enthalpy and entropy Flashcards

1
Q

why are solid ionic compounds stable and have a high melting point

A
  • strength of ionic bonds ,electrostatic attractions between opposityley charged ions in the ionic lattice structure
  • this creates a substantial energy barrier must be overcome to break down the lattice
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2
Q

what is lattice enthalpy

A

measure of the strength of ionic bonding in a giant ionic lattice

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3
Q

what is the definition of lattice enthalpy

A
  • enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
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4
Q

is lattice enthalpy exothermic or endothermic and why

A
  • involves the ionic bond formation from seperate gaseous ions
  • it is exothermic change and value for enthalpy change will always be negative
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5
Q

describe the two different routes of the born haber cycle

A
  • route 1 - formation of gaseous ions, formation of gaseous atom , lattice enthalpy
  • route 2 - lattice formation from elements
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6
Q

describe the processes in route 1 in the born haber cycle

A
  • formation of gasoues atoms - changing the elements in their standard states into gaseous atoms, this change is endothermic as it involves bond breaking
  • formation of gaseous ions - changing the gaseous atoms into positive and negative gaseous ions , overall change is endothermic
  • lattice formation - changing the gasoues ions into the solid ionic lattice
    this is the lattice enthalpy and is exothermic
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7
Q

describe the processes in route 2 of the born haber cycle

A
  • converts the elements in thir standard state directly to the ionic lattice, one enthalpy change
  • enthalpy change of formation
  • this is exothermic
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8
Q

what is the standard enthalpy change of formation

A

is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states

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9
Q

what is the compound formed when standard enthalpy change of formation occurs

A

ionic compound in its solid lattice

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10
Q

what is the standard enthalpy change of atomisation

A

is the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions

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11
Q

is the standard enthalpy change of stomisation endothermic or exothermic and why

A
  • endothermic
  • bond are broken to form gaseous atoms
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12
Q

what is the first ionisation energy

A
  • enthalpy change required to remove one electron from each atom in one mole of gasouess atoms to form one mole of gasoues 1+ ions
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13
Q

are ionisation energies endothermic or exothermic explain why

A
  • endothermic because energy is required to overcome the attraction between a negative electron and the positive nucleus
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14
Q

what is electron affinity the opposite of ionisation energy

A
  • measures the energy to gain electrons
  • ionisation energy measures the energy to lose electrrons
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15
Q

what is the first electron affinity

A

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

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16
Q

are first electron affinities endothermic or exothermic and why

A
  • exothermic as electron being added is attracted in towards the nucleus
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17
Q

are second electron affinities are exothermic and endothermic and explain why

A
  • endothermic
  • second electron being gained by a negative ion which repels the electron away
  • so energy must be put in to force the negativley charged electron onto the negative ion
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18
Q

describe the process of dissolving ionic compounds

A
  • water molecules able to break up the giant ionic lattice struture
  • overcome the strong electrostatic attractions between oppositley charged ions
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19
Q

what is the standard enthalpy of solution

A
  • enthalpy change that takes place when one mole of solute dissolves in a solvent
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20
Q

what happens if the solvent is water in the standard enthalpy change of solution

A
  • ions from the ionic lattice finish up surrounded with water molecules as aqueous ions
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21
Q

is the enthalpy chane of solution be endothermic or exothermic

A
  • can be both
  • the attraction exists in three dimensions so there will alo be ions and water molecules above and below the plane of the paper
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22
Q

describe how water molecules attract the ions in the ionic lattice

A
  • negative diple on the oxygen atom is attracted to the positive ion
  • the hydrogen atoms is attracted to the negative ion
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23
Q

describe the dissolving process

A
  • when a solid ionic compound dissolved in water two processes take place:
  • the ionic lattice breaks up
  • water molecules are attracted to and surround the ions
24
Q

what are the two types of energy changes involved in dissolving

A
  • ionic lattice is broken up forming sepetrate gaseous ions
  • opposite energy from lattice energy which forms the ionic lattice from gaseous ions
  • the seperate gaseous ions interact with polar water molecules to form hydrated aqueous ions the energy change involved is the enthalpy change of hydration
25
Q

what is the enthalpy change of hydration

A
  • the enthalpy change that accompanies the dissolving of gasoues ions in water to form one mole of aqueous ions
26
Q

is the enthalpy change of solution exothermic or endothermic

A
  • an be both depending on the relative sizes of lattice enthalpy and the enthalpy of hydration
27
Q

