Module 3 - [ch9] (enthalpy) Module 5 - [ch22a] (lattice energy)

Question 1

what is a system in a chemical reaction

Answer 1

atoms and bonds involved in a chemical reaction

Question 2

explain law of conservation

Answer 2

energy in an isolated system remains the same
energy cannot be created nor destroyed. only transferred from one form to another

Question 3

endothermic reaction

Answer 3

energy taken in to break bonds
ΔH is positive

Question 4

What is meant by the term exothermic? [1]

Answer 4

energy released to make bonds
ΔH is negative

Question 5

activation energy

Answer 5

minimum energy required for a reaction to take place

Question 6

standard conditions

Answer 6

1atm - 101kPa
298K - 25 degrees

Question 7

what is standard state

Answer 7

the state an element/compound exists in at standard conditions

Question 8

Define the term enthalpy change of formation. [2]

Answer 8

energy change when1 mole of substance is formed from its constituent elements in their standard state under standard conditions

Question 9

define the term enthalpy change of combustion [2]

Answer 9

change in energy when 1 mole of substance is completely combusted

Question 10

enthalpy change of neutralisation

Answer 10

energy change when 1 mole of water is produced from a neutralisation reaction

Question 11

enthalpy change of reaction

Answer 11

energy change associated with a given reaction

Question 12

enthalpy change {}

Answer 12

Q = mcΔT
energy change = mass * specific heat capacity * change in temperature

shc = 4.18g^-1K^-1

Question 13

what are the advantages to using a bomb calorimeter

Answer 13

minimises heat loss to surroundings
pure oxygen used to ensure complete combustion

Question 14

Suggest three reasons why standard enthalpy changes of combustion determined
experimentally are less exothermic than the calculated theoretical values [3]

Answer 14

• heat is released to surroundings
• non-standard conditions
• incomplete combustion (reaction may not go to completion)

Question 15

average bond enthalpy [2]

Answer 15

average energy required to break 1 mole of bonds in gaseous molecules

Question 16

Use ideas about the enthalpy changes that take place during bond breaking and bond
making to explain why some reactions are exothermic. [2]

Answer 16

bond breaking absorbs energy whereas bond making releases energy
in some reactions more energy is released than absorbed, these reactions are exothermic

Question 17

State le Chatelier’s principle [1]

Answer 17

The position of equilibrium will shift so as to minimise the effect of any change in conditions

Question 18

It is very difficult to determine the standard enthalpy change of formation of
hexane directly. Suggest a reason why [1]

Answer 18

many different hydrocarbons would form
OR
activation energy very high

Question 19

Why do bond enthalpies have positive values? [1]

Answer 19

bond breaking is endothermic so energy has to be put in to break a bond

Question 20

Write the equation, including state symbols, that represents the standard
enthalpy change of formation for carbon monoxide, CO [2]

Answer 20

C(s) + 1/2O2(g) -> CO(g)

Question 21

what is Hess’ law

Answer 21

enthalpy change is independent of route taken

Question 22

what is lattice enthalpy

Answer 22

measure of the strength of the ionic bonding in a giant ionic lattice
the enthalpy change when 1mole of an ionic compound is formed from its gaseous ions under standard conditions

Question 23

born haber cycle

Answer 23

gaseous ions at top
/\ |
| |
gaseous atoms |
/\ | lattice energy
| |
elements in |
standard states |
| |
| |
\/ \/
ionic lattice

Question 24

define standard enthalpy of atomisation

Answer 24

the enthalpy change when 1 mole of gaseous atoms are formed from the elements in its standard state under standard conditions

Question 25

why is the value of enthalpy of atomisation always positive

Answer 25

endothermic process bonds are broken to form gaseous atoms

Question 26

Atomisation of : sodium chlorine

Answer 26

Na(s) -> Na(g) 1/2 Cl2(g) -> Cl(g)

Question 27

why are ionisation energies always positive

Answer 27

energy is required to overcome the attraction between a negative electron and the positive nucleus endothermic

Question 28

what is electron affinity

Answer 28

measure of energy to gain electrons ( opposite of ionisation energy )

Question 29

first electron affinity

Answer 29

the energy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 30

why are first electron affinities exothermic

Answer 30

electron being added is attracted in towards the nucleus

Question 31

why are second electron affinities endothermic

Answer 31

a second electron is being gained by a negative ion which repels the electron energy must be put in to overcome this repulsion and force another electron in

Question 32

define standard enthalpy change of solution

Answer 32

the enthalpy change that takes place when one mole of a solute dissolves in a solvent

Question 33

in an enthalpy change of solution, when would the products be in their aqueous state and why

Answer 33

when the solvent is water ions from ionic lattice surrounded by water molecules delta positive H+ attracted to negative ions delta negative O~ attracted to positive ions

Question 34

in enthalpy of solution what mass is used

Answer 34

mass of the solution mass of the water + mass of the solid

Question 35

define enthalpy change of hydration

Answer 35

the energy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions

Question 36

describe the dissolving process of a solid ionic compound

Answer 36

the ionic lattice breaks up water molecules are attracted to and surround the ions

Question 37

lattice energy born haber cycle , fewer steps

Answer 37

Gaseous ions | | | | | | hydration | | | \/ | aqueous ions | /\ | | dissolving ( solution ) \/ | ionic lattice

Question 38

2 factors affecting lattice enthalpy

Answer 38

ionic size ionic charge

Question 39

what is the effect of ionic size in ionic compounds

Answer 39

as ionic radius increases attraction between ions decreases lattice energy increases (less negative) melting point decreases

Question 40

what is the effect of ionic charge in ionic compounds

Answer 40

as ionic charge increases attraction between ions increases lattice energy becomes more negative ( decreases ) melting point increases

Question 41

what are the factors affecting hydration

Answer 41

ionic size ionic charge

Question 42

how does ionic charge affect hydration

Answer 42

as ionic charge increases attraction with water molecules increases hydration energy becomes more negative

Question 43

how does ionic size affect hydration

Answer 43

increasing ionic radius decreased attraction between ion and water molecules hydration energy less negative

Question 44

how much energy is required to dissolve an ionic compound in water

Answer 44

attraction between ions in the ionic lattice must be overcome energy = to lattice enthalpy needed

Question 45

when would a compound dissolve (in terms of enthalpies)

Answer 45

if the sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy compound should dissolve enthalpy change of solution will be exothermic