Coursework: Enthalpy change of combustion Flashcards
What is Standard Enthalpy change of Combustion (ΔHoc)?
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
What is ‘standard conditions’?
- 298 K (25ºC) temperature.
- 100 KPa (1 atm) pressure.
- 1 mol.dm-3 concentration (when applicable).
Why is there a regular trend in ΔHºc for alcohols?
- Each successive alcohol in the homologous series increases in length by a -CH2- group, which adds an extra C-C bond as well as 2 C-H bonds that need to be broken during combustion, so the energy required for bond breaking increases regualrly down the homologous series.
- Each successive alcohol forms an extra CO2 and H2O as products of combustion, containing 2 C=O bonds and 2 C-H bonds, so energy released from bond formation also increases regualrly down homologous series.
- Overall enthalpy change therefore gets more exothermic regularly down the homologous series.
Why are ΔHºc different from the ΔH calculated using average bond enthalpies?
- Average bond enthalpies are only averages and they do not take into account different bonding environments that may lead to slightly different bond enthalpies.
- Average bond enthalpies are for gaseous bonds. Not all species are gaseous under standard conditions, therefore extra enthalpy changes will take place when converting the gaseous species into its standard state.
What is the formula for calculating percentage errors in measurements?
Percentage error = (total uncertainty associated with apparatus / measurement made)*100
When might procedural errors be significant?
When even taking the % errors in measurement into account doesn’t get the actual values into the range of the measured value. When the actual error > error in measurement.
How do you decrease the percentage errors in measurement without changing the apparatus?
Make the actual measurement greater, especially for the least accurate piece of apparatus.
How would you modify an experimental setup to decrease the percentage errors in measurement?
Replace the least accurate piece of apparatus with a more accuate piece of apparatus, one with smaller divisions on the scale of measurement.
What are the possible sources of procedual errors in the method?
- Heat may be lost to the surroundings from the spirit burner as well as th water itself.
- The alcohol doesn’t undergo complete combustion.
- The alcohol/ water may have evaporated which will take heat energy out of the system.
- Conditions are not standard.
- Apparatus may also be heated and will take a certain amount of energy to heat.
What effect would obtaining a bigger reading for the mass of alcohol have on the final value for ΔHc?
It would give a less exothermic ΔHc since ΔHc= Q / No. of moles of alcohol. The bigger the moles of alcohol, the smaller (less negative) the ΔH, the less exothermic the ΔHc.
What would measuring less water have on the value for ΔHc?
Less water was heated, so less alcohol was used to heat the alcohol. Because ΔHc = Q / No. of moles of alcohol, the smaller the mass of alcohol, the greater the ΔHc; so the reaction would be more exothermic.
What effect would boiling the water have on the final value for ΔHc?
As the water boils, energy is used up to convert the water from a liquid to a gas which escapes the system. This means that the temperature rise would be smaller for the amount of alcohol that was used up. Becuse Q = mcΔT, value obtained for Q would be smaller if ΔT was smaller and because ΔHc = Q / No. of moles of alcohol, ΔHc would be smaller.
What effect would accidently using an alcohol with greater Mr for calculations have on the final value for ΔHc?
Because No. moles of alcohol = mass of alcohol / Mr of alcohol; the greater the Mr, the smaller the No. of moles of alcohol. Because ΔHc = Q / No. of moles of alcohol, the smaller the No. of moles of alcohol, the greater the ΔHc.
