Thermodynamics Flashcards
What is enthalpy change?
The heat energy change at a constant pressure
What does Hess’s law state?
The enthalpy change of a chemical reacyion is independent to the route taken
What is the enthalpy of lattice dissociation?
The enthalpy change to separate one mole of a solid ionic compound into its gaseuous ions
What is the symbol of enthalpy lattic of dissociation?
ΔH ⦵ LD
Is the enthalpy of lattice dissociation positive or negative? Why?
It is positive, as bonds are being broken
What is enthalpy lattice pf formation?
The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
What is the symbol for enthalpy lattice of formation?
ΔH ⦵ LF
Is the enthalpy lattice of formation positive or negative?
Negative, as bonds are formed so energy is required
What is the relationship between enthalpy lattic of dissociatipn and formation?
They will be equal in magnitude buthave the opposite sign (if one is negative the other is positive)
What is the standard enthalpy lf formation? And its symbol
The enthalpy change when one mole of a compound is formed from its constituent elements, when all reactants and products are in the gaseous state
ΔH ⦵ f
What is the standard enthalpy of atomisation and its symbol?
The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state
ΔH ⦵ at
What is bond enthalpy?
The heat energy required to break one mole lf a given covalent bond in a molecule in the gaseous state
What is ionisatipn energy?
The energy rewuired to remove one mole of electrons from one mole of atoms on teh gaseous state.
What is the symbol for the first and second ionisation energy?
ΔH ⦵ IE1/2
What is the second ionisation energy?
The enthalpy change when one mole of 2+ gaseous ions are formed from one mole lf gaseous 1+ ions
What is the first electron affinity and its symbol?
ΔH ⦵ EA1
The enthalpy change when one mole of geseous 1- ions are formed from one mole of gaseous atoms
What is the second electron affinity and its symbol?
ΔH ⦵ EA2
The enthalpy change when one mole of 2- ions are formed from one mole of 1- ions in the gasupus state
What are born harber cycle diagrams?
Diagrams that use hess’s law to calculate the enthalpy of lattice dissociation and formatiom
Using born harber cycle diagrams, what is teh equation for the enthalpy of lattice formation?
ΔH ⦵ formation - ( sum of all other enthalpy changes)
Using the born harber cycle diagrams, what is the equation for enthalpy of lattice dissociation?
- ΔH ⦵ LF
Or
(Sum of all other enthalpy changes) - ΔH ⦵ formation
What is the perfect ionic model?
A model where ions are perfect spheres with only electrostatic forces of attraction
Why might a value from the born harber cycles not be accurate?
Born harber cycles assume that the ionic lattice has the ‘perfect ionic model’ when in reality they sometimes have some covalent characters
How do theoretical and experimental (from the born harber cycle) values indicate how perfect an ionic model is?
A perfect ionic model will have very similar experimental and theoritical values, so will have pure ionoc bonding
What are two characteristics of perfect ionic models?
Both ions are spherical
The charge within the model is evenly distributed
How do ionic models show partial covalent bonding?
a positive ion attracts electrons in the negative ion, because it is small and highly charged.
Therefore the ions are no longer spherical and the charge is not evenly distributed
What is polarisatipn in ionic compounds?
When the ions in a compound are distorted (no longer spherical) because the positive ipn attracts the electrons in the negative ion
What affects the polarising power of ions in an ionic lattice?
Positive ions that are higher in charge and smaller are best at polarising negative ions
Negative ions that are higher in charge and larger are the easiest to polarise
What characteristics do ionoc compounds that have covalent characteristics show?
They have low solubility
Melting points and electrical conductivity are lower than expected
What is the enthalpy of solution and its equation?
The enthalpy change when one mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution
ΔH ⦵ sol
What is the enthalpy of hydration and its symbol?
The enthalpy change when one mole of aqueous ions are formed from one mol of geseous ions
What factors affect the lattice enthalpy lf formation and how?
Size of the ion- if size increases, attraction between oppositely charged ions decreases so the enthalpy is less exothermic
Charge- if more charged, attraction between opposite ions increases so the enthalpy is more exothermic
What factors affect affect the hydration enthalpy and how?
Size lf ion- if the size of the ion increases, the attraction between the delta positove hydrogen in water and the negative ion decreases
So enthalpy is less exothermic
Charge lf ion- if charge increases, the attraction between water and ion increases so the enthalpy is more exothermic
What is entropy? And what is its symbol?
S- a measure of the disorder in a system
Is entropy positive or negative? Why?
There is always a degree of disorder because particles always have energy, therefore the values for S are always positive
What are the units for entropy?
J mol-1 k-1
Joules/ mol/ kelvin
What four factors determine the entropy of a substance?
State of matter, dissolving, mixtures, number of particles
How does state of matter affect entropy of a substance?
Solids are ordered, so have a low entropy
Gases are very disordered so have a high entropy
What is the difference of entropy between a solution and a solid and solvent?
The solution is more disordered, so the entropy is higher
How does the number of particles affect entropy?
As the number of particles increases, disorder increases so entropy increases
What is the equation for entropy change of a reaction?
Entropy change = total entropy of products- total entropy of reactants
What does a positive entrophy change indicate?
An increase in disorder
What does a negative entrophy change indicate?
A decrease in disorder
Is a negative or positive entropy change more favourable in a reaction?
A positive entropy change is more favourable
What are units for enthalpy change?
KJmol-1
Why is a negative enthalpy change favourable?
Because heat is given out, which causes the entropy of the surroundings to increase. This is favourable
What is the equation for the enthalpy change of a reaction?
Enthalpy of formation of products - enthalpy of formation of reactants
What is Gibbs free energy and feasibility?
States whether a reaction is feasable or not at any given temperature
What is the equation for gibbs free energy change? What are its units?
∆G = ∆H of surroundings - T ∆S of system
Using Gibbs free energy, when will a reactipn be feasable?
When ∆G is zero or negative
Why can an endothermic reaction take place spontaneously at room temperature?
A spontaneous reaction will only occur if ∆G is 0 or below 0
If ∆H of surroundings is positive (and so unvarourable) yet the T ∆S of the reaction is also positive and greater than ∆H, a negative ∆G will still form
How do you calculate the temperature at which a reaction becomes feasable?
A reaction becomes feasable when ∆G is zero or below.
Take ∆G as 0:
T= ∆H/ ∆S
What temperature is required when ∆H is favourable yet ∆S is unfavourable? Why
Low temp
H is negative and S is negative
For G to be negative, - TS must be smaller in magnitude than H
What temperature is required when ∆H is unfavourable yet ∆S is favourable? Why
High temperature
S is positive and H is positive
For G to be negative, TS must be larger than H
Why might a reaction not occur,despite it being favourable?
Because its activation energy is too high and the rate of reaction is too slos
