negative of q.

Energy per Mole Calculations

• The calculated heat of reaction is for a small fraction of a mole of sodium carbonate or sodium bicarbonate (approximately 2.25 to 2.5 grams).
• Convert grams to moles to find the energy per mole.
• Units guide the calculation: joules per mole (j/mol) or kilojoules per mole (kj/mol).
• Divide the energy in kilojoules by the moles of sodium carbonate or sodium bicarbonate to get the energy per mole.

Adjustments for Stoichiometry

• If the reaction requires doubling (as indicated in the balanced equation), double the \Delta H value.

Ethical Considerations and Lab Safety

• The instructor emphasizes ethical concerns:
  • Cautioning against cheating and copying, as it will eventually be revealed.
  • Mentioning statistical analysis to identify potential instances of academic dishonesty.
  • The instructor stresses the importance of wearing goggles for safety during the lab.

Calculations walkthrough

• The mass of hydrochloric acid should be used for calculations rather than the mass of the solution.
• Values for the variables are: mass = \tilda 49 grams. Should be around \tilda 48 to \tilda 50 grams splitting the difference.
• In calculation with provided values, the heat of the reaction is negative for the second reaction.
• The \Delta H value does eventually have to be multiplied by 2.