Energetics (I and II)

Question 1

how do you calculate bond enthalpies

Answer 1

sum of bonds broken - sum of bonds made

Question 2

why are calculated bond enthalpies different from actual enthalpies

Answer 2

Because bond enthalpies only apply for gaseous compounds

Question 3

Why do experimental values differ from theoretical values?

Answer 3

Calculations assume ions are perfectly spherical

A lot of ionic compounds have a degree of covalent character

Question 4

Definition of (standard ) enthalpy change of formation

Answer 4

Enthalpy change when one mole of a compound from its elements (under standard conditions or 298K and 100kPa)

Question 5

Definition of enthalpy change of combustion

Answer 5

Enthalpy change when one mole of a substance is completely brunt in oxygen

Question 6

Define exothermic reaction

Answer 6

When energy is transferred from system to surroundings

Usually results in temp increase

Question 7

Define endothermic reaction

Answer 7

Energy is transferred from surroundings to the system

Usually results in temp decrease

Question 8

Define enthalpy

Answer 8

The heat energy transferred between systems and surroundings at constant pressure

Question 9

Define enthalpy change of reaction

Answer 9

Enthalpy change when moles of the reactants as stated in the chemical equation90-= react together under standard conditions

Question 10

Define Hess’ Law

Answer 10

Value of enthalpy change is independent of the route taken to obtain the value providing the initial and final conditions are the same

Question 11

Define bond enthalpy

Answer 11

Enthalpy change released to broke one mole of a bond when bond is in a gaseous state

Question 12

define average bond enthalpy

Answer 12

Enthalpy change when one mole of a bond is broken and bond is averaged over several molecules and bond is in gaseous state

Question 13

define enthalpy change of neutralisation

Answer 13

enthalpy change when one mole of water is produced when an acid and alkali react together under standard conditions

Question 14

X and Y axis of an enthalpy level/ reaction profile diagrams

Answer 14

y axis: Enthalpy, H

x axis: Progress of the reaction

Question 15

relationship between enthalpy of solution, enthalpy of hydration and lattice energy

Answer 15

enthalpy of sol= enthalpy of hydration - lattice energy

Question 16

relationship between solubility and enthalpy of solution

Answer 16

the more exothermic enthalpy of solution, the more likely the molecule is soluble in aqueous solution

Question 17

how do you calculate values from a born haber cycle?

Answer 17

enthalpy of formation= all the other values

when you rearrange the equations, change the signs

Question 18

how to determine enthalpy change of combustion?

Answer 18

sum of reactants- sum of products

Question 19

how to determine enthalpy change of formation?

Answer 19

sum of products- sum of reactants

Question 20

errors in calorimetric method for measuring enthalpy change of reaction?

Answer 20

density of solution taken to be the same as water
specific heat capacity is approx.- assumed to be the same as water
energy lost to the surroundings
Thermometer inaccurate
Impure

Question 21

errors in calorimetric for measuring enthalpy change of combustion

Answer 21

the specific heat capacity of the calorimeter is ignored
incomplete combustion
evaporation of fuel and water
energy lost to the surrondings

Question 22

how to make sure when calculating enthalpy of reaction you determine max temperature change?

Answer 22

use more concentrated acid
use for finely powered solid
ensure the reaction is mixed properly
measure the temperature before adding the second reagent and then measure at regular intervals

Question 23

define ‘first electron affinity’

Answer 23

enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous -1 ions

Question 24

why is first electron affinity exothermic

Answer 24

because the ion is more stable than the atom and there is an attraction between the nucleus and the electron

Question 25

why is second electron affinity endothermic

Answer 25

because it take energy to overcome the repulsive force between the negative ion and the electron

Question 26

example of an exothermic reaction

Answer 26

bond making
combustion
enthalpy of hydration
(enthalpy of solution is substance is soluble)

Question 27

example of endothermic reaction

Answer 27

bond forming

dissociation reactions

Question 28

what makes an ionic lattice insoluble

Answer 28

if the lattice energy of dissociation is larger than hydration enthalpies

Question 29

units of delta G

Answer 29

KJ mol-1

Question 30

units of delta S

Answer 30

J mol-1 K-1

Question 31

Equation for delta G using k

Answer 31

Delta G= -RTlnk

Question 32

Why does CaO exist but not Ca2O3

Answer 32

Because the 3rd ionisation energy is higher is not being compensated by a higher lattice energy

Question 33

Define enthalpy change of solution

Answer 33

Enthalpy change when 1 mole of an ionic solid is completely dissolved in water to form an infinitely dilute solution

Question 34

Define enthalpy change of hydration

Answer 34

Enthalpy change when one mole of a gaseous in completely hydrated

Question 35

Factors that affect delta H of hydration

Answer 35

Size of the ions

Charges of the ions

Question 36

what’s the effect on enthalpy of combustion if the alcohol was blown out later than expected

Answer 36

it would be less exothermic

as there would be a great amount of heat loss

Question 37

what’s the effect on enthalpy of combustion if the initial recorded temperature is higher than a previous experiment?

Answer 37

it would be less exothermic

as the energy would be absorbed to boil the water and boiling is an endothermic reaction

Question 38

what’s the effect of using a copper beaker instead of a glass beaker on the accuracy of the experiment?

Answer 38

copper is a better conductor than glass so more heat would be transferred to the glass

Question 39

what’s the effect of blowing out a flame on enthalpy change of combustion value?

Answer 39

more evaporation
more mass is lost
less negative value of enthalpy of combustion

Question 40

why is extrapolation required to determine maximum temp change?

Answer 40

reaction is not instantaneous so some heat is lost as mixture heats up

Question 41

how can experiment be amended to allow for a value of maximum temp change to determined without extrapolation

Answer 41

use finely powered reagents as it increases the rate of reaction (it has no effect on the final amount of products however)