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OCR A a-level chemistry > Module 3 - [ch9] (enthalpy) Module 5 - [ch22a] (lattice energy) > Flashcards

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

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1
Q

what is a system in a chemical reaction

A

atoms and bonds involved in a chemical reaction

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2
Q

explain law of conservation

A

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

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3
Q

endothermic reaction

A

energy taken in to break bonds
ΔH is positive

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4
Q

What is meant by the term exothermic? [1]

A

energy released to make bonds
ΔH is negative

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5
Q

activation energy

A

minimum energy required for a reaction to take place

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6
Q

standard conditions

A

1atm - 101kPa
298K - 25 degrees

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7
Q

what is standard state

A

the state an element/compound exists in at standard conditions

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8
Q

Define the term enthalpy change of formation. [2]

A

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

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9
Q

define the term enthalpy change of combustion [2]

A

change in energy when 1 mole of substance is completely combusted

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10
Q

enthalpy change of neutralisation

A

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

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11
Q

enthalpy change of reaction

A

energy change associated with a given reaction

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12
Q

enthalpy change {}

A

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

shc = 4.18g^-1K^-1

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13
Q

what are the advantages to using a bomb calorimeter

A

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

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14
Q

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

A
  • heat is released to surroundings
  • non-standard conditions
  • incomplete combustion (reaction may not go to completion)
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15
Q

average bond enthalpy [2]

A

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

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16
Q

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

A

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

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17
Q

State le Chatelier’s principle [1]

A

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

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18
Q

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

A

many different hydrocarbons would form
OR
activation energy very high

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19
Q

Why do bond enthalpies have positive values? [1]

A

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

20
Q

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

A

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

21
Q

what is Hess’ law

A

enthalpy change is independent of route taken

22
Q

what is lattice enthalpy

A

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

23
Q

born haber cycle

A

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

24
Q

define standard enthalpy of atomisation

A

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

25
Q

why is the value of enthalpy of atomisation always positive

A

endothermic process
bonds are broken to form gaseous atoms

26
Q

Atomisation of :
sodium
chlorine

A

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

27
Q

why are ionisation energies always positive

A

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

28
Q

what is electron affinity

A

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

29
Q

first electron affinity

A

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

30
Q

why are first electron affinities exothermic

A

electron being added is attracted in towards the nucleus

31
Q

why are second electron affinities endothermic

A

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

32
Q

define standard enthalpy change of solution

A

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

33
Q

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

A

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

34
Q

in enthalpy of solution what mass is used

A

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

35
Q

define enthalpy change of hydration

A

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

36
Q

describe the dissolving process of a solid ionic compound

A

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

37
Q

lattice energy born haber cycle , fewer steps

A

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

38
Q

2 factors affecting lattice enthalpy

A

ionic size
ionic charge

39
Q

what is the effect of ionic size in ionic compounds

A

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

40
Q

what is the effect of ionic charge in ionic compounds

A

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

41
Q

what are the factors affecting hydration

A

ionic size
ionic charge

42
Q

how does ionic charge affect hydration

A

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

43
Q

how does ionic size affect hydration

A

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

44
Q

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

A

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

45
Q

when would a compound dissolve
(in terms of enthalpies)

A

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

OCR A a-level chemistry (11 decks)

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