5.2.2 Enthalpy and Entropy Flashcards
what is the symbol of entropy
S
what is entropy
the dispersal of energy within the chemicals making up the chemical system
what does greater the entropy result in
- greater the entropy
- greater the dispersal of energy
- the greater the disorder
what is the natural tendency of particles
- natural tendency for particles to spread out
- rather than to stay concentrated in one place
- e.g. gas spreading through a room
what is entropy measured in
JK-1mol-1
- so greater the entropy, greater the energy is spread out per kelvin per mole
in general, what is the order of entropy, and why is this only general
- 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
at 0K, what is the value of entropy
- at 0K, all substances have entropy 0
what happens to the entropy value above 0K
- entropy value is positive in all substances
- as energy becomes dispersed amongst particles
- the more chaotic the system, the greater the entropy value
what happens to the value of entropy if a system changes to become more random
- the energy spreads out more
- so the entropy change ΔS will be positive
what happens to the entropy value if a system becomes less random
- the energy becomes more concentrated
- so the entropy change ΔS will be negative
how can we predict entropy change
- predict for equations involving physical or chemical changes
- we can compare the physical states
- and amount of gas on either side
how does entropy change as we change states
- entropy INCREASES when changing state to give a more random arrangement of particles
- so when solid goes from liquid to gas
why does entropy change when we change states
- BECAUSE:
- melting and boiling increases the randomness of particles
- so energy is spread out more
- so ΔS is positive
how does entropy change when we change the number of gas molecules
- 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
what is standard entropy S°
the entropy of one mole of a substance under standard conditions (100kPa/298K)
- every substance has one
- can be found in a data book
what are the units of standard entropy
JK-1mol-1
- ALWAYS positive
how do you calculate standards entropy change ΔS°
- 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
what is feasibility
whether a reaction is actually able to happen and is energetically feasible
what is another word for energetically feasible
spontaneous
why will a reaction happen
if the products have a lower overall energy than the products
what is free energy
the overall change in energy during a chemical reaction
- ΔG
what 2 energies is free energy made up of
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
what is Gibbs’s equation
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
how do you calculate ΔG from Gibbs’ equation
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
what is the equation for ΔH if you need to use an equation
ΔH= mcΔT
- J, not kJ, divide by 1000 to get the value needed for the equation
- mass is that of the SOLUTION
what happens if a reaction is feasible with Gibbs’ equation
- there is a decrease in free energy
- so ΔG<0
what do you need to remember about units in the Gibbs’ equation
- 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
at room temperature, what has more of an effect on Gibbs’ equation
- 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
what is feasibility dependent on using the Gibbs’ equation energies
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
how do you calculate minimum temperature required for a reaction to take place
- at minimum temp, ΔG is equal to 0, so ΔH-TΔS = 0 too, so rearrange equation to
- 0=ΔH/TΔS
what is an example of an endothermic reaction taking place at room temperature
the dissolving of ionic compounds in water at room temperature, even though endothermic
- so still can be feasible
- and can calculate value of ΔG
why are there limitations to predicting feasibility
many reactions which should take place due to Gibbs’ equation (ΔG<0), do not seem to take place
what is an example of a reaction which can be predicted to be feasible, but isn’t
the combustion of hydrogen peroxide into water and oxygen
- even though ΔG is negative, does not seem to spontaneously combust
what is an explanation for hydrogen peroxide not spontaneously combusting
- 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
what else needs to be considered other than free energy change when considering feasibility
- although the sign of ΔG is a good indicator of thermodynamic feasibility
- it does not take into account of kinetics and rate of reaction
