entropy book
5.2.2 Enthalpy and Entropy For 2013-2014 S for melting vs. boiling
Discuss
How do chemists work out whether a reaction will happen or not?
There are various ways including equilibrium constants, standard electrode potentials and enthalpy values. But how are these related? There is a more fundamental concept which links all these together.
Remember?
Enthalpy is a measure of heat energy and ΔH is the enthalpy change which we studied at AS.
Questions
1. Which sign for ΔH would you expect to see for a reaction that happens spontaneously, such as a piece of sodium reacting with chlorine gas?
2. Explain your answer.
3. Draw an enthalpy profile for this reaction
But some reactions that happen spontaneously are endothermic
Demonstration see powerpoint slides
1. ammonium chloride and barium hydroxide
2. citric acid and sodium hydrogencarbonate (Alka-Seltzer).
So there must be something more going on to explain why endothermic reactions occur.
Entropy
Entropy (S) is a measure of the disorder (or chaos or randomness) of a system. The entropy of the universe is increasing. Things become more stable when they become more disorderd (Second Law of Thermodynamics for the Physicists!)
Think of a typical teenage bedroom:
Question
Can you think of other everyday examples where there is a high entropy?
See powerpoint slides
Entropy measures how much the energy of motion of the particles in a substance changes per K of temperature.
For each degree K the temperature rises above zero K (at zero K the substance has no entropy), a substance will gain a certain amount of energy per mole and this will differ from substance to substance.
Entropy values (S) are always positive and usually quite small so we measure them in J not kJ.
Note: The units of entropy are J mol-1 K-1.
Eg. Copper has an entropy (S) of 33.3 J mol-1 K-1
Questions
1. How much energy is
Discuss
How do chemists work out whether a reaction will happen or not?
There are various ways including equilibrium constants, standard electrode potentials and enthalpy values. But how are these related? There is a more fundamental concept which links all these together.
Remember?
Enthalpy is a measure of heat energy and ΔH is the enthalpy change which we studied at AS.
Questions
1. Which sign for ΔH would you expect to see for a reaction that happens spontaneously, such as a piece of sodium reacting with chlorine gas?
2. Explain your answer.
3. Draw an enthalpy profile for this reaction
But some reactions that happen spontaneously are endothermic
Demonstration see powerpoint slides
1. ammonium chloride and barium hydroxide
2. citric acid and sodium hydrogencarbonate (Alka-Seltzer).
So there must be something more going on to explain why endothermic reactions occur.
Entropy
Entropy (S) is a measure of the disorder (or chaos or randomness) of a system. The entropy of the universe is increasing. Things become more stable when they become more disorderd (Second Law of Thermodynamics for the Physicists!)
Think of a typical teenage bedroom:
Question
Can you think of other everyday examples where there is a high entropy?
See powerpoint slides
Entropy measures how much the energy of motion of the particles in a substance changes per K of temperature.
For each degree K the temperature rises above zero K (at zero K the substance has no entropy), a substance will gain a certain amount of energy per mole and this will differ from substance to substance.
Entropy values (S) are always positive and usually quite small so we measure them in J not kJ.
Note: The units of entropy are J mol-1 K-1.
Eg. Copper has an entropy (S) of 33.3 J mol-1 K-1
Questions
1. How much energy is