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Enthalpy

Edexcel AS-level ChemistryTopic 18 of 2121 lessons

Revision notes for Enthalpy in Edexcel Chemistry (8CH0). Every lesson in BrainCake comes with R.E.C.I.P.E. recall steps, a quick check and spaced reviews.

  1. Chemical and Heat EnergyThe first law of thermodynamics states that energy cannot be created or destroyed, only transferred.
  2. Enthalpy and Enthalpy ChangeEnthalpy is a measure of the total energy of a chemical system.
  3. Exothermic and Endothermic ReactionsIn an exothermic reaction heat energy is transferred from the system to the surroundings.
  4. Standard ConditionsStandard conditions are a pressure of 100kPa and a temperature of 298K Enthalpy changes measured under standard conditions are called standard…
  5. Standard Enthalpy Change of CombustionThe enthalpy change at 100 kPa and 298K when one mole of a substance is completely burned in oxygen, with all reactants and products in their…
  6. Experimental Determination of Enthalpy Change of CombustionA known volume e.g.
  7. Evaluating Sources of ErrorThe data book value for methanol is -726 kJmol-1.
  8. Standard Enthalpy Change of NeutralisationThe standard enthalpy change of neutralisation ΔneutHθ is the enthalpy change that accompanies the reaction of an acid with a base to form one mole…
  9. Experimental Determination of Standard Enthalpy Change of NeutralisationWhen carrying out the enthalpy change of a reaction between two aqueous solutions, the immediate surrounding is the solution itself, so a…
  10. Standard Enthalpy Change of FormationThe standard enthalpy change of formation ΔfHθ is the enthalpy change that takes place when one mole of a compound is formed from its elements under…
  11. Enthalpy Change of a ReactionThe standard enthalpy change of reaction ΔrHθ is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical…
  12. Hess's LawIf a reaction can take place by two routes the total enthalpy change for each route is the same provided that the starting and finishing conditions…
  13. Hess's Law Example 2Worked example: enthalpy change of formation from combustion data Methane cannot be made directly from carbon and hydrogen in a calorimeter, so its…
  14. Hess's Law Example 3Worked example: enthalpy change of reaction from formation data When enthalpy changes of formation are given, the Hess cycle is built with the…
  15. Hess's Law Example 4Worked example: a reaction that cannot be measured directly Calcium carbonate only decomposes when heated strongly, so its enthalpy change cannot be…
  16. Hess's Law Example 5Worked example: hydrogenation of ethene from combustion data Calculate the enthalpy change for: C2H4(g) + H2(g) → C2H6(g) Data: ΔcHθ[C2H4(g)] =…
  17. Hess's Law Example 6Worked example: formation of ethanol from combustion data Calculate ΔfHθ for ethanol: 2C(s) + 3H2(g) + ½O2(g) → C2H5OH(l) Data: ΔcHθ[C(s)] = -393.5…
  18. Hess's Law Example 7Worked example: using two measured reactions The enthalpy change for hydrating anhydrous copper(II) sulfate cannot be measured directly, because it…
  19. Hess's Law Example 8Worked example: combining equations Sometimes you are given whole equations with their enthalpy changes instead of a table of data.
  20. Hess's Law Example 9Worked example: formation data with elements and multiples The thermite reaction produces molten iron and is used to weld railway lines: 2Al(s) +…
  21. Hess's Law Example 10Worked example: finding an unknown enthalpy change of formation Hess's law can be used backwards.

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