Topic 5: Energetics

• Thermodynamics


  The study of energy and how it is interconverted
• First law of thermodynamics

  Energy can be converted from one form to another and the total amount of energy for a given system will remain constant
• Energy can be neither created nor destroyed; it can only be converted between different forms
• Chemical potential energy


  Energy stored in the chemical bonds of reactants and products
• Heat (q)


  A form of energy that is transferred from a warmer body to a cooler body, as a result of the temperature gradient
• Absolute zero (0 K)


  The temperature at which all motion of the particles theoretically stops and the entropy (S) of a system reaches its minimum possible value
• Absolute temperature (in kelvin)


  Proportional to the average kinetic energy of the particles of matter
• System


  The reactants, products, and any solvents in a chemical reaction
• Surroundings


  The apparatus that contains the reaction, thermometers or other measuring devices, the laboratory, and everything external to the reacting substances
• Bond dissociation energy


  The energy required to break chemical bonds
• Enthalpy (H)


  A state function, any change in value is independent of the pathway between the initial and final measurements
• Thermochemistry


  The study of heat changes that occur during chemical reactions
• Enthalpy change (ΔH)


  The heat transferred by a closed system during a chemical reaction
• Exothermic reaction


  A chemical reaction in which heat is transferred from the system to the surroundings, with a negative ΔH
• Endothermic reaction


  A chemical reaction that absorbs heat from its surroundings, with a positive ΔH
• Specific heat capacity (c)

  The amount of heat needed to raise the temperature of 1 g of a substance by 1 °C or 1 K
• Thermic reaction


  Enthalpy of the products is greater than that of the reactants. The products are described as being energetically less stable than the reactants
• Endothermic reaction


  Enthalpy of the products is less than that of the reactants. The products are described as being energetically more stable than the reactants
• Potential energy


  Energy stored in a system by virtue of the position or arrangement of its parts
• Reaction pathway


  The sequence of steps by which reactants are converted into products
• ∆H


  Change in enthalpy
• Reactants


  The starting substances in a chemical reaction
• Products


  The substances formed as the result of a chemical reaction
• Specific heat capacity is an intensive property that does not vary in magnitude with the size of the system being described
• Specific heat capacity is used to calculate the heat q of a system using the relationship: q = mc∆T
• Coffee-cup calorimeter


  • Systematic errors that can be analysed and the effect of their directionality assessed
  • Measured change in enthalpy for a reaction will always be lower than the actual value, as heat will be transferred between the contents and the surroundings in every experiment
• Experimental method to determine ΔT

  1. Measure mass of 25 cm3 of 1.0 mol dm-3 CuSO4 solution

  2. Record temperature of solution every 30 seconds for up to 3 minutes, or until a constant temperature is achieved

  3. Introduce powdered zinc and commence stirring

  4. Continue to take temperature readings for up to 5 minutes after the maximum temperature has been reached

  5. Produce a temperature versus time graph to determine the change in temperature
• Assumptions and errors in the coffee-cup calorimeter method
• Loss of heat from the system to the surroundings is the main source of error in this experiment and one that is difficult to quantify
• Calculation of molar enthalpy change

  1. q = mc∆T

  2. Amount of CuSO4 = 1.37 g / 65.38 g/mol = 0.0210 mol

  3. Molar enthalpy change = 4.69 kJ / 0.0210 mol = 223 kJ/mol
• Enthalpy change of formation (∆Hf)


  The energy change upon the formation of 1 mol of a substance from its constituent elements in their standard state
• Enthalpy change of combustion (∆Hc)

  The heat evolved upon the complete combustion of 1 mol of substance
• Calculating enthalpy change of reaction using enthalpy of formation


  ∆H reaction = ∑(∆Hf products) - ∑(∆Hf reactants)
• Enthalpy of formation values

  • C6H6(l): +49.0 kJ/mol

  CO2(g): -393.5 kJ/mol

  H2O(l): -285.8 kJ/mol

  
• Experimental method to find enthalpy change of combustion

  Determine initial mass of spirit burners

  Determine mass of 30 cm3 of water

  Place beaker/calorimeter on tripod with spirit burner beneath

  Determine and record initial temperature of water

  Allow alcohol to burn until 30°C temperature change achieved or for 2 minutes

  Extinguish spirit burner and re-weigh
• Assumptions in the enthalpy of combustion experiment
• Measuring energy changes


  1. Allow each alcohol to burn until a temperature change of 30°C is achieved

  2. Allow each alcohol to burn for a period of 2 minutes

  3. Extinguish each spirit burner by replacing the cap

  4. Re-weigh each one and record the change in mass of the alcohol
• q


  Heat energy absorbed or released
• 0.0312 kg × 4.18 kJ kg-1 K-1 × 30.0 K = 3.91 kJ
• Data-logging equipment can be used to record temperature changes accurately and perform data analysis and graphing