U2-2 - Reaction Feasibility Flashcards
Standard values (states, enthalpy, etc) are taken at a temperature of …
298 K (25 °C)
Standard state
The most stable state at a pressure of 1 atm at 298 K.
Standard enthalpy of formation
The enthalpy change when 1 mol of a substance is formed from its elements in their standard states.
For pure elements in their standard states, ∆H°f = …
0
Entropy
A measure of the degree of disorder of a system
Symbol for entropy
S
Which has the highest entropy:
solids, liquids or gases?
Gases
(particles free to move = high disorder)
Which has the lowest entropy:
solids, liquids or gases?
Solids
(particles can only vibrate, not move freely = low disorder)
What happens to entropy as temperature increases?
Entropy increases (more kinetic energy = more disorder)
State the 2nd law of thermodynamics
The total entropy of a reaction system and its surroundings always increases for a spontaneous process.
What is a spontaneous process?
A process which will happen without having to add heat.
The products are favoured at the conditions under which the reaction is occurring.
State the 3rd law of thermodynamics
At 0 K the entropy of a perfect crystal is 0.
Units for entropy (S)
J K-1 mol-1
Why does this equation not represent an enthalpy of formation?
2 mol of product (must be 1 mol)
Why does this equation not represent an enthalpy of formation?
Br is not in its standard (liquid) state