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Chemistry A-Level > 5.2 energy > Flashcards

5.2 energy Flashcards

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1
Q

define lattice enthalpy?

A

energy needed to form 1 mole of an ionic compound from its gaseous ions

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2
Q

is lattice enthalpy always endo or exo why?

A

exothermic because bonds are being formed

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3
Q

the more exothermic the lattice enthalpy….

A

…the stronger the ionic bonding

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4
Q

give the lattice enthalpy equation of magnesium chloride

A

Mg²⁺(g) + 2Cl⁻(g) → MgCl₂(s)

must have GAS SYMBOLS

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5
Q

what are first and second electron affinity? are they endo or exo?

give oxygen as an example

A

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)

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6
Q

what is enthalpy of atomisation? is it endo or exo?

give magnesium and chlorine as an example

A

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)

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7
Q

in a born haber cycle, which arrows mean endo and which arrows in exo?

A

upwards arrows are endo

downwards arrows are exo

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8
Q

trends in lattice enthalpy:

what happens to LE as you move down a group? why?

A

down group = LESS exothermic

  • ionic radius increases
  • attraction between ions weakens
  • less energy released when bonds form
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9
Q

trends in lattice enthalpy:

what happens to LE as charge of metal ion increases?

A

charge increases = MORE exothermic

  • charge of ion increases AND metal ion gets SMALLER
  • attraction between ions get stronger
  • more energy released when bonds form
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10
Q

define and give the symbol for enthalpy of solution

A

ΔsolH - enthalpy change when 1 mole of a solute dissolves in water

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11
Q

give the equation for enthalpy of solution of NaCl

A

NaCl (s) → Na+ (aq) + Cl- (aq)

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12
Q

is enthalpy of solution endo or exo? why?

A

could be endo or exo because bonds are broken but then bonds are also made with polar solvent

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13
Q

define and give the symbol for enthalpy of hydration

A

ΔhydH - enthalpy change when 1 mole of gaseous ions are dissolved in water

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14
Q

give the equation for enthalpy of hydration of F- and Na+ ions

A

F- (g) → F- (aq)

Na+ (g) → Na+(aq)

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15
Q

is enthalpy of hydration exo or endo? why?

A

always EXOTHERMIC because bonds are made with the polar water molecules

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16
Q

what happens to enthalpy of hydration as you go down a group? why?

A

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
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17
Q

what is entropy?
what is the symbol for entropy?
what are the units?

A 
measure of dispersal pf energy
measure of disorder in a system
more disorder = higher entropy
symbol: S
units: JK-1mol-1
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18
Q

what is the entropy at 0K/-273 degC?

A

0

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19
Q

how does entropy change as you move through the states of matter?

A

gases are most disordered so have highest entropy

solids have least disorder so have lowest entropy

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20
Q

how does entropy change when a substance dissolves?

A

entropy increases

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21
Q

in a reaction equation, how can you tell if entropy has increased or decreased overall?

A

more moles of gas on product side = increase

more moles of gas on reactants side = decrease

22
Q

how do you calculate ΔS of a reaction?

A

ΣS of product - ΣS of reactants

23
Q

what does it mean when:
ΔS is +ve?
ΔS is -ve?

A

+ve: more disordered

-ve: less disordered/ordered

24
Q

what is meant by a reaction being ‘feasible’ ?

A

if a reaction is feasible, it can occur

25
Q

what is the symbol for free energy?

A

G

26
Q

give the free energy change/Gibbs equation

A

ΔG = ΔH - TΔS

temp in K

27
Q

for which values of ΔG is a reaction feasible? for which values of G is a reaction NOT feasible?

A

feasible - ΔG < 0

NOT feasible - ΔG > 0

28
Q

even if ΔG < 0, what can prevent a reaction from occurring?

A

if activation energy is too high so rate is too slow so the reaction won’t take place

29
Q

what are the colour changes for the following half equations:

  1. Cr2O72- –>Cr3+
  2. KMnO4 –> Mn2+
A
  1. orange to green

2. purple to pale pink/colourless

30
Q

in what order do you assign oxidation states of the following:
hydrogen, carbon, oxygen and fluorine

A 
fluorine
hydrogen
oxygen
carbon
FHOC
31
Q

what are the four steps of balancing half equations?

A
  1. find the before and after oxidation state of the target element
  2. balance the oxidation states by adding electrons to one side
  3. balance the charges by adding H+’s to the same side with the electrons. REMEMBER EACH ELECTRON COUNTS AS ONE -VE CHARGE
  4. if needed, balance hydrogens by adding water molecules to one side of the equation
32
Q

what are the four steps of combining half equations?

A
  1. write out 2 half equations
  2. multiply the equations up so that the number of electrons in each equation is the same
  3. add/subtract the 2 equations so that the electrons cancel out
  4. collect like terms and if needed cancel out any repeated species on each side
33
Q

what is electrode potential? what is the symbol? what is it measured in?

A

measure of how easy it is for reduction to take place
symbol under standard condition: E^⦵
measured in volts (V)

34
Q

what does a more positive electrode potential value tell you about a metal?

A

more positive E = easier to reduce

35
Q

more reactive metals are _____ likely to ___ electrons so they have a more ______ electrode potential value

A

more, lose, negative

LOW ELECTRODE POTENTIAL = MORE REACTIVE METAL

36
Q

what is the value of standard electrode potential

A

0 V

37
Q

what type of half cell is used to calculate the standard electrode potential? what is the half equation of this half cell?

A

H2/H+ half cell

H2 —> 2e- + 2H+

38
Q

what standard conditions must be used in the standard electrode potential half cell?

A

298 K/25ºC
1 atm/100kPa
1.0 moldm-3/1.0 M

39
Q

what type of electrode and wire must be used in the standard electrode potential half cell? why?

A

platinum electrode and wire

  • not reactive so won’t react
  • provides good contact between H2 and H+
40
Q

define standard electrode potential

A

voltage produced when a half cell is connected to to a standard hydrogen half-cell

41
Q

describe the set up for the standard electrode potential

A
  • 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
42
Q

how do you work out the overall voltage output of a half cell?

A

find the difference between the electrode potentials of the two half cells

43
Q

what are the two limitations of electrochemical cells?

A
  • 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

44
Q

what happens to E value if you:

  • increase conc on LHS of equation?
  • decrease conc on LHS of equation?
A
  • equilibrium shifts to the right, where more reduction takes place, so E INCREASES
  • equilibrium shifts left, where less reduction takes place, so E DECREASES
45
Q

what happens to E value if you:

  • increase conc on RHS of equation?
  • decrease conc on RHS of equation?
A
  • equilibrium shifts left, less reduction is taking place, E DECREASES
  • equilibrium shifts right, more reduction is taking place, E INCREASES
46
Q

to make something a stronger oxidising agent, what must happen to the E value?

A

E value must increase, more reduction is happening so the oxidising agent is stronger

47
Q

if E values of two metals are very close together, what can happen under non-standard conditions?

A

one value can increase and one value can decrease enough that the lower one becomes the higher one so the reverse reaction happens

48
Q

how do you find the overall equation given the two half equations and E values for each metal?

A
  • 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
49
Q

define storage cell

A
  • 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
50
Q

what can a fuel cell be fuelled by? weigh up all the 3 options

A

hydrogen, methane or methanol

  • hydrogen is dangerous because it is explosive
  • methane and methanol produce CO2 but are safer than hydrogen

Chemistry A-Level (13 decks)

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