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Inorganic Chemistry Y13 > 5.2.2 Enthalpy and Entropy > Flashcards

5.2.2 Enthalpy and Entropy Flashcards

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1
Q

what is the symbol of entropy

A

S

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2
Q

what is entropy

A

the dispersal of energy within the chemicals making up the chemical system

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3
Q

what does greater the entropy result in

A
  • greater the entropy
  • greater the dispersal of energy
  • the greater the disorder
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4
Q

what is the natural tendency of particles

A
  • natural tendency for particles to spread out
  • rather than to stay concentrated in one place
  • e.g. gas spreading through a room
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5
Q

what is entropy measured in

A

JK-1mol-1

  • so greater the entropy, greater the energy is spread out per kelvin per mole
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6
Q

in general, what is the order of entropy, and why is this only general

A
  • solids have the smallest entropy value
  • liquids are greater
  • gases have the greatest entropy value
  • only general, substances in each state can have varying entropies
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7
Q

at 0K, what is the value of entropy

A
  • at 0K, all substances have entropy 0
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8
Q

what happens to the entropy value above 0K

A
  • entropy value is positive in all substances
  • as energy becomes dispersed amongst particles
  • the more chaotic the system, the greater the entropy value
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9
Q

what happens to the value of entropy if a system changes to become more random

A
  • the energy spreads out more
  • so the entropy change ΔS will be positive
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10
Q

what happens to the entropy value if a system becomes less random

A
  • the energy becomes more concentrated
  • so the entropy change ΔS will be negative
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11
Q

how can we predict entropy change

A
  • predict for equations involving physical or chemical changes
  • we can compare the physical states
  • and amount of gas on either side
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12
Q

how does entropy change as we change states

A
  • entropy INCREASES when changing state to give a more random arrangement of particles
  • so when solid goes from liquid to gas
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13
Q

why does entropy change when we change states

A
  • BECAUSE:
  • melting and boiling increases the randomness of particles
  • so energy is spread out more
  • so ΔS is positive
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14
Q

how does entropy change when we change the number of gas molecules

A
  • reactions producing gas increase in entropy:
  • production of gas increases the disorder of particles
  • so energy is spread out more
  • and ΔS is positive
  • vice verse for a decrease in the number of gas molecules
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15
Q

what is standard entropy S°

A

the entropy of one mole of a substance under standard conditions (100kPa/298K)

  • every substance has one
  • can be found in a data book
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16
Q

what are the units of standard entropy

A

JK-1mol-1
- ALWAYS positive

17
Q

how do you calculate standards entropy change ΔS°

A
  • subtract the total entropies of the products from that of the reactants
  • ΔS° = sum of S°(product) - S°(reactants)
  • REMEMBER your stoichiometry values, and to multiply values when taking from the data book
18
Q

what is feasibility

A

whether a reaction is actually able to happen and is energetically feasible

19
Q

what is another word for energetically feasible

A

spontaneous

20
Q

why will a reaction happen

A

if the products have a lower overall energy than the products

21
Q

what is free energy

A

the overall change in energy during a chemical reaction
- ΔG

22
Q

what 2 energies is free energy made up of

A

1) ΔH: enthalpy change, referring to the heat transfer between the chemical system and the surroundings
2) TΔ: the entropy change at the temperature of the reaction, referring to the dispersal of energy within the system itself

23
Q

what is Gibbs’s equation

A

finds ΔG through the relationship between ΔH and TΔS
- ΔG = ΔH-TΔS

  • ΔG = free energy change
  • ΔH = enthalpy change with surroundings
  • T= temperature in Kelvin
  • ΔS = entropy change of the system
24
Q

how do you calculate ΔG from Gibbs’ equation

A

1) calculate ΔS° using the equation as before
2) calculate ΔH° , either from the data given in table and in same way as for S, or using the constant gas equation if from an experiment
3) convert all units over, so C to Kelvin and Jmol-1 to kJmol-1 for S
4) input into the equation

25
Q

what is the equation for ΔH if you need to use an equation

A

ΔH= mcΔT

  • J, not kJ, divide by 1000 to get the value needed for the equation
  • mass is that of the SOLUTION
26
Q

what happens if a reaction is feasible with Gibbs’ equation

A
  • there is a decrease in free energy
  • so ΔG<0
27
Q

what do you need to remember about units in the Gibbs’ equation

A
  • typically, ΔH is much larger than ΔS so dominated the equation, and
  • ΔH is measured in kJmol-1
  • ΔS is usually measured jmol-1, as is a much smaller value
  • NEED to remember to convert the value of ΔS into kJmol-1, by dividing by 1000, before you can input it into the equation
28
Q

at room temperature, what has more of an effect on Gibbs’ equation

A
  • ΔH is much larger than TΔS, so ΔG is largely dependent on this
  • but as temperature increases, the other factor will obviously have more of an effect
29
Q

what is feasibility dependent on using the Gibbs’ equation energies

A

the balance between the 2 energies used
- to check whether the at high/low quantities a reaction is feasible, need to consider and make table yourself, just find out yourself

30
Q

how do you calculate minimum temperature required for a reaction to take place

A
  • at minimum temp, ΔG is equal to 0, so ΔH-TΔS = 0 too, so rearrange equation to
  • 0=ΔH/TΔS
31
Q

what is an example of an endothermic reaction taking place at room temperature

A

the dissolving of ionic compounds in water at room temperature, even though endothermic
- so still can be feasible
- and can calculate value of ΔG

32
Q

why are there limitations to predicting feasibility

A

many reactions which should take place due to Gibbs’ equation (ΔG<0), do not seem to take place

33
Q

what is an example of a reaction which can be predicted to be feasible, but isn’t

A

the combustion of hydrogen peroxide into water and oxygen
- even though ΔG is negative, does not seem to spontaneously combust

34
Q

what is an explanation for hydrogen peroxide not spontaneously combusting

A
  • it has a VERY large activation energy
  • so has a very low rate
  • so although does not take place spontaneously, if left for a very long time, it would combust
35
Q

what else needs to be considered other than free energy change when considering feasibility

A
  • although the sign of ΔG is a good indicator of thermodynamic feasibility
  • it does not take into account of kinetics and rate of reaction

Inorganic Chemistry Y13 (7 decks)

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