5.2 energy Flashcards
define lattice enthalpy?
energy needed to form 1 mole of an ionic compound from its gaseous ions
is lattice enthalpy always endo or exo why?
exothermic because bonds are being formed
the more exothermic the lattice enthalpy….
…the stronger the ionic bonding
give the lattice enthalpy equation of magnesium chloride
Mg²⁺(g) + 2Cl⁻(g) → MgCl₂(s)
must have GAS SYMBOLS
what are first and second electron affinity? are they endo or exo?
give oxygen as an example
first electron affinity is ability for an atom to gain an electron - exothermic
second electron affinity ability for a 1- atom to gain another electron - endthermic
both in gaseous forms
first: O(g) + e⁻ → O⁻(g)
second: O⁻(g) + e⁻ → O⁻²(g)
what is enthalpy of atomisation? is it endo or exo?
give magnesium and chlorine as an example
energy required to turn an element from it’s standard state to gaseous atoms - endo cos breaking bonds
Mg(s) → Mg(g)
Cl₂(g) → 2Cl(g)
in a born haber cycle, which arrows mean endo and which arrows in exo?
upwards arrows are endo
downwards arrows are exo
trends in lattice enthalpy:
what happens to LE as you move down a group? why?
down group = LESS exothermic
- ionic radius increases
- attraction between ions weakens
- less energy released when bonds form
trends in lattice enthalpy:
what happens to LE as charge of metal ion increases?
charge increases = MORE exothermic
- charge of ion increases AND metal ion gets SMALLER
- attraction between ions get stronger
- more energy released when bonds form
define and give the symbol for enthalpy of solution
ΔsolH - enthalpy change when 1 mole of a solute dissolves in water
give the equation for enthalpy of solution of NaCl
NaCl (s) → Na+ (aq) + Cl- (aq)
is enthalpy of solution endo or exo? why?
could be endo or exo because bonds are broken but then bonds are also made with polar solvent
define and give the symbol for enthalpy of hydration
ΔhydH - enthalpy change when 1 mole of gaseous ions are dissolved in water
give the equation for enthalpy of hydration of F- and Na+ ions
F- (g) → F- (aq)
Na+ (g) → Na+(aq)
is enthalpy of hydration exo or endo? why?
always EXOTHERMIC because bonds are made with the polar water molecules
what happens to enthalpy of hydration as you go down a group? why?
going down a group - it becomes LESS EXOTHERMIC
- down the group ionic radius increases
- reduces electrostatic attraction to water molecules
- less energy released during hydration
what is entropy?
what is the symbol for entropy?
what are the units?
measure of dispersal pf energy measure of disorder in a system more disorder = higher entropy symbol: S units: JK-1mol-1
what is the entropy at 0K/-273 degC?
0
how does entropy change as you move through the states of matter?
gases are most disordered so have highest entropy
solids have least disorder so have lowest entropy
how does entropy change when a substance dissolves?
entropy increases
in a reaction equation, how can you tell if entropy has increased or decreased overall?
more moles of gas on product side = increase
more moles of gas on reactants side = decrease
how do you calculate ΔS of a reaction?
ΣS of product - ΣS of reactants
what does it mean when:
ΔS is +ve?
ΔS is -ve?
+ve: more disordered
-ve: less disordered/ordered
what is meant by a reaction being ‘feasible’ ?
if a reaction is feasible, it can occur
what is the symbol for free energy?
G
give the free energy change/Gibbs equation
ΔG = ΔH - TΔS
temp in K
for which values of ΔG is a reaction feasible? for which values of G is a reaction NOT feasible?
feasible - ΔG < 0
NOT feasible - ΔG > 0
even if ΔG < 0, what can prevent a reaction from occurring?
if activation energy is too high so rate is too slow so the reaction won’t take place
what are the colour changes for the following half equations:
- Cr2O72- –>Cr3+
- KMnO4 –> Mn2+
- orange to green
2. purple to pale pink/colourless
in what order do you assign oxidation states of the following:
hydrogen, carbon, oxygen and fluorine
fluorine hydrogen oxygen carbon FHOC
what are the four steps of balancing half equations?
- find the before and after oxidation state of the target element
- balance the oxidation states by adding electrons to one side
- balance the charges by adding H+’s to the same side with the electrons. REMEMBER EACH ELECTRON COUNTS AS ONE -VE CHARGE
- if needed, balance hydrogens by adding water molecules to one side of the equation
what are the four steps of combining half equations?
- write out 2 half equations
- multiply the equations up so that the number of electrons in each equation is the same
- add/subtract the 2 equations so that the electrons cancel out
- collect like terms and if needed cancel out any repeated species on each side
what is electrode potential? what is the symbol? what is it measured in?
measure of how easy it is for reduction to take place
symbol under standard condition: E^⦵
measured in volts (V)
what does a more positive electrode potential value tell you about a metal?
more positive E = easier to reduce
more reactive metals are _____ likely to ___ electrons so they have a more ______ electrode potential value
more, lose, negative
LOW ELECTRODE POTENTIAL = MORE REACTIVE METAL
what is the value of standard electrode potential
0 V
what type of half cell is used to calculate the standard electrode potential? what is the half equation of this half cell?
H2/H+ half cell
H2 —> 2e- + 2H+
what standard conditions must be used in the standard electrode potential half cell?
298 K/25ºC
1 atm/100kPa
1.0 moldm-3/1.0 M
what type of electrode and wire must be used in the standard electrode potential half cell? why?
platinum electrode and wire
- not reactive so won’t react
- provides good contact between H2 and H+
define standard electrode potential
voltage produced when a half cell is connected to to a standard hydrogen half-cell
describe the set up for the standard electrode potential
- a beaker with H+ ion solution of 1.0M
- bell jar in the beaker with H2 gas coming in at the top at 1 atm
- platinum wire in bell jar connected to a platinum electrode which is submerged in the solution
how do you work out the overall voltage output of a half cell?
find the difference between the electrode potentials of the two half cells
what are the two limitations of electrochemical cells?
- standard conditions in a lab are not often used
- rate of reaction is too slow so reaction will take too long due to high activation energy
what happens to E value if you:
- increase conc on LHS of equation?
- decrease conc on LHS of equation?
- equilibrium shifts to the right, where more reduction takes place, so E INCREASES
- equilibrium shifts left, where less reduction takes place, so E DECREASES
what happens to E value if you:
- increase conc on RHS of equation?
- decrease conc on RHS of equation?
- equilibrium shifts left, less reduction is taking place, E DECREASES
- equilibrium shifts right, more reduction is taking place, E INCREASES
to make something a stronger oxidising agent, what must happen to the E value?
E value must increase, more reduction is happening so the oxidising agent is stronger
if E values of two metals are very close together, what can happen under non-standard conditions?
one value can increase and one value can decrease enough that the lower one becomes the higher one so the reverse reaction happens
how do you find the overall equation given the two half equations and E values for each metal?
- the half equation with the higher E value proceeds with the forward reaction
- the other half equation proceeds with the reverse reaction
- balance out the electrons
- add/subtract to cancel electrons
define storage cell
- an electrochemical cell that can be recharged
- a redox reaction takes place in it
- when the reaction is finished, it can b reversed using electricity and then the cell can be used again
what can a fuel cell be fuelled by? weigh up all the 3 options
hydrogen, methane or methanol
- hydrogen is dangerous because it is explosive
- methane and methanol produce CO2 but are safer than hydrogen
