Topic 13- Further energetics Flashcards
what does Hess’ law state?
the enthalpy change of a reaction is independent of the route taken
what is standard enthalpy of atomisation?
The enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions
define first ionisation energy
The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
define second ionisation energy
The enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
define first electron affinity
The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions
define second electron affinity
The enthalpy change when one mole of gaseous -1 ions gains one mole of electrons to form one mole of gaseous 2- ions
define lattice enthalpy of formation
The enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
define enthalpy of hydration
The enthalpy change when one mole of gaseous ions becomes hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)
define enthalpy of solution
The enthalpy change when one mole of a solute dissolves completely in a solvent to infinite dilution
define mean bond dissociation enthalpy
The enthalpy change when one mole of ( a certain type of) covalent bonds is broken, with all species in the gaseous state
what factors affect the lattice energy of an ionic compound?
- size of ions
- charge of ions
how can you increase the lattice enthalpy of a compound? why does this increase it?
- smaller ions, since the charge centres will be closer together
- increased charge, since there will be a greater electrostatic force of attraction between the oppositely charged ions
- increasing the charge of the anion has a much smaller effect than increasing the charge on the cation, since increasing anion charge also has the effect of increasing ionic size
How can Born-Haber cycles be used to see if compounds could theoretically exist?
use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable. (was used to predict the existence of the first noble gas containing compound)
what actually happens when a solid is dissolved in terms of interactions of the ions with water molecules?
- break lattice —> gaseous ions, dissolve each gaseous ion in water.
- the aqueous ions are surrounded by water molecules *which has a permanent dipole due to O-H polar bond)
what is the perfect ionic model?
- assumes that the ions are perfectly spherical
- even charge distribution (100% polar bonds)
why is the perfect ionic model often not accurate?
- ions aren’t perfectly spherical
- polarisation often occurs when small cations or large anions are involved (so the ionic bond gain covalent character)
- some lattices are not regular and the crystal structure can differ
which kind of bonds will be the most ionic? why?
between large positive ions and small negative ions e.g. CsF
define the terms spontaneous and feasible
if a reaction is spontaneous and feasible, it will take place on its own accord, does not take into account rate of reaction
what is a spontaneous process?
a spontaneous process is one that takes place without continuous intervention from us
is a reaction with a positive or negative enthalpy change more likely to be feasible?
negative (exothermic)
what is entropy?
entropy is a measure of the dispersal of energy in a system which is greater when the system is more disordered
what is the symbol of entropy?
S
which is more disordered- solid or gas?
gas
what is the unit of standard entropy?
J K-1 mol-1
how does temperature affect entropy?
- increased temperature increases entropy
- the particles have more energy so they move more. therefore particle arrangement becomes more random- so higher entropy
when a solid ionic lattice is dissolved in solution what happens to entropy?
entropy increases because the ions are more disordered
how does change in number of gas molecules in a reaction affect entropy?
increase in number of gas molecules= increase in entropy
decrease in number of gas molecules= decrease in entropy
what is the equation to calculate entropy change?
entropy of reaction= sum of entropy of products - sum of entropy of reactants
write the gibb’s free energy equation
gibbs energy= enthalpy change - temperature (K) x entropy of reaction
what is the second law of thermodynamics?
entropy (of an isolated system) always increases, as it is overwhelmingly more likely for molecules to be disordered than ordered
what does the value for Gibb’s free energy of a reaction show?
- if G < 0, reaction is feasible
- if G=0, reaction is just feasible
- if G > 0, reaction is not feasible
what is significant of the temperature at which G=0?
this is the temperature in kelvin at which the reaction becomes feasible
how would you calculate the temperature at which a reaction becomes feasible?
T= enthalpy change/ entropy
what are the limitations of using G as an indicator of whether a reaction will occur?
Gibbs free energy only tells you whether a reaction is feasible. It does not take in the rate of reaction (the kinetics). Many reactions that are feasible at a certain temperature have such a slow rate that almost no reaction is occurring at all.
what is another equation for Gibbs free energy?
Gibbs energy= -RT ln K (gas + equilibrium constant, log and temperature)
why is entropy zero at 0K?
no disorder- molecules/ atoms aren’t moving or vibrating and cannot be arranged in any other way. Maximum possible state of order
what are the 2 key things to look out for to decide if entropy increases/decreases/stays relatively constant?
- number of moles (more moles made the higher the entropy)
- going from solid to liquid/ gas or liquid to gas
how is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant?
the heat that is given out escapes to the surroundings
Define lattice energy
the energy change when one mole of an ionic compound/solid is formed from its gaseous ions
