Physical Chemsitry 2.1 - Reaction Feasibility - Enthalpy of Formation and Entropy Flashcards
What is the standard enthalpy of formation (ΔHf standard) of a substance?
the enthalpy change when one mol of a substance is formed from its elements in their standard statres
What is the standard state of a substance?
it’s most stable state (usually 298k and 1 ATM pressure - unless otherwise specified)
The standard enthalpy change of a ___ can be calculated from the enthalpy of ___ of the ___ and ___.
What is the equation for this? *in db
substance, formation, reactants, products.
ΔH f. = ∑ΔH f. (products) - ∑ΔH f. (reactants)
Elements ___ ___ have an enthalpy of formation.
Compounds ___ have an enthalpy of formation, and they’re given on page ___ of the data booklet.
do not.
do, page 11
What is ΔH f. measured in?
kJ/mol
Entropy is a measure of…
the degree of disorder in a reaction system
Entropy is measured in ___ and has the symbol ___.
J/k/mol (Jk-1mol-1), S
For calculations, the units for entropy must be converted from ___ to ___.
J/k/mol to kJ/k/mol
The standard entropy of a system is its entropy at ___ and ___.
298k and 1 ATM pressure (1 atmosphere)
The greater the degree of disorder, the greater the ___.
Solids have a ___ disorder, and glasses have a ___ disorder.
entropy.
low, high
Entropy increases as ___ increases, and is also affected by the ___ of a substance.
temperature, state
There is a rapid increase in entropy when a substance ___, and an even bigger one when it ___.
melts, boils
State the first law of thermodynamics.
- Energy cannot be created or destroyed (there is a fixed amount in our universe)
State the second law of thermodynamics.
- The total entropy of a reaction (system and surroundings) always increases for a spontaneous process.
State the third law of thermodynamics.
- The total entropy of a perfect crystal at absolute zero (0k) is 0.
What is a spontaneous process?
one that can take place on its own (without any input) irrespective of how long it takes
Heat energy released by the reaction ___ into the surroundings increases the ___ of the surroundings.
Heat energy absorbed by the reaction ___ from the surroundings increases the ___ of the system.
system, entropy (and energy).
system, entropy (and energy)
The standard entropy of a substance is the ___ value for a substance in its ___ ___.
entropy, standard state
The ___ symbol means ‘standard’.
Degree (∆Hf°)
The standard entropy change (ΔS°) is the ___ in ___ starting from the ___ ___ of the reactants.
change, entropy, standard states
The change in standard entropy for a reaction system can be calculated from the ___ ___ of the reactants and products (just like enthalpy), using the equation….
Standard entropy, ΔS. = ∑ΔS°(products) - ∑ΔS°(reactants)
Remember that entropy values are provided ___ ___!
per mole!
Which factors affect the entropy of substances?
temperature,
state,
number of moles,
size of molecule (rarely relevant),
degree of mixed-ness,
degree of dissolved-ness,
strength of intermolecular bonds (IMPORTANT)
For a spontaneous reaction, ΔS must be _ve.
positive
At a certain temperature, the substance with the greatest degree of disorder will be the one in a different ___, or with the weakest ___ ___.
state, intermolecular bonds
Reactions that result in a decrease in entropy either have less moles of ___ than ___, or have products in a different ___ to reactants.
product, reactant, state
