Physical Chemsitry 2.1 - Reaction Feasibility - Enthalpy of Formation and Entropy

Question 1

What is the standard enthalpy of formation (ΔHf standard) of a substance?

Answer 1

the enthalpy change when one mol of a substance is formed from its elements in their standard statres

Question 2

What is the standard state of a substance?

Answer 2

it’s most stable state (usually 298k and 1 ATM pressure - unless otherwise specified)

Question 3

The standard enthalpy change of a ___ can be calculated from the enthalpy of ___ of the ___ and ___.
What is the equation for this? *in db

Answer 3

substance, formation, reactants, products.
ΔH f. = ∑ΔH f. (products) - ∑ΔH f. (reactants)

Question 4

Elements ___ ___ have an enthalpy of formation.
Compounds ___ have an enthalpy of formation, and they’re given on page ___ of the data booklet.

Answer 4

do not.
do, page 11

Question 5

What is ΔH f. measured in?

Answer 5

kJ/mol

Question 6

Entropy is a measure of…

Answer 6

the degree of disorder in a reaction system

Question 7

Entropy is measured in ___ and has the symbol ___.

Answer 7

J/k/mol (Jk-1mol-1), S

Question 8

For calculations, the units for entropy must be converted from ___ to ___.

Answer 8

J/k/mol to kJ/k/mol

Question 9

The standard entropy of a system is its entropy at ___ and ___.

Answer 9

298k and 1 ATM pressure (1 atmosphere)

Question 10

The greater the degree of disorder, the greater the ___.
Solids have a ___ disorder, and glasses have a ___ disorder.

Answer 10

entropy.
low, high

Question 11

Entropy increases as ___ increases, and is also affected by the ___ of a substance.

Answer 11

temperature, state

Question 12

There is a rapid increase in entropy when a substance ___, and an even bigger one when it ___.

Answer 12

melts, boils

Question 13

State the first law of thermodynamics.

Answer 13

1. Energy cannot be created or destroyed (there is a fixed amount in our universe)

Question 14

State the second law of thermodynamics.

Answer 14

2. The total entropy of a reaction (system and surroundings) always increases for a spontaneous process.

Question 15

State the third law of thermodynamics.

Answer 15

3. The total entropy of a perfect crystal at absolute zero (0k) is 0.

Question 16

What is a spontaneous process?

Answer 16

one that can take place on its own (without any input) irrespective of how long it takes

Question 17

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.

Answer 17

system, entropy (and energy).
system, entropy (and energy)

Question 18

The standard entropy of a substance is the ___ value for a substance in its ___ ___.

Answer 18

entropy, standard state

Question 19

The ___ symbol means ‘standard’.

Answer 19

Degree (∆Hf°)

Question 20

The standard entropy change (ΔS°) is the ___ in ___ starting from the ___ ___ of the reactants.

Answer 20

change, entropy, standard states

Question 21

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….

Answer 21

Standard entropy, ΔS. = ∑ΔS°(products) - ∑ΔS°(reactants)

Question 22

Remember that entropy values are provided ___ ___!

Answer 22

per mole!

Question 23

Which factors affect the entropy of substances?

Answer 23

temperature,
state,
number of moles,
size of molecule (rarely relevant),
degree of mixed-ness,
degree of dissolved-ness,
strength of intermolecular bonds (IMPORTANT)

Question 24

For a spontaneous reaction, ΔS must be _ve.

Answer 24

positive

Question 25

At a certain temperature, the substance with the greatest degree of disorder will be the one in a different ___, or with the weakest ___ ___.

Answer 25

state, intermolecular bonds

Question 26

Reactions that result in a decrease in entropy either have less moles of ___ than ___, or have products in a different ___ to reactants.

Answer 26

product, reactant, state