Bond Enthalpies and Entropies

Question 1

What is an enthalpy?

Answer 1

Unit of heat gained or lost. = -Q/n KJ/mol

Question 2

What is an enthalpy of combustion?

Answer 2

Heat produced when one mole of a substance is burned completely in standard conditions

Question 3

What is an enthalpy of formation?

Answer 3

Heat produced when one mole of a substance is formed from elements in standard states.

Question 4

What is an enthalpy of reaction?

Answer 4

the total enthalpy of a reaction. Total = sum of reactions - sum of the products. Difficult to measure due to side reactions and slow rates

Question 5

What is an enthalpy of ionization?

Answer 5

The energy needed to remove the first valence electron from one mole of metal in a gas state to form its ions.
= E/mol

Question 6

What is an enthalpy of electron affinity?

Answer 6

Energy that is transferred by the addition of one mole of electrons to one mole of gaseous atoms.

Question 7

What is an enthalpy of atomization?

Answer 7

Heat exchanged when one mole of gaseous atoms of an element is formed from the elements in its standard state.

Question 8

What is a lattice enthalpy?

Answer 8

Energy absorbed when one mole of a solid ionic compound is decomposed to form its gaseous ions.
H(latt) = sum of all processes (IE, EA, F)

Question 9

What is an enthalpy change of solution?

Answer 9

Change in energy when one mole of a substance is dissolved in a large excess of a pure solvent.

Question 10

What is an enthalpy change in hydration?

Answer 10

Change when one mole of gaseous ions is added to water to form a dilute solution.

Question 11

Definition of energy

Answer 11

a measure of an ability to do work in joules.

Question 12

Thermal energy definition

Answer 12

energy available to a substance as a result of the motion of its molecules.

Question 13

Definition of chemical potential energy

Answer 13

Energy store in chemical bonds (enthalpy)

Question 14

What is the difference between an open, closed and isolated system?

Answer 14

Open: Can exchange matter and energy with the surroundings
Closed: Can only exchange energy with the surroundings
Isolated: Neither energy nor matter can be exchanged with the surroundings.

Question 15

What is an exothermic reaction?

Answer 15

Transfers energy to the surroundings, occurs in most chemical reactions, all combustion and neutralization reactions. Has a negative enthalpy as there is a loss of heat in the system.

Question 16

What is an endothermic reaction?

Answer 16

Thermal energy is absorbed from the surroundings, occurs in a few chemical reactions. Has a positive enthalpy as the system gains heat.

Question 17

What are the conditions for standard enthalpy changes?

Answer 17

1. T = 298K or 25 degrees
2. 100 kPa
3. [ ] = 1 mol/dm^3
4. all substances are in their standard states (most thermodynamically stable state)

Question 18

What is the heat change equation?

Answer 18

Q = mc(delta)t
Q = heat change
m = m (g)
c = specific heat capacity
t = temp

Question 19

Enthalpy relationship of products and reactants

Answer 19

It is expected that the H of products is less than the H of the reactants because compounds are more stable at a lower E.

Question 20

What are some sources of energy for combustion calorimetry?

Answer 20

• Heat lost to surroudings
• On complete combustion
• Heat gained by the metal calorimeter cup

Question 21

What is polystyrene calorimetry?

Answer 21

Reactions in solution. Single or double displacement reactions are common. Heat is still lost to surroundings.

Question 22

What is Hess’s law?

Answer 22

Application of the law of conservation of energy. Implies that no process is 100% efficient and a measured difference may occur. States: The value of any change of H of reaction can be written in steps that equals the sum of the change of enthalpy (direction changes mean -ve or +ve).

Question 23

What is a bond enthalpy?

Answer 23

The energy required to break one mole of a covalent bond in gaseous state under standard conditions

Question 24

Why do we use average bond enthalpies?

Answer 24

Because occasionally when the same bond type is broken it can have different enthalpies as it is affected by the bonds and atoms around it.

Question 25

Trends in bonds

Answer 25

Shorter bonds require more energy to break, related to electronegativity difference

Question 26

Making bonds

Answer 26

An exothermic reaction, energy is given off. We can track breaking and formation of bonds through:
(delta)H = sum of bonds broken (reactants) - sum of bonds formed (products)

Question 27

What is the Born-Haber cycle?

Answer 27

The energy cycle based on Hess’s law, represents the formation of an ionic compound from its elements in its standard states.

Question 28

Process of a Born-Haber cycle

Answer 28

Starts at enthalpy of formation - enthalpy of atomization of each reactant - first and second enthalpy of ionization - first enthalpy of electronic affinity which goes down - second enthalpy of electron affinity which goes back up - lattice enthalpy is the sum of all these steps

Question 29

What are the factors affecting the lattice enthalpy?

Answer 29

1. Charges: larger charges implies greater attraction which implies higher lattice enthalpy
2. Size of ions: Bigger ions imply a larger separation between the nucleus of the cation and the e cloud of the anion. size and lattice enthalpy is inversely proportional.

Question 30

What is an entropy?

Answer 30

A measure of disorder or the dispersal of E in a system. Measured in J/K/mol

Question 31

Laws of Thermodynamics

Answer 31

1. Law of the conservation of energy
2. The entropy of the universe must always increase

Question 32

Predicting changes in entropy

Answer 32

Effect of changing state:
Solid to gas means an increase in entropy. Gas to solid means a decrease in entropy.

Effect of temperature change:
higher temp - higher movement - higher disorder

Effect of change in # of particles:
mole coefficients, more particles = more disorder, doubling particles doubles entropy

Effect of mixing particle:
breaking a substance into its ions

Question 33

Quantifying the change in entropy of the surroundings

Answer 33

2nd law of thermodynamics states that S of the system and surroundings must add to greater than 0.
S of surroundings = -Hsys/T
S total = S sys - Hsys/t

Question 34

Explain spontaneity

Answer 34

Disorder in the system is sufficient to compensate for the order created in the surroundings by an endothermic reaction, therefore, the reaction will occur spontaneously for most temperatures

Question 35

Gibbs free energy formula

Answer 35

Change in G of system = Change in H of sys - T (change in S of system)

• Change in G is free E available to do work
• T(change of S) is unfree E
• Change in H is total energy change at a constant temp

Question 36

How to determine spontaneity?

Answer 36

If G = positive the reaction is endo and not spontaneous
If G = negative the reaction is exo and spontaneous

Question 37

What is Gibbs free energy of formation?

Answer 37

Free E changes when 1 mole of a compound is formed from its element in a standard state under SATP

Question 38

How to determined the percentage error for a combustion rxn?

Answer 38

% = |theo-exper/theo| ×100