1.8 Thermodynamics
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What is the definition of Enthalpy Change?
Change in heat energy at constant pressure
What are standard conditions?
298 K and 100kPA
What is Hess’ law?
That enthalpy change is independent to route taken
What is the definition of mean bond dissociation?
Enthalpy change To break bonds in a gaseous molecule Averaged over a range of compounds
Why are given values of Mean bond dissociation not the same as experimental values?
It’s averaged over a range of compounds and not specific to the bond
What is the definition of enthalpy of atomisation?
Enthalpy change To form 1 mole of gaseous atom From its element in its standard state
What is the definition of electron affinity?
Enthalpy change To form 1 mole of gaseous -1 ions From 1 mole of gaseous atoms
What is the definition of ionisation energy?
The enthalpy change To form 1 mole of gaseous +1 ions from 1 mole of gaseous atoms
What is the definition of enthalpy of formation?
The enthalpy change To form 1 mole of a substance With all substances in their standard states
What is the definition of enthalpy of combustion?
The enthalpy change To combust 1 mole of a substance in excess O2 With all substances in their standard states
What is the definition of enthalpy of hydration?
The enthalpy change To form 1 mole of aqueous ions From 1 mole of gaseous ions
What is the definition for enthalpy of lattice formation?
The enthalpy change To form 1 mole of solid ionic compound From its gaseous ions
What are the main problems with the energetics required practical?
Heat loss Incomplete combustion when using calorimeter
What is the relationship between bond dissociation and enthalpy of atomisation?
Atomisation is half bond dissociation
What are the two ways of finding lattice enthalpy?
Born Haber cycles Perfect ionic model
What does the perfect ionic model assume?
Pure ionic bonding Ions are point charges Purely electrostatic
What do close theoretical and experimental lattice enthalpy values state?
Pure ionic bond Ions are point charges No covalent character
What does greatly different Theoretical and experimental lattice enthalpy values suggest?
Positive ion has a high charge density Polarising Covalent character
Why is the hydration of ions exothermic?
Because bonds are being formed
What are two points always mentioned when talking about ionic bond strengths?
Size and charge
Why is theoretical value of LE lower than experimental?
Most ionic compounds have some covalent character Then explain why the compound isn’t a perfect ionic model e.g. polarising,Hugh charge density etc.
Why is the first electron affinity exothermic but the second endothermic?
1st electron affinity forms a more stable ion so there is an attraction between the nucleus and the electron 2nd electron affinity is endothermic because more energy is required to overcome the repulsive force of the electron and the negative ion .
What is the gibbs free energy equation?
Delta G = Delta H - TdeltaS
What is entropy?
The amount of disorder in a reaction e.g. gas is high disorder and solid is low disorder Can also refer to the change in moles?