Thermdynamics Flashcards
Define enthalpy change?
Heat energy change at a constant pressure.
Define standard enthalpy change?
Enthalpy change measured under standard conditions.
Define enthalpy of formation?
Enthalpy change when one mole of a compound is formed from it’s elements with all reactants and products in their standard states.
Define enthalpy of atomisation?
Enthalpy change when one mole of gaseous atoms is formed from an element in it’s standard state.
Define first ionisation enthalpy?
The enthalpy change when one mole of gaseous 1+ ions is formed from it’s gaseous atoms.
Define first electron affinity?
The enthalpy change when one mole of gaseous 1- ions are formed from gaseous atoms.
Define bond dissociation enthalpy?
The enthalpy change when one mole of covalent bonds is completely separated to form gaseous atoms.
Define lattice enthalpy of formation?
The enthalpy change when one mole of solid ionic compound is formed from gaseous ions.
Define lattice enthalpy of dissociation.
The enthalpy change when one mole of solid inc compounds dissociates into gaseous ions.
How can born hater cycles be sued to work out lattice enthalpies?
- Elements converted on gaseous atoms.
- Electrons lost/gained to form ions.
- Ions attract to form a lattice.
- Write start/finish and add downward arrows and subtract upwards arrows if x is on the right.
State why some ionic compound display covalent character?
- +ve ins attract the outer electrons of the -ve ions.
- Electron clouds are polarised leading to distorted charge clouds.
What is the perfect ionic model?
Ions being perfect spheres with evenly distributed charge.
In what scenarios is covalent character more likely for an ionic compound?
- cation has a high charge density
- anion has a low charge density
How does covalent character changes the lattice enthalpy?
- Larger than expected.
- As it requires more entry to break/form.
What two factors des the strength of lattice enthalpy depend on and how?
Size and charge of ions, larger size weaker attraction, larger charge, larger attraction.
What happens when NaCl dissolves?
- Water molecules are polar.
- Na+ attracted to delta - O atoms.
- Cl- ions are attracted to delta + H atoms.
Define enthalpy of solution.
The enthalpy change when one mole of ionic compound dissolve in water.
Define enthalpy of hydration.
The enthalpy change when one mole of aqueous ions is formed from gaseous ions.
Use enthalpy cycles, qualitatively describe how we can work out enthalpy of solution.
- Ionic lattice broken down into gaseous ions.
- Gaseous ions hydrated by polar water molecules.
What two factors affecting solubility of an ionic compound?
Higher charge leads to greater attraction leading to increase in enthalpies.
Larger ions lead to less charge density leading to decreased enthalpies.
What is entropy?
- A measure of disorder in a system.
- In physical and chemical changes, processes move spontaneously to form order to disorder i.e. entropy will increase.
What factors can affect entropy?
- Physical state.
- Dissolving.
- Number of particles.
What is the formula of entropy?
Entropy change = entropy of products - entropy of reactants
Negative entropy change.
Reaction unlikely.
