Oceans 5: Entropy Flashcards
Symbol for entropy
S
Definition of entropy
Entropy = quantity ( S ) which represents the degree of disorder in a system
Units of entropy
J mol^-1 K^-1
J / mol K
What is the relationship between entropy and degree of disorder
Increase entropy = increase degree of disorder
Why are entropy values always greater than 0
Due to vibration of movement of particles
How does temperature effect entropy
Increase temp = increase entropy
What does entropy increase when temperature increases
Because heating generally results in particles becoming more disordered
Compare the entropies of a gas, liquid and solid
gas > liquid > solid
Compare the entropies of mixtures and deprecate substances ( eg. Pure substance)
mixture > separate
Compare the entropies of large molecules & small molecules
larger molecules > smaller molecules
What 2 things are higher values of entropy favoured by
- Spreading out of molecules
2. Higher temp
More ordered structure = ??? entropy
Give example of substance with low entropy
more ordered structure = lower entropy
Eg. Diamond & graphite = very low entropy
Equation of entropy change ( delta S sys )
sys = system= where reaction takes places ie. beaker / test tube
Delta S sys = sum of PRODUCTS - sum of REACTANTS
What does a +ve delta S represent
Increase in entropy
What does a -ve delta S represent
Decrease in entropy
What kind of reaction are more favoured, increase or decrease in entropy?
Reactions where there is an increase in entropy are more favourable i.e disorder = good
How can you predict changes in entropy by looking at an equation
By using
- no. of moles on each side
- state symbols ( be careful when dealing with aqueous compounds = one mol of each ion )
Predict whether entropy will increase or decrease from the equation below
• NaCl ( s ) —> NaCl ( aq )
- entropy increases
- 1 mole solid —> 2 mol aq ( 1 mol Na+(aq) + 1 mol Cl-(aq) )
Give equation for total entropy change ( delta S total )
delta S total = delta S sys + delta S surr
Total = system + surroundings
Give equation for enthalpy change of surroundings (delta S surr)
delta S surr = - delta H / T
- delta H = enthalpy change = Joules ( KJ -> J = x1000 )
- T = temperature = Kelvins ( °c —> K =+273)
What conditions are required for a chemical change to be feasible
Delta S total = +ve
What does a -ve delta S total tell you about a chemical change reaction
-ve = not feasible
When does the minimum temperature for a reaction to be feasible occur
Delta S total = 0
Definition of feasible
- Feasible = reaction does occur
- Not feasible = reaction doesn’t occur
How do you calculate enthalpy of reaction give enthalpies of formation
Using a Hess cycle ( arrows towards equation )
Delta H1 = -delta H2 +delta H3
