chapter 5 - entropy

what is entropy?

• a measure of disorder

• entropy always increases during a reaction

are substances more stable when they have more or less disorder?

• greater disorder

• this means that everything tends to increase its disorder

  • this is why phone cables get tangled in your pocket

  • also why NaCl dissolves in water - its entropy increases

entropy technical definition?

• the number of ways molecules can be arranged, and the number of ways their energy can be arranged between them

• it is given the symbol ‘S’

  • a big and positive value of S indicates a large disorder

factors affecting entropy?

• temperature

• physical state

• number of moles

temperature?

• an increase in temperature increases the speed the particles move at

• this increases disorder and so entropy increases

physical state?

• the molecules in gases are free to move

• this means that they have a higher entropy than solids and liquids

• in the same way, liquids have higher entropy than solids

number of moles?

• there are more ways you can arrange 10 particles than 2 particles

• so the more moles of substance, the greater the entropy

when is a reaction more likely to happen?

• if the reaction increases the number of moles of substance

• if the reaction produces gas and consumes solids

• if the reaction is exothermic

how to calculate entropy change?

• you must find the difference between the standard entropies of the products and the reactants

example entropy change

If the entropy of nitrogen and ammonia gas is 192JK^-1mol^-1 and the entropy of hydrogen gas is 131JK^-1mol^-1, what is the entropy change?

• Entropy change = (2 x 192) - (3 x 131) − 192 = −201JK^-1mol^-1

  • this fits well with the observation that the Haber process does not produce much ammonia

what is free energy?

• a simple way to determine if a reaction if feasible

  • free energy includes both entropy and enthalpy

for a reaction to happen, what must occur?

• the total entropy of everything involved must increase

what is gibbs free energy?

• ΔG = ΔH - TΔS

  • if ΔG is negative, the overall entropy increases and a reaction will happen

  • (for a reaction to happen, ΔG must be negative

  • negative ΔG = feasible reaction

    
    

what would happen if the reaction has a negative ΔH and a positive ΔS?

• reaction will always be feasible

if the reaction has a positive ΔH when will it only be feasible?

• if the ΔS is positive, and larger than it

how to calculate when a reaction switches from feasible to unfeasible?

• by setting ΔG equal to zero

• to find this temperature, rearrange the equation

things to remember when using the gibbs free energy change?

• both the entropy and enthalpy change should be in joules