Unit 1 Inorganic Chemistry: Key Area 5 - Reaction Feasibilty Flashcards
What is thermochemistry?
The study of changes in energy which occur during chemical reactions.
What is the first law of thermodynamics?
- The total energy of the Universe (System and Surroundings) is constant.
- Energy can be converted from one form to another while the total energy remains the same.
- Energy can neither be created nor destroyed.
What is the symbol for a standard reaction?
()°
What is standard state?
Most stable state of the substance under standard conditions (usually 298K = 25⁰C and pressure of 1 atm).
Usuwiw
The standard enthalpy of formation, ΔH°,is the enthalpy change when one mole of a f
substance is formed from its elements in their standard states.
What does the standard enthalpy of formation of elements always equal?
0
What is the equation for the standard enthalpy?
The standard enthalpy of formation(f) of products - reactants:
ΔH° = ΣΔH°f(products) - ΣΔH°f(reactants)
What do elements in their standard state have?
ΔH°f = 0
What is entropy?
The entropy (S) of a system is a measure of the degree of disorder of the system.
What does a greater degree of entropy represent?
A greater degree of disorder
What does the second law of thermodynamics state?
That the total entropy of a reaction system and its surroundings always increases for a spontaneous process.
What is the third law of thermodynamics?
The Third Law of Thermodynamics provides a reference against which entropies can be measured:
- “the entropy of a perfect crystal at 0K is zero”
- The entropy then increases as heat energy is supplied.
- Entropy increases as temperature increases.
How does temperature effect entropy?
Entropy increases as temperature increases.
When is there a rapid increase in entropy and an even more rapid change in entropy?
There is a rapid increase in entropy at the melting point of a substance and an even more rapid and larger change in entropy at the boiling point.
How does entropy increase from a solid ➡️ liquid ➡️ solid?
The disorder increases
How does entropy change from smaller molecules to larger molecules?
Entropy increases (larger molecules have greater energy).
How does heat energy released by the reaction system into the surroundings affect entropy?
Increases the entropy of the surroundings.
How does heat energy absorbed by the reaction system from the surroundings affect entropy?
Decreases the entropy of the surroundings.
What are the units for standard entropy?
JK-1 mol-1
What is the standard entropy of a substance?
The standard entropy of a substance is the entropy value for the substance in its standard state.
What is the calculation for standard entropy?
ΔS° = ΣΔS°(products) - ΣΔS°(reactants)
How does entropy change when a substance is dissolved?
The substance has greater entropy when it is dissolved.
How does molecular shape like linear to trigonal pyramidal affect entropy?
Greater vibrations in trigonal pyramidal compared to linear increases the entropy, therefore trigonal pyramidal has higher energy.
How does mixtures affect entropy?
Mixtures have greater randomness and therefore increase entropy.
What is a spontaneous reaction?
A reaction is said to be spontaneous if it occurs without being driven by some outside force.
How do spontaneous reactions affect entropy?
Spontaneous reactions increase the total entropy of the universe.
What are examples of spontaneous reactions?
- Rusting
- Salt dissolving
- Ice melting
- Fruit ripening
How does water freezing affect entropy?
- Water freezing leads to a decrease in entropy within the system.
- Being exothermic, however, leads to an increase in entropy in the surroundings.
- Water freezing is a spontaneous process whenever there is an overall increase in entropy.
How does ice melting affect entropy?
- Being endothermic leads to a decrease in entropy in the surroundings.
- There must then be an increase in entropy within the system.
- Ice melting is a spontaneous process whenever there is an Overall Increase In Entropy.
What is the Gibbs free energy change?
The change in Gibbs free energy (ΔG) combines entropy and enthalpy into a single equation to describe the spontaneity of a process at constant temperature and pressure.
What is the equation of Gibbs free energy?
ΔG° = ΔH° - TΔS°
What does a ΔG<0 represent?
The reaction is spontaneous - Feasible.
What does a ΔG>0 represent?
The reaction is non-spontaneous - Not Feasible.
What does the T stand for in the Gibbs free energy equation?
Temperature in kelvin
What does a negative value of ΔG mean?
- That a reaction can happen but does not mean it will happen.
- The reaction is thermodynamically feasible but could be kinetically unfavourable.
- i.e. it is very slow due to a high activation energy.
What are the 2 ways of Gibbs free energy for a feasible reaction?
ΔG(-) ΔH(-) TΔS(+) or ΔG(-) ΔH(+) TΔS(TΔS > ΔH)
What are the 2 ways of Gibbs free energy for a non-feasible reaction?
ΔG(+) ΔH(+) TΔS(-) or ΔG(+) ΔH(-) TΔS(TΔS < ΔH)
What is a feasible reaction?
A feasible reaction is one that tends towards the products rather than the reactants.
What is the critical temperature of a reaction?
The temperature at which the reaction is feasible.
What is the equation for the critical temperature of a reaction?
T = ΔH° / ΔS° as we take ΔG° = 0
What is the equation for the critical temperature of a reaction?
T = ΔH° / ΔS°
At what value of ΔG is equilibrium?
0 (when the Gibbs energy of the products is equal to the Gibbs energy of the reactants).
What will happen to a reversible reaction until the composition is reached where ΔG = 0?
reversible reaction will proceed spontaneously until the composition is reached where ΔG = 0.