what are the general properties of ionic compounds

A
  • high metling and boiling points
  • soluble in polar solvents
  • conduct electricity when molten or in aqueous solutions
28
Q

what is a role of ionic compounds

A
  • coat the inside of furnaces
29
Q

describe the effect of ionic size on melting point

A
  • ionic radius increases
  • attraction between ions decreases
  • lattice energy is negative
  • melting point decreases
30
Q

describe the effect of ionic charge on melting point

A
  • ionic charge increases
  • attraction between ions increase
  • lattice energy becomes more negative
  • melting point increases
31
Q

why are metal oxides used to coat the inside of furnaces

A
  • very exothermic lattice enthalpies and very high melting points
32
Q

describe how ionic radius affects hydration energies

A
  • ionic radius increases
  • attraction betwen ion and water molecules decrease
  • hydration energy less negative
33
Q

desribe the effect of ionic charge on hydration enthalpy

A
  • ionic charge increases
  • attraction with water molecules increases
  • hydration energy becomes more negative
34
Q

describe how you can predict solutbility

A
  • requires a quantity of energy equal to the lattic enthalpy
  • if the sum of hydration enthalpues is larger than the magnitude of lattice enthalpy the overall enthalpy change (enthalpy change of solution) will be exothermic and compound should dissolve
35
Q

how cn entropy be used to explain things that occur naturally

A
  • gas spreading through a room
  • heat from a fire spreading through a room
  • ice meltingn in a hot room
36
Q

describe entropy

A
  • energy is dispersd and become more spread out
  • natural tendency for energy to spreat our rather than concentrated in one place
  • greater the entropy the greater the dispersal of energy the greater the disorder
37
Q

what is the term entropy mean

A
  • dispersal of energy within chemicals making up the chemical system
38
Q

what are the units of entropy

39
Q

describe the correlation of entropies with states

A
  • solids have the smallest entropies
  • liquids have greater entorpies
  • gases have the greatest entropies
40
Q

how do you predict entropy changes

A
  • at 0K there would be no energy and all substances would have an entropy value of 0
  • above 0K energy becomes dispersed amongst the particles and all substances have a positive entropy
  • systems that are more chaotic have a higher entropy value
41
Q

describe how you can determine if an entropy change is positive or negative

A
  • if a system changes to become more random energy can be spread out more - there will be a positive entropy change
  • if a system changes to become less random energy becomes more concentrated - entropy change will be negative
42
Q

what happens when susbtances change state from a solid to a liquid to a gas

A
  • it entropy increases
  • melting and boiling increae the randomness of particles
  • nergy is spread out more and entropy is positive
43
Q

does entropy increase or decrease from a solid to a liquid

44
Q

does entropy increase or decrease from a liquid to a gas

45
Q

why does entropy increases from solid->liquid->gas

A
  • changes is state give a more random arrangement of particles
46
Q

describe what happens to entropy when calcium carbonate reacting with hydrochloric acid

A
  • increase in entropy
  • production of a gas increase and the disorder of particles
  • energy is spread out more and entropy is positive
47
Q

describe what happens to the entropy when nitrogen and hydrogen gas react

A
  • 4 moles of gas -> 2 moles of gas
  • there is a decreases in the randomness of particles
  • energy is spread out less and entropy is negative
48
Q

what is the standard entropies and why is it standard

A
  • entropy of one mole of a substance under standard conditions
  • units JK-1Mol-1
  • are always positive
49
Q

how do you calculate entropy changes

A
  • entropy of products - entropy of reactants
50
Q

what does the term feasibility mean

A
  • used to describe wether a reactions isabel to happen and is energetically feasible
  • spontaneous - energetically feasible
51
Q

what is free energy

A
  • overall change in energy during a chemical reaction called free energy change
  • enthalpy change - heat transfer between the chemical system and the surroundings
  • entropy change at the temperature of the reaction - dispersal of energy within the chemical system itself
52
Q

what is gibbs equation

A

enthalpy change - (temeprature x entropy change)

53
Q

describe the link of a reaction being feasible and free energy

A
  • free energy < 0
  • the value for enthalpy larger than entropy
54
Q

describe how endothermic reactions take place at room temeprature

A
  • some ionic compounds idssolve in water at room temperaure in endothermic processes cooling down the water
55
Q

describe the limitations of predications made for feasibility and how it is overcome

A
  • very large activation energy resulting in a very slow rate
  • catalyst is used allows the reaction to take place via an alternative route with a lower activation energy
  • reduced energy barrier then allows the reaction to take place