Module 5.2 - Energy Flashcards
(97 cards)
What is meant by the term ‘lattice enthalpy’?
The enthalpy change that accompanies the formation of one mole of an ionic lattice from its gaseous ions under standard conditions
What is meant by the term ‘standard enthalpy change of formation’?
The enthalpy change that accompanies the formation of 1 mole of a compound from its elements
What is meant by the term ‘first ionisation energy’?
The energy change that accompanies the removal of 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
What energy changes occur during the formation of an ionic lattice?
- when oppositely charge ions attracted to one another to form giant ionic lattice, theres a large lowering of energy from strong attraction
- although energy required to form ions is huge, the lowering of energy when forming lattice more than compensates for this
- hence why giant ionic lattices have v strong ionic bonds and v high melting and boiling points
What are the features of lattice enthalpy?
- ions it forms from are both gaseous
- 1 mol of substance formed
- enthalpy change is negative (energy given off to surroundings)
- ionic lattice formation is exothermic
What does a more exothermic lattice enthalpy indicate about the strength of the ionic bonds in the lattice and therefore the melting and boiling points?
- more exothermic = stronger ionic bonds = stronger electrostatic interactions
- high mp and bp as more energy to overcome interactions present
What type of ion would cause the most negative lattice enthalpy and why?
- small ions with large charges
- charges cause large electrostatic forces
- smaller ions can get closer together
Why can’t lattice enthalpy be directly measured, and how would you work it out instead?
- it’s impossible for one mole of an ionic lattice to form from its gaseous ions
- use a Born-Haber cycle
What are the key features of a Born-Haber cycle?
- continuous cycle formed that can start at the elements and end at the elements
- includes 1 step showing formation of 1 mole of solid ionic lattice from its gaseous ions; the lattice enthalpy
- remaining steps show intermediate changes that correspond to key enthalpy changed that can be measured
- lattice enthalpy can be calculated by Hess’ law (if a reaction can take place by more than one route and the initial and final conditions are the same, the Toal enthalpy change for each route is the same
What enthalpy changes usually take place during in a Born-Haber cycle (especially for lattice enthalpy)?
- standard enthalpy change of formation
- standard enthalpy change of atomisation
- first/second ionisation energy
- first/second electron affinity
Is the standard enthalpy change of formation for an ionic lattice usually endothermic or exothermic and why?
- exothermic
- bonds are made
Give an equation for the standard enthalpy change of formation of potassium chloride.
K(s) + 1/2Cl2(g) –> KCl(s)
Is standard enthalpy change of atomisation endothermic or exothermic and why?
Endothermic as bonds have to be broken
What does ‘standard enthalpy change of atomisation’ mean?
One mole of gaseous atoms formed from its element in its standard state
Give the standard enthalpy change of atomisation of chlorine and potassium.
K(s) –> K(g)
1/2Cl2(g) –> Cl(g)
What does ‘first electron affinity’ mean?
One mole of gaseous 1- ions formed from gaseous atoms (each one gains an electron)
Give the equation for the first electron affinity of chlorine.
Cl(g) + e- –> Cl-(g)
Is first/second ionisation energy usually exothermic or endothermic and why?
Endothermic as electron being lost has to overcome attraction from nucleus to leave atom
Is first electron affinity exothermic or endothermic and why?
- exothermic
- electron is attached into outer shell of an atom by nucleus (bond formed)
Is second electron affinity exothermic or endothermic and why?
- endothermic
- electron repelled by the 1- ion and this repulsion has to be overcome
What equation can you use to use Hess’ law on Born-Haber cycles?
sum of clockwise enthalpies = sum of anticlockwise enthalpies
What is meant by the term ‘enthalpy change of solution’?
The enthalpy change that is associated with dissolving 1 mole of solute in a solvent to form an infinitely dilute solution (i.e. completely dissolved in solvent)
What is meant by the term ‘enthalpy change of hydration’?
The enthalpy change that associates dissolving 1 mole of gaseous ions in water
What 2 processes take place when an ionic solid dissolves?
- ionic lattice breaks down
- free ions become part of the solution (hydration)