Thermodynamics, Rate Equations Flashcards
What are the orders for concentration’s influence on rate of reaction?
if no effect —>
Zero order A^0
if rate doubles when concentration doubles (direct proportion) —>
First Order A^1
if rate quadruples when concentration doubles —>
Second Order A^2
The rate constant…
- Different for every reaction
- Varies with temperature
- units are different for every reaction
What is Enthalpy of Lattice Formation?
Enthalpy change when 1 mole of ionic lattice/solid ionic compound is formed from its gaseous ions.
Why may the calculation of enthalpy for lattice formation based on the perfect ionic model give a smaller numerical value than the calculated value? e.g in the case of AgI
The Covalent character of the molecule
They may not be completely ionic.
the forces/bonds holding the matrix together are stronger.
Enthalpy change of formation
The enthalpu change when 1 mole of a compound is formed from it’s elements in their standard states under standard conditions.
(2Cs + 2H2aq –> C2H6aq)
Lattice enthalpy of formation
The enthalpy change when 1 mole of a solid ionic compound is formed from it’s gaseous ions under standard conditions.
Ca2+g + 2Cl-g –> CaCl2s
- exothermic
Lattice enthalpy of dissociation
The enthalpy change when 1 MOLE of solid ionic compound is dissociated into it’s gaseous ions under standard conditions.
CaCl2s –> Ca2+ + 2Cl-g
Enthalpy change of dissociation
The enthalpy when 1 mole of bonds of the same type of molecule in the gaseous state is broken
F2g –> 2Fg
Enthalpy change of 1st ionisation energy
The enthalpy change of when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms
- endothermic
Enthalpy change of atomisation
The enthalpy change when 1 mole of gaseous atoms are made from an element in it’s standard state.
1/2F2(g) –> F(g)
- endothermic
Enthalpy change of 2nd ionisation
The enthalpy change when 1 mole of gaseous 2+ ions are made from 1 mole of gaseous 1+ ions.
Ca+(g) –> Ca2+
- endothermic
1st electron affinity
The enthalpy change when 1 mole of gaseous atoms form 1 mole of gaseous 1- ions.
e.g. O(g) –> O-(g)
- exothermic
Why do we use Born Haber cycles?
Born-Haber cycles are useful to calculate lattice enthalpies. This is because we can’t calculate directly from experiments.
2nd electron affinity
- The enthalpy change when 1 mole of gaseous 1- ions form 1 mole of gaseous 2- ions.
e.g. O-(g) –> O2-(g)
-endothermic
Why may the theoretical Lattice enthalpies and experiment Lattice enthalpies differ?
- The covalent character of the ionic compound ( the amount at which the positive ions polarise the negative ions)
- can result in greater experimental values for lattice enthalpy than theoretical ( when there is high covalent character)
- The larger the ion the greater the distortion
Theoretical Lattice Enthalpies and Experimental Lattice Enthalpies
- Theoretical lattice enthalpies can be calculated from data assuming a perfectly ionic model.
- during experimental testing you will find that you will not get the exact value you get from the theoretical lattice enthalpies
- This means that the compound doesn’t follow the perfectly ionic model and has some covalent characteristics.
- Most of the time the positive ion distorts the charge distribution in the negative ion. ( The positive ion polarises the negative ion + the more polarisation the greater the covalent character)
Enthalpy change of solution
The enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure NO FURTHER enthalpy change is observed upon further dilution.
For a Substance to dissolve… (enthalpy change of solution)
- substance bonds must break (endothermic)
- New bonds formed between the solvent and substance (exothermic)
Why may an ionic compound not dissolve?
For an ionic compound to dissolve the new bonds formed during the hydration of the freely moving ions must be equivalent or greater in strength than the ionic bonds broken.
- If not then substance is unlikely to dissolve
How do you calculate the enthalpy change of solution
- using lattice dissociation enthalpy
- using enthalpy of hydration
(use the hess’ law structure)
The sum of the two is equal to the enthalpy of solution.
Enthalpy of Hydration
- Enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions. / Enthalpy change when 1 mole of gaseous ions form 1 mole of aqueous ions.
(exothermic)
Define Entropy
Entropy is the measure of disorder in a system.
The more disorder there is the higher the level of entropy
things to remember when calculating gibbs free energy
- when calculating D S make sure it’s S = products - reactants and that you include balancing numbers in calculation
- making sure you convert your entropy value from JK-1mol-1 to KJK-1mol-1 ( or you can convert your enthalpy to KJmol-1 but former is easier)
- making sure your temperature is in KELVIN ( it’s not temp change so it matters that it’s kelvin)
Why may endothermic reactions still be spontaneous?
- a calculated positive entropy can tell you a reaction is feasible
- increasing entropy is energetically favourable and some reactions that are enthalpically unfavourable (endothermic) can still spontaneously (feasibly) react if changes in entropy overcome changes in enthalpy