Energetics + Thermodynamics

Question 1

Enthalpy Change

Answer 1

? The Energy transferred to or from the system when a chemical change happens. In constant pressure

Question 2

Standard Enthalpy of Combustion

Answer 2

? The enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions, all substances in the standard state.

Question 3

Standard Enthalpy of Formation

Answer 3

? The enthalpy change when one mole of a substance is formed from its elements under standard conditions, all substances in their standard state.

Question 4

Standard state

Answer 4

? Whether it is a solid, liquid or gas @ 298 K(25oC) 1cm3 = 1g

Question 5

Calorimetry

Answer 5

? Process of measuring the heat change for a given chemical reaction

Question 6

Bond dissociation enthalpy

Answer 6

? The enthalpy change when one mole of COVALENT bonds are broken with all the species in the gaseous state. All enthalpies are endothermic.

Question 7

Mean bond enthalpy

Answer 7

? The enthalpy change when one mole of covalent bond is broken averaged over a range of compounds with everything in the gas phase. If this is a diatomic molecule, you need to define bond enthalpy instead.

Question 8

Hess Law

Answer 8

? The enthalpy change in a reaction is independent of the route taken

Question 9

Lattice enthalpies

Answer 9

? THe measure of the strength of the forces between ions in an ionic solid.
Factors that increase attraction
- High charge
- Small size

Question 10

Enthalpy of atomisation

Answer 10

? Enthalpy changes when one mole of gaseous atom is formed from its elements in its standard states. (may not happen in standard conditions)

Question 11

Electron affinity (opposite of ionisation)

Answer 11

? Enthalpy changes when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous ions

Question 12

Lattice formation enthalpy

Answer 12

? The enthalpy of lattice formation is the standard enthalpy change when 1 mole of an solid ionic lattice is formed its constituents ions in gaseous form

Question 13

Born Haber Cycle

Answer 13

? Used to calculate some to the types of enthalpy changes from experimental data.
? Special type of Hess Cycle that represents the formation of an ionic compound from its elements in a series of steps.

Question 14

Enthalpy of Hydration

Answer 14

? The enthalpy change when one mole of gaseous ions dissolves to form one mole of aqueous ions.

Question 15

Enthalpy of Solution

Answer 15

? The enthalpy changes when one mole of a solid crystal is dissolved in a large excess of solvent

Question 16

Entropy

Answer 16

? Measure of disorder

? Number of ways molecules can be arranged and the number of ways their energy can be arranged between them.

Question 17

Free Energy

Answer 17

? A simple way to determine if the reaction is feasible (includes both enthalpy and entropy)

Question 18

Gibbs free energy

Answer 18

? A measure of whether the reaction is spontaneous (depends on the experimental conditions)

Question 19

What do the letters in this equation represent? q=mcΔT

Answer 19

q - heat transferred
m - mass of the substance
c - specific heat capacity
ΔT - change in temperature

Question 20

How does Calorimetry work?

Answer 20

Dry the cup after washing
Put the polystyrene in a beaker for insulation and support
Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
Clamp thermometer into place making sure the thermometer bulb is immersed in the solution
Measure the initial temperatures of the solution or both solutions (if 2 are used). Do this at regular intervals of 2-3 min (or as instructed by the method)
At minute 3/6 transfer the second reagent to the cup. Solid reagent is used, then weigh the solid then add it into the cup.
Stir the mixture (ensures that all of the solution is at the same temperature)

Question 21

What are the errors in calorimetry

Answer 21

Energy transfer to the surroundings
Reaction or dissolving may incomplete
Density of solution is taken to be same as water
Assumes all solution to have a SHC of water

Question 22

How do you calculate ΔH

Answer 22

Energy (BB) - Energy (FB)

Question 23

Why do the mean bond values from the book differ from the ones calculated in the Hess cycle.

Answer 23

Remember that calculated values of enthalpy of combustions will
be more accurate if calculated from enthalpy of formation data than
if calculated from average bond enthalpies. This is because
average bond enthalpy values are averaged values of the bond
enthalpies from various compounds.

Question 24

Define Bond dissociation enthalpy

Answer 24

The bond dissociation enthalpy is the standard molar enthalpy change
when one mole of a covalent bond is broken into two gaseous
atoms (or free radicals)

Question 25

Define first ionisation energy

Answer 25

The first ionisation enthalpy is the enthalpy change required to
remove 1 mole of electrons from 1 mole of gaseous atoms to form
1 mole of gaseous ions with a +1 charge

Question 26

Equation for first ionisation energy

Answer 26

X (g) –> X+ (g) + e-

Question 27

Enthalpy of lattice dissociation

Answer 27

? The Enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form.

Question 28

Second electron affinity

Answer 28

? The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.

Question 29

Trends in Lattice enthalpies

Answer 29

Size -

Charge

Question 30

Describe a perfect ionic model

Answer 30

Theoretical lattice enthalpies assumes a perfect ionic model where the ions
are 100% ionic and spherical and the attractions are purely electrostatic.

Question 31

Why does calcium chloride have a formula of CaCl2, not CaCl or CaCl3.

Answer 31

Calculate the enthalpy of formation for each case. The one with the most exothermic enthalpy of formation will be the one that will be formed as it is the most thermodynamically stable.

Question 32

Factors affecting entropy

Answer 32

1. Temperature
2. Physical state
3. number of moles

Question 33

Equation for entropy

Answer 33

Total ΔS = ΣSsystem + ΣSsurroundings

Question 34

Unit for entropy

Answer 34

J K^-1 mol^-1

Question 35

Equation for Gibbs free energy

Answer 35

ΔG = ΔH - TΔS

the reaction will be spontaneous if G<0

Question 36

Define spontaneous

Answer 36

A process that will proceed on its
own without any external influence.

(e.g. diffusion)

Question 37

what happens to ∆G during phase changes?

Answer 37

As physical phase changes like melting and boiling are

equilibria, the ∆G for such changes is zero

Question 38

An equation to calculate enthalpy change of solution

Answer 38

ΔH solution = Δ HL dissociation + ΣΔ hydH