Thermodynamics pt 2. Flashcards
Why can enthalpy of solution not be measured directly?
Difficult to measure temp rise of solid
Is enthalpy of solution a chemical or physical change?
Physical - no new substance made
Why can dissolved ions no longer attract eachother?
Water gets in between them, so they can no longer attract eachother
What is the enthalpy change of solution?
The enthalpy change when 1 mole of ionic solid is dissolved completely in water to form an infinitely dilute solution under standard conditions
Why is the enthalpy of solution of some metal oxides not found in data books?
Metal oxides react with water to form insoluble metal hydroxides
Why is the bond enthalpy of NaCl not found in data books?
NaCl is ionic. Bond enthalpies are for covalent compounds
What is the enthalpy change of hydration?
The enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions
How does water surround positive cations?
Water is polar.
Ion-dipole attraction between positive ion and delta- on oxygen in H2O molecules.
What factors affect ion-dipole attraction?
Ionic radius
Ionic charge
What type (exothermic or endothermic) of reaction would be expected from an ion-dipole attraction?
Exothermic
Energy released from ion-dipole attraction
How does ionic charge affect enthalpy change of hydration?
More exothermic, so stronger ion-dipole attraction, so more exothermic
How does ionic radius affect enthalpy change of hydration?
Smaller ionic radius, stronger the ion-dipole attraction, so more exothermic
Why can enthalpy of solution not be measured directly?
Difficult to measure temp of a solid
What is the formula for enthalpy change of solution?
ΔHsoln = ΔHLE + ΣΔHhyd (dissociation data given)
ΔHsoln = ΣΔHhyd - ΔHLE (formation data given)
What must be the condition to for a substance to dissolve in water?
Enthalpy change of hydration must be greater than enthalpy change of lattice dissociation ( can also dissolve by heating)
What is a spontaneous reaction?
One which proceeds on its own without any external influence
Why are most reactions that occur freely exothermic?
Products are more thermodynamically stable than reactants, so spontaneous (feasible)
Why do some spontaneous reactions not occur (even when ΔG is less than or equal to 0)?
Ea of reaction is too high or rate of reaction is too slow
What is entropy?
The degree of disorder in a system
What are the factors affecting entropy?
Dissolving - increases entropy, as particles can move more freely.
Number of moles - more particles means more entropy,
Physical state - most entropy in gasses, disorder increases as gasses have more way of arranging themselves
Ar - more electros hence more energy to distribute
When does a substance have 0 entropy?
0 K
Describe the movement of particles when the entropy of a substance is 0
Stationary
Describe the arrangement of particles when a substance has an entropy of 0
State of perfect order
What is the general relationship between temperature and entropy (from graph)?
S increases with temp
Line slopes upwards as temp increases as particles start to move
Why does boiling lead to a larger entropy change than melting?
Bigger change in disorder
Why is entropy 0 at 0 K?
Particles are stationary and in a state of perfect order.
What is the formula for entropy change?
SPAR
ΔS = Σ(Sproducts) - Σ(Sreactants)
Positive change - increase in disorder
Negative change - decrease in disorder
What is the equation to calculate Gibbs free energy change?
ΔG = ΔH - TΔS
What does the value of ΔG tell us about a reaction?
If negative or equal to 0, the reaction is feasible, if positive, the reaction is not feasible
How do you find the minimum temperature at which a reaction is feasible?
Set ΔG to 0
What does a positive/negative gradient of a Gibbs graph tell us?
Positive - negative ΔS
Negative - positive ΔS
