Thermodynamics Flashcards
What is the standard enthalpy of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions, with all reactants and products being in their standard states
What is the first ionisation enthalpy?
Standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions, each with a single positive charge
What is the standard enthalpy of atomisation?
Enthalpy change when one mole of gaseous atoms are formed from an element in its standard state
What is mean bond enthalpy?
Enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals averaged over a range of compounds
What is the first electron affinity?
Standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single negative charge, under standard conditions
What is lattice formation enthalpy?
Standard enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
What is lattice dissociation?
Standard enthalpy change when one mole of solid ionic compound dissociated into its gaseous ions
What is the standard enthalpy of hydration?
The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions
What is the standard enthalpy of solution?
The standard enthalpy change when one mole of solute dissolves to form its aqueous ions
What two key factors determine how exothermic a lattice enthalpy is?
Charge on the ions
Size of the ion (ionic radius)
How to increase the lattice enthalpy of a compound? (2)
The greater the charge on the ion, the greater its attraction to an oppositely charged ion
The smaller the ion the greater the attraction to an oppositely charged ion
What does a more negative lattice enthalpy of formation show about the ionic bonds?
What does a more positive lattice enthalpy of dissociation show? (1)
Stronger ionic bonds
What is covalent character in ionic bonding? (1)
Some of the electron density is shared
What characteristics must a compound have to show covalent character? (2)
A positive ion which is small and highly charge (i.e. is very polarising)
A negative ion which is large and highly charged (i.e. is very polarisable)
What is the perfect ionic model?
Compounds which display purely ionic bonding with no covalent character and ions are point charges
Explain the meaning of the term ‘perfect ionic model’ (2)
Ions have point charges
Only electrostatic attractions between oppositely charged ions
What is the Born Haber experimental model (2)
Takes into account covalent character and ions are polarisable
If an ionic compound has no covalent character what does this mean about the lattice enthalpy of formation (1)
Lattice enthalpy of formation calculated by perfect ionic model is equal to the lattice enthalpy of formation calculated by experimental model
If an ionic compound has covalent character what does this mean about the lattice enthalpy of formation and why (2)
Lattice enthalpy of formation calculated by perfect ionic model is less exothermic than the lattice enthalpy of formation calculated by experimental model
Experimental Born Haber allows for covalent character and predicts stronger bonding
What 3 factors determine if energy is feasible? (3)
Temperature
Enthalpy
Entropy
What is entropy (1)
A measure of the amount of ‘disorder’ in a system
How to calculate entropy change (1)
sum of entropy of product - sum of entropy of reactants
What equation demonstrates the relationship between temperature, enthalpy and entropy? (1)
Gibbs Free Energy = enthalpy change - (temperature x entropy change)
When is a reaction feasible? (1)
When Gibbs free energy is equal to or less than zero
How can you rearrange the Gibbs free equation to make temperature the subject of the formula? (1)
temperature = enthalpy change / entropy change
What happens to Gibbs free energy when the enthalpy change is negative and entropy change is positive and what happens if temperature is increased? (2)
Gibbs free energy is always negative at any temperature
If temperature is increased Gibbs free energy becomes more negative as T∆S gets bigger
What happens to Gibbs free energy when the enthalpy change is positive and entropy change is negative and what happens if temperature is increased? (2)
Gibbs free energy is always positive at any temperature
If temperature is increased Gibbs free energy becomes more positive as T∆S gets bigger
What happens to Gibbs free energy when the enthalpy change is negative and entropy change is negative and what happens if temperature is increased? (3)
∆G is becoming more positive because T∆S gets bigger
∆G will be negative when ∆H>T∆S
Gibbs free energy is negative at low temperatures
What happens to Gibbs free energy when the enthalpy change is positive and entropy change is positive and what happens if temperature is increased?
∆G is becoming more negative because T∆S gets bigger
∆G will be negative when ∆H<T∆S
If temperature is increased Gibbs free energy becomes more negative
What is second ionisation energy?
Enthalpy change when one mole of electrons is removed from one mole of gaseous +1 ions to form one mole of gaseous +2 ions
What is second electron affinity?
Enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions
Compare the general entropy values for solids, liquids and gases
solids < liquids < gases
What are the limitations of using G as an indicator of whether a reaction will occur (2)
Gibbs free energy only indicates if a reaction is feasible
It does not take into account the rate of reaction
Why is entropy zero at 0 kelvin (2)
No disorder - molecules have no kinetic energy so are not moving/vibrating and cannot be arranged in any other way
How is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant (1)
The heat that is given out escapes to the surroundings
