Topic 8 and 13: Energetics I and II Flashcards
Standard enthalpy of a reaction
The enthalpy change of a reaction when completed under standard conditions of 100kPa and 298K
Enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its constituent elements in standard state under standard conditions
Enthalpy change of combustion
The enthalpy change when one mole of a substance burns completely with excess oxygen under standard conditions.
Enthalpy change of neutralisation
The enthalpy change to form one mole of water when an acid and alkali reacts under standard conditions
Problems with the calorimetry experiment
Heat loss to the surroundings from lack of insulation
Not complete combustion –> less exothermic for incomplete combustion
Evaporation of fuel after weighing
Experiment not carried out under standard conditions
What is Hess’ law?
Enthalpy change for any reaction is independent of the route at which the reaction takes place, provided that the initial and final conditions for products and reactants are the same.
How to quickly calculate standard enthalpy of combustion ?
ΔHf of products - ΔHf of reactants
Why should Hess Cycle be used instead of experiments?
- Cannot measure temperature change of solid substances (also for decomposition)
- Unable to know how much water to add exactly
Definition of Bond enthalpy
The energy needed to break one mole of covalent bonds in gaseous molecules under standard conditions for a certain reaction
Definition of mean (average) bond enthalpy
The energy needed to break one mole of covalent bonds in gaseous molecules under standard conditions, averaged from bond enthalpies in range of similar molecules
Limitations of calculating with mean bond enthalpy
- For gaseous atoms, so enthalpy of vaporisation not included
- Not specific to the molecules and reaction involved
Definition of lattice energy
The energy change when one mole of ionic solid is formed from its gaseous ions
Always endothermic
Measure of ionic bond strength
Definition of enthalpy change of atomisation
The energy change to form one mole of gaseous atoms from its element in its standard state under standard conditions
Definition of enthalpy of electron affinity
The enthalpy change to add one mole of electron to one mole of gaseous atoms to form one mole of gaseous 1- ions
Definition of enthalpy change of solution
The enthalpy change when one mole of solid is dissolved in sufficient solvent to give an infinitely dilute solution
Definition of enthalpy change of hydration
The enthalpy change when one mole of gaseous ions dissolve in water to form one mole of aqueous ions.
Why do lattice energy differ from experiment and theoretical?
Theoretical assumes 100% Ionic character which may not be true for certain compounds
If it differs, it means the compound displays some covalent characteristics
Effect of ionic charge and ionic radius on lattice energy
Higher charge density = more endothermic
Effect of ionic charge and ionic radius on enthalpy change of hydration
Higher charge density = more exothermic
How to calculate enthalpy of solution
-Lattice energy + hydration of cation + hydration of anion
Definition of Entropy
The measure of disorder or randomness
How to consider standard entropy?
The state of a substance (g>l>s)
Number of atoms in the substance (more atoms = more entropy)
How to predict entropy in a reaction
More disorder = Positive change of entropy system
Less disorder = negative change of entropy system
What is the relationship between temperature and entropy
Entropy increases as temperature increases, larger increase between changes of state
How to find entropy of systems
ΔSsystems = ΔSproducts - ΔSreactants
How to find total entropy
ΔStotal = ΔSsystem + ΔSsurroundings
How to find Entropy of surroundings
ΔSsurroundings = -ΔH/T
When is a reaction thermodynamically feasible?
When ΔStotal is positive or ΔG is negative
What is Gibb’s Free energy equation?
ΔG = ΔH - TΔSsystems
ΔG = -RT lnK
Thermodynamic stability
A mixture/chemical is thermodynamically stable if there is no tendency for the reaction to occur
Kinetic stability
Feasible reactions may not occur as the mixture is kinetically inert. This means the Ea is too high that the reaction is too slow
