1.4 energetics

• what is enthalpy

  the amount of energy produced by one mole of a substance
• what is enthalpy change

  change in heat content at constant pressure
• what is standard state

  • standard pressure = 1 atmosphere
  • standard temperature = 298k (25)
  • standard concentration = 1 mol/dm-1
  • elements in their standard state
• what is standard enthalpy change of formation

  enthalpy change to produce 1 mole of a compound from its constituent elements in their standard state
• enthalpy formation of an element is always 0
• what is standard enthalpy of combustion

  enthalpy change that occurs when 1 mole of fuel burns completely in oxygen under standard conditions with all its reactants / products in standard state
• what is standard enthalpy change of neutralisation

  enthalpy change that occurs when 1 mole of water is formed from its constituent ions under standard conditions
• specific heat capacity is the amount of energy required to raise the temperature of 1g of a substance by 1 degree
• • specific heat capacity equation
  • q = m c Δ T
  • energy transferred (j) = mass (g)× specific heat capacity ( jg$^-1$/k$^-1$ × change in temperature
• list all the ways energy can escape in a calorimeter

  • evaporates via the surface of the liquid
  • energy absorbed by the beaker
  • energy escapes in air due to distance
  • fuel may evaporate before burning
• ways to reduce energy loss using a calorimeter

  • copper beaker reduces energy loss
  • using a lid
  • draught excluder
  • reducing distance between can and flame
  • using a bomb calorimeter reduces all of these
• label the calorimeter

  A) thermometer

  B) beaker

  C) water

  D) spirit burner

  
• steps : (calorimeter)

  1. measure the initial mass of the fuel so you can minus it from its final mass (initial-final)
  2. measure the volume of water in the colorimeter
  3. burn the fuel to warm up the water
  4. measure the temperature change of water (initial-final)

  
• how to find enthalpy change of a reaction

  1. the enthalpy change of a reaction can be found by measuring the temperature change in a reaction
  2. the heat energy given or taken is used to heat or cool a known mass of water and given specific heat capacity
  3. use the specific heat capacity equation to find q (energy given out)
  4. to find enthalpy change you substitute into this equation

  enthalpy change (per mole) = q / number of moles reacting
• heat loss is a major problem with calorimetry and can lead to errors in results.
• ways to reduce errors in calorimetry

  • reaction taking place in solutions

  1. reaction is carried out in an insulated beaker and temperature change is measured
  2. the reaction must be fast so that the maximum temperature is reached quickly before it starts to cool
• ways to reduce errors in calorimetry

  • flame calorimeter
• 
  

  • flame calorimetry

  1. usually used to find the enthalpy of combustion of substance ( usually a fuel )
  2. the fuel which is in a bottle with a wick is burned so that the heat is passed to water which it heats
  3. sometimes the specific heat capacity is used but in more accurate flame calorimeters the actual heat capicity of the flame can be determined
• bond enthalpy is the enthalpy change to break one mole of covalent bonds in the gas phase
• mean bond enthalpy is the energy required to break one mole of bond averaged over a range of compounds
• The heat capacity of water is 4.18 J g-1K-1
• common exothermic reactions:

  • combustion of fuels
  • oxidation of carbohydrates
• standard enthalpy change of formation

  
• standard enthalpy change of combustion

  
• If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together
• errors using calorimetry

  • Energy losses from calorimeter
  • Incomplete combustion of fuel
  • Incomplete transfer of energy
  • Evaporation of fuel after weighing
  • Heat capacity of calorimeter not included
  • Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment E
• what is an exothermic change?

  when energy is transferred from system to the surroundings so products have less energy than reactants
• what are common oxidative exothermic reactions?

  1. combustion of fuels
  2. oxidation of carbohydrates like glucose is respiration
• what is an endothermic reaction?

  when heat is transferred from the surroundings into the system , the products have more energy than the system, this requires a heat input like the thermal decomposition of calciumcarbonate
• what is the standard enthalpy of formation and its symbol?

  enthalpy change when 1 mole of a compound is produced from its constituent elements under standard conditions (ΔfH°)
• what is the standard enthalpy change of combustion and its symbol?

  enthalpy change when 1 mole of a substance burns completely in excess oxygen under standard conditions(ΔcH°)
• what are the standard conditions? (4)

  1. 100kpa
  2. 1 moldm^-3
  3. standard state in room temp
  4. 298k (25 degrees celcius)
• what is the standard enthalpy of neutralisation?

  when an acid and alkali react to form 1 mole of water under standard conditions
• how do you measure energy change? (give units)

  Q(j) = m(g)c(jg-1k-1)ΔT(k)
• what is the calorimetric method?

  when substances are mixed in an insulated container and temperature is measured
• general method for calorimetry ?

  1. washes the equipment (cup and pipettes etc) with the solutions to be used dry after
  2. put polystyrene cup in a beaker for insulation and support
  3. Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
  4. clamp thermometer into place making sure the thermometer bulb is immersed in solution
  5. measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
  6. add second reagent in
  7. Record temperature every minute after addition for several minutes
• what happens if the reaction is too slow in calorimetry?

  the reaction will occur slowly so temperature rise will be hard to obtain as the solution is simultaneously cooling
• what are ways to counteract a slow reaction in calorimetry?

  by plotting readings at regular intervals and then extrapolating the temperature curve to the time the reactants were added
• what are errors of calorimetry?

  1. assume the heat capacity of the solution in the same as water
  2. assume the density of the solution is the same as water
  3. ignore heat absorbed by apparatus
  4. energy loss to the surroundings
  5. reaction or dissolving of substances may be slow or incomplete
• what are errors in the calorimetry of combustion ?

  1. incomplete combustion of the fuel
  2. fuel evaporates after weighing
  3. energy loss from calorimetry
  4. heat capacity of calorimetry not used
  5. measurements not taken under standard conditions