thermodynamics content Flashcards
What is the equation linking lattice enthalpy of formation, enthalpy of solution and hydration enthalpies?
Enthalpy of hydration= enthalpy of formation + enthalpy of solution
What does the lattice enthalpy of a compound indicate?
The strength of the bonding- the greater the lattice enthalpy, the stronger the bonding
How does the size and charge of the ion affect the lattice enthalpy?
Generally speaking, smaller ions and/or ions with higher charges have stronger attractions and so greater lattice enthalpy
How do we find lattice enthalpy?
Using a Born-Haber cycle
Why does theoretical value of lattice enthalpy vary from the experimental value (i.e from Born Haber)?
Theoretical assumes charge is perfectly spherical and no covalent character
The bigger the difference between experimental and theoretical, the more covalent character
What is covalent character?
When there is a lot of distortion of the ions in an ionic compound so the ions are not perfectly spherical
What type of ions cause distortion and what type of ions are easily distorted?
Positive ions that are small and/or highly charged are good at distorting negative ions, particularly ones that are large and/or highly charged
What is entropy?
Entropy (S) is disorder. The more disordered or random something is, the greater the entropy
What are the units of entropy?
J mol^-1 K^-1
What states have the most entropy and the least?
Gases have the most whereas solids have the least
What is the 2nd law of thermodynamics?
Over time entropy will naturally increase
What is the 3rd law of thermodynamics?
The entropy of a substance is zero at absolute zero and increases with temperature
How does the entropy change of a substance vary with temperature?
Higher the temperature, the faster particles vibrate and so greater the entropy. Big increases in entropy on state changes (melting/boiling), and entropy change from liquid to gas is greater than entropy change from solid to liquid due to large amount of disorder in gas compared to liquids/solids
Why do structures like diamond and graphite have very low entropies?
Very highly structured
How do we calculate the entropy change ∆S for a reaction?
∆S = sum of S products - sum of S reactants
When is ∆S positive/negative?
In reactions where there is an increase in entropy, ∆S is positive
In reactions where there is a decrease in entropy, ∆S is negative
What is the equation for Gibbs free energy change ∆G?
∆G = ∆H - T∆S
When is a reaction feasible?
When ∆G is less than or equal to zero
What does feasible mean?
The reaction can take place
When does a substance melt/boil?
When ∆G=0
