Thermodynamics - Born Haber Cycles Flashcards
What is the symbol for enthalpy change?
Delta H
What are standard conditions?
298K and 100kPa and all solutions at a concentration of 1moldm-3 and all substances present in their standard states
What is enthalpy change?
The heat energy change at constant pressure
Define enthalpy of lattice dissociation
The enthalpy change when one mole of an ionic compound is separated into its component gaseous ions
Is lattice dissociation endo or exothermic and why?
Endothermic because breaking bonds requires energy
Define enthalpy of lattice formation
The enthalpy when one mole of an ionic compound is formed from its constituent ions in the gaseous state
Is lattice formation exo or endothermic?
Exothermic bc making bonds releases energy
Define enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements when all the reactants and products are in their standard states under standard conditions
Define enthalpy of atomisation of an element
The enthalpy for the formation of one mole of gaseous atoms from the element in its standard state
Define bond dissociation enthalpy
The enthalpy change when one mole of a covalent bond is broken under standard conditions in the gaseous state
Is bond dissociation enthalpy endo or exothermic?
Endothermic
Define first ionisation enthalpy
The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to produce one mole of gaseous ions with a single positive charge
Define second ionisation enthalpy
The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a single positive charge to form one mole of gaseous ions with a 2+ charge
Define third ionisation enthalpy
The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a 2+ charge to form one mole of gaseous ions with a 3+ charge
Define first electron affinity
The enthalpy change when one mole of gaseous atoms forms one mole of negative ions with a single negative charge
Define second electron affinity
The enthalpy change when one mole of gaseous ions with a single negative charge forms one mole of gaseous ions with a double negative charge
What two assumptions are made by the perfect ionic model?
Bonding in a compound is 100% ionic
The ions are perfect spheres and are not distorted
What does a difference in experimental lattice enthalpy and calculated lattice enthalpy show?
The compound has some covalent character
How do you calculate the enthalpy of solution?
Enthalpy of lattice dissociation + enthalpy of hydration
Define enthalpy of solution
The enthalpy change when one mole of solute dissolves in water
Define enthalpy of hydration
The enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions
Are enthalpy of hydration values exo or endothermic and why?
Exothermic bc energy is released when ions are attracted to the polar water molecules
What does the value of the lattice enthalpy show?
A measure of the strength of the ionic bond
What does the strength of the ionic bond depend on?
Charge of the ion and their size
What is mean bond enthalpy?
The enthalpy needed to break one mole of a covalent bond averaged across many compounds containing that bond
Why is the bond enthalpy for diatomic molecules not averaged?
Because the bond only occurs in the diatomic molecules
Why are the values for second electron affinities positive?
Because there is repulsion between the negative ion and the negative electron being added
In a Born Haber cycle, what do arrows going up represent?
Endothermic processes
In a Born Haber cycle, what do arrows going down represent?
Exothermic processes
Why do smaller ions have a higher lattice enthalpy value?
Smaller so they pack more tightly together
Why do ions with a higher charge have a higher lattice enthalpy value?
Form stronger ionic bonds bc more attraction between the ions
Why is there a more negative enthalpy of hydration for smaller ions with the same charge?
Bc higher charge density so negative charge on the ion attracts the H+ on the water more strongly
What is the first law of Thermodynamics?
Energy cannot be created or destroyed
What is enthalpy?
The amount of chemical energy in a system at a constant pressure
What is the standard state of a substance?
Its pure, most stable form at 100kPa at a given temperature
What is the standard enthalpy change?
The enthalpy change under standard conditions of 298K and 100kPa with all reactants and products in their standard states
What is the standard enthalpy change of formation of an element?
0
What does Hess’s Law state?
If a reaction can take place by more than one route, the overall enthalpy change is the same, regardless of the route, as long as the final and initial conditions are the same
