Enthalpy. Flashcards
Enthalpy change equation.
H(products)-H(reactants)
Exothermic vs endothermic.
Exothermic change=heat leaves the system delta H is negative
Endothermic change=heat enters the system delta H is positive
Revise enthalpy profile diagrams.
Activation energy definition.
Minimum energy required for a reaction to take place.
Revise enthalpy profile diagrams.
Standard conditions and states.
100kPa
298K/25C
1moldm^-3
Standard state=physical state of a substance under standard conditions.
Standard enthalpy change of reaction definition.
Enthalpy change that accompanies a reaction in molar quantities shown in the chemical equation under standard conditions w/ everything in standard states.
Enthalpy change of formation definition.
Enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
Elements have no change in enthalpy.
Enthalpy change of combustion definition.
Enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions.
Enthalpy change of neutralisation definition.
The energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O under standard conditions.
-57kjmol^-1.
H+ + OH- –>H2O.
Calculating energy change.
q=mcdeltaT
m=mass
c=specific heat capacity
Delta T=temp change of surroundings
Reasons for calculations inaccuracy.
Heat loss to other surroundings
Incomplete reactions/combustion
Evaporation
Non-standard conditions.
Average bond enthalpy definition.
Energy required to break one mole of bonds in gaseous molecules
Energy always required to break bonds
Always endothermic
Positive enthalpy values.
Bond breaking.
Energy required
Endothermic
Delta H is positive.
Bond making.
Energy released
Exothermic
Delta H is negative.
Enthalpy changes from average bond enthalpies.
Sum of bond enthalpies in reactants - sum of bond enthalpies in products
Hess’ law.
Allows enthalpy changes to be calculated indirectly.
If a reaction can take place by two routes, and starting and finishing conditions are the same, the total enthalpy change is the same for each route.
