Thermodynamics Flashcards
What is lattice enthalpy?
A measure of the strength of the forces between the ions in an ionic lattice
What attractions form between positive and negative ions?
Electrostatic attractions
How are electrostatic attractions and lattice enthalpy linked?
The stronger the electrostatic attractions the greater the lattice enthalpy
What is lattice dissociation?
- Bond breaking - +ve- Endothermic
What is lattice association?
- Bond Formation- -ve- Exothermic
What factors affect lattice enthalpy?
1 - Charge on the ions2 - Ionic radius (sheilding)
How does ionic charge and ionic radius affect lattice enthlapy?
The greater the charge and the smaller the radius the stronger the electrostatic attractions therefore the greater the lattice enthalpy (high charge density)
What is a Born-Haber cycle?
A type of Hess’s law cycle that enables us to calculate lattice enthlapy
What is the enthalpy change of formation?
Formation of 1 mole of a substance from its constituent elements in standard states
What is the enthalpy change of atomisation?
The standard atomisation enthalpy is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state. Enthalpy change of atomisation is always positive.
What is the enthalpy change of ionisation energy?
(Metals) The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions
What is the enthalpy change of electron affinity?
(Non-Metals) The energy required to add an electron to one mole of gaseous atoms
What is the comparison of the born-haber and the perfect ionic model?
A comparison of experimental and theoretical calculated lattice enthalpy values
How are the two models different?
Born haber uses experimental values to calculate lattice enthalpy and takes into account covalent character whereas the perfect ionic model assumes tehre is no covalent character, perfect ionic bonding and perfectly spherical ion.
What is covalent character?
The movement of electrons causing the partial sharing of electrons in an ionic bond, it is increased when there is greater difference in charge and size of the ions
When is there the biggest difference in experimental and theoretical values?
When covalent character is greater
What is enthalpy of solution?
The enthalpy change when 1 mole of an ionic substance dissolves in water to give a solution of infinite dilution, e.g. a solid salt broken down into its ions in an aqueous state
How is enthalpy of solution calculated?
enthalpy of lattice dissociation + the sum of all enthalpies of hydration
What is enthalpy of lattice dissociation always?
Endothermic because bonds are being broken so delta H is positive
What is the enthalpy of hydration?
The enthalpy change when 1 mole of gaseous ions forms aqueous ions
What is enthalpy of hydration always?
Exothermic because it is the formation of bonds therefore delta H is negative
How can you compare two elthalpies of solution?
The most negative enthalpy is most likely to be soluble because it releases energy when dissolved
What is entropy?
a measure of disorder in a system measured in J.K^-1.Mol^-1
When do you get an increase in entropy?
- Solid -> Liquid (melting)- Liquid -> Gas (boiling)- Dissolving- When theres an increase in number of moles
How is entropy of a system calculated?
The sum of the entropy of the products - the sum of the entropy of the reactants
Which reactions are more feasible?
Reactions with a positive entropy
What is Gibbs free energy?
A measure of if a reaction is feasible
What must delta G be, in order for a reaction to be spontaneous?
Below 0
What is the delta G equation?
delta G = delta H - (T x delta S)
What are the units of delta G?
J.Mol^-1
What are the units of delta S?
J.K^-1.Mol^-1
What are the units of delta H?
J.Mol^-1
What are the units of T?
K
How do you calculate the temperature of feasibility?
Assume delta G is 0
Why might a reaction not occur even if it is feasible?
A reaction may not occur if there is a high activation energy
What does delta G vary with?
Temperature
How is the Gibbs free energy equation linked to the line equation?
y = delta Gmx = - delta S x Tc = delta H
What happens when delta S is positive?
- Gradient is negative- Increase in entropy
What happens when delta S is negative?
- Gradient is positive- Decrease in entropy
In the Gibbs free energy graph, how is - delta s calculated?
From the graph, calculate the gradient
In the Gibbs free energy graph, what does the x intercept represent?
The temperature at which the reaction becomes feasible.
In the Gibbs free energy graph, how is gradient, T and dekta G linked?
The greater the gradient, the greater effect T has on delta G
In the Gibbs free energy graph, what does the y intercept represent?
The delta H for the reaction
