Week 6 (Heat Flow (Calorimetry, Hess Law), Galvanic Cells)

• Heat flow, q


  Measure of heat flow
• Calorimetry


  Science of measuring heat

  
• DH is proportional to mole
• Dissolving sodium nitrate in water

  1. NaNO3(s) → NaNO3(aq)

  2. ΔH = +21.0 kJ mol–1
• Sodium nitrate dissolving in water

  Temperature of water rises or falls
• Determining enthalpy change


  1.00 g of solid NaNO3 dissolved in water
• Relationship between heat and temperature

  • Flow of energy
  • Measure of average kinetic energy

  
• Finding q and therefore ΔrH

  Relate q, the flow of energy in/out of a system and the change in temperature of the system, or the surroundings
• What should be understood and done by the end of the lecture

  • Understand calorimetry relates heat and temperature
  • Define and use Specific Heat Capacity to find q
  • Find q and ΔH heat of reaction, or enthalpy change of a reaction using Coffee Cup calorimeter
  • Find q and ΔH heat of reaction, or enthalpy change of a reaction using Bomb calorimeter, using Ccal the calorimeter constant
  • Write the thermochemical equation

  
• Heat capacity


  Quantity of heat needed to change the temperature of a specified amount (gram or n) of material by 1oC (ΔT = 1oC = 1K)
• Hess' Law


  Predict ΔH for a given reaction, using thermochemical data of related reactions

  
• Hess' Law - Predicting enthalpy change of reactions that can't be measured

  1. Find another path to get from the reactants to products in the target reaction

  2. Add the values of ΔH = ΔH (iii) - ΔH (ii)

  
• Specific heat capacity


  Heat capacity per gram, amount of energy needed to raise the temperature of 1 g of the substance by 1oC (or 1K)
• Specific heat capacity values

  • water: 4.18
  • aluminium: 0.89
  • carbon: 0.71
  • iron: 0.45
  • mercury: 0.14
  • sand: 0.48

  
• Enthalpy change, ΔHreaction is a state function, meaning the path we take will not change the final value of ΔHreaction
• Molar heat capacity


  Energy required to heat 1 mole of a substance by 1oC (=1K)
• If the reaction is reversed


  The sign of the ΔH is reversed, while the magnitude remains the same
• Heating 200.0 mL of water from 20.0oC to 100.0oC


  1. q = C. m. ΔT

  2. q = 4.18 × (200.0 ×1.000) × 80.0

  3. q = 66900 J = 66.9 kJ
• Galvanic cell


  Energy source
• If the coefficients of a balanced reaction have been multiplied by an integer x

  ΔH must be multiplied by x
• Unattributed images taken from Cengage Learning resources Mahaffy et al, HACR and Zumdahl et al
• Heating 200.0 g block of iron from 20.0oC


  Tf = ?, m=200.0 g, Ti = 20.0oC, q = 7.5 kJ, C(iron) = 0.45 JoC-1 g-1
• Chemistry see LMS for details
• General Method for solving Hess' Law problems

  1. Look for reactions containing the desired reactants, and rearrange reaction (and amounts) to place that species on the lefthand side of reaction

  2. Look for reactions containing the desired products, and rearrange reaction (and amounts) to place that species on the righthand side of reaction

  3. Add the reactions together, cancelling any species in the same state that appear on the left and right of the equation
• C2H4 (g) + H2O (l) → C2H5OH (l)
• Determining molar heat capacity of water

  1. Specific heat capacity = 4.18 J g-1 oC-1

  2. Mass of 1.00 mole of water = 18.0 g

  3. Energy required to heat 1 mole (18.0 g) of H2O by 1.00oC
• Balance a redox reaction equation in acidic solution

  1. Balance all elements except hydrogen and oxygen

  2. Balance oxygen atoms using H2O molecules

  3. Balance hydrogen atoms using H+ ions

  4. Balance charge, using electrons
• Calorimetry


  Science of measuring heat, based on observation of temperature changes when a body absorbs or releases heat

  
• Half cell


  Part of a galvanic cell where a redox reaction takes place

  
• Redox chemistry


  Electron transfer reactions

  
• Coffee Cup Calorimeter


  Measures temperature changes and relates this to the heat associated with a chemical reaction
• Salt bridge


  Allows flow of ions between half cells to maintain electrical neutrality
• C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(g), ΔrHo= -2803 kJ mol-1
• Determining change in enthalpy for neutralisation reaction

  1. 50.0 mL of 1.00 M HCl solution at 25.0oC combined with 50.0 mL of 1.00 M NaOH solution at 25.0oC

  2. Temperature increases to 31.9oC

  3. Specific heat of solution = 4.18 J g-1 0C-1, density of solution = density of water, no heat loss
• Redox reaction in a galvanic cell

  1. Oxidation at the anode

  2. Reduction at the cathode
• Redox reactions


  • Rusting: 4Fe (s) + 3O2 (g) → 2Fe2O3 (s)
  • Capturing energy(sunlight) as carbohydrates in plants: 6 CO2(g) + 6 H2O(g) → C6H12O6(s) + 6 O2(g)
  • 'Burning' carbohydrates for energy in the body: C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(g)
  • 'Burning' gas for heat (combustion): CH4(g) + 2O2(g) → CO2(g)+ 2H2O(l)
• Writing thermochemical equation for neutralisation reaction


  1. Net ionic equation

  2. Calculate number of moles of reactants

  3. Calculate heat released to the surroundings = the solution: qsurroundings = Csolution .msolution .ΔTsolution

  
• Oxidation


  Reaction with oxygen, losing electrons
• Determine the change in enthalpy for the neutralisation reaction, DH in kJ mol-1 , and write the thermochemical equation for the reaction.
• Sketch a galvanic cell

  Draw the cell diagram from the equation for the cell reaction