15.2 Entropy and spontaneity (HL)
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Entropy
Measure of the distribution of energy among particles in a system
What does greater entropy imply?
That there are more ways for the energy to be dispersed
Spontaneous process
Occurs without the addition of energy, other than that required to overcome the initial activation energy barrier
Which state of matter has the highest entropy?
Gases have highest entropy as energy can be distributed in more ways
What happens when energy is dispersed?
It becomes less able to “do work” otherwise known as low-quality energy
Does entropy increase or decrease when going from solids to gases? Is entropy change positive or negative?
Entropy increases and so change is positive
Does increase in amt. of moles of gas lead to increase or decrease in entropy?
Increase in entropy so change is positive
Unit for entropy
Jk/mol
Standard entropy
Consequential entropy change from heating the substance from absolute 0K to thermodynamic standard temp. of 298K
How can you calculate change in entropy?
Total entropy of products - Total entropy of reactants
How do you calculate total entropy?
Entropy of system + entropy of surroundings
Does a spontaneous reaction have a positive or negative entropy change?
Positive
What is happening when there is an increase in entropy of surroundings?
Heat is being released from the system
What is the equation for entropy of surroundings? How does this relate to entropy of the universe?
• Entropy of surroundings= - (Enthalpy change of process)/Abs. temperature (K) • Entropy change of universe = Entropy change of system - (enthalpy change of process)/Abs. temp.
Using equation for entropy of universe, calculate Gibbs free energy
> -TEntropy change of universe= Change in enthalpy - TEntropy change of system Gibbs free energy change= Change in enthalpy - T*Entropy change of system
Gibbs free energy
Energy associated with chemical reaction that can be used to do work. Total entropy change of universe
Expansion work
Due to change in volume of system by expansion or contraction
Position of equilibrium and why
When there is max value of entropy and min value of Gibbs free energy because there is maximum disorder and less “useful work”
What happens during position of equilibrium?
The reaction will not proceed
Factors which increase disorder
• Increased number of gaseous particles • Mixing of particles • Change of state to greater distance (Solid–>gas) • Increased particle movement (eg. Increase temp.)
Gibbs free energy change of formation
Free energy change when 1 mole of a compound is formed from its elements under standard conditions
Equation for Gibbs free energy change of reaction
Gibbs free energy change (Products)- Gibbs free energy change (Reactants)
Which reaction occurs despite not being spontaneous or energetically favorable?
Decomposition of calcium carbonate
Are products or reactants more energetically stable?
Reactants are more energetically stable than products