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chemistry > 5.5 Enthalpy and Entropy > Flashcards

5.5 Enthalpy and Entropy Flashcards

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1
Q

what is enthalpy change of atomisation

A

the enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state
Na(s) -> Na(g)

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2
Q

what is first ionisation energy

A

energy required to remove the 1st electron from each atom in 1 mole of atoms in its gaseous state
Na(g) -> Na+(g) + e-

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3
Q

what is electron affinity

A

enthalpy change when every atom in 1 mol of gaseous atom acquires an electron to form 1 mol of gaseous ions

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4
Q

what does first electron affinity involve?

A

the electron being gained experiencing an attraction to the nucleus
is an exothermic reaction

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5
Q

what does second electron affinity involve?

A

the electron being gained experiencing repulsion by the negatively charged ions

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6
Q

what is enthalpy of formation

A

the enthalpy change when 1 mole of a compound is made from its elements in their standard states
H2(g) + 1/2O2(g) -> H2O(l)

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7
Q

what does Hess’ law state

A

the enthalpy change for a chemical reaction depends only on the initial and final states and is independent on the path followed.
or
whatever route in which the reaction proceeds the overall enthalpy change for the reaction will be the same.

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8
Q

Born-Haber cycles

A

used primarily as a means of calculating lattice enthalpies which cannot otherwise be measured directly
made of many separate sub-processes each with their own enthalpies

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9
Q

what is the process of the Born-Haber cycle

A
  1. atomisation
  2. 1st ionisation
  3. atomisation
  4. electron affinity
  5. lattice enthalpy
  6. formation
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10
Q

what are exothermic reactions

A 
release energy (get hot)
makes bonds
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11
Q

what are endothermic reactions

A 
absorb energy (get cold) 
breaks bond
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12
Q

why do solid ionic lattices form

A

not only as a result of the transfer of electrons, but also the huge release in energy that occurs when the 2 ions of opposite charge combine to form a solid

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13
Q

what is lattice enthalpy

A

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

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14
Q

how do you calculate standard enthalpy change of solution

A
  1. calculate energy change (q) in kJ using q=mcT
  2. calculate amount in moles of the thing that’s being dissolved
  3. calculate standard enthalpy change of solution using △H=q/n
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15
Q

what is enthalpy change of solution

A

the enthalpy change for 1 mole of solute dissolving

the process of dissolving can be endo or exothermic

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16
Q

what happens when a solid dissolves- breakdown

A
  1. breakdown of the ionic lattice into monatomic gas ion
    opposite to lattice enthalpy
    K+ + Cl- -> KCl(s)
    KCl(s) -> K+ + Cl-
17
Q

what happens when a solid dissolves- hydration

A

hydration of gaseous ions by water molecules
ions bond to water
is exothermic
slight + ion are attracted to slight - oxygen
slight - ion are attracted to slight + hydrogen

18
Q

what is enthalpy change of hydration

A

enthalpy change for 1 mole of gaseous ions dissolving in water
energy released when the ions form bonds with water molecules
different ions have different enthalpy changes

19
Q

what happens when a solid dissolves

A

breaking the lattice is endothermic

water interacting with the gaseous ions is exothermic

20
Q

what happens if the lattice is….

A

if lattice enthalpy > enthalpy of hydration, then the enthalpy of solution will be endothermic
if lattice enthalpy < enthalpy of hydration, then the enthalpy of solution will be exothermic

21
Q

what is the relation between ionic size of atoms and lattice enthalpy

A

as the radius gets smaller, there is an increased attraction, as the ions closer together.
this means that lattice enthalpy increases

22
Q

what is the relation between charge and lattice enthalpy

A

as charge increases, there is more lattice enthalpy. this is because it can form stronger bonds and so a larger attraction
ion charge is more important than ionic size

23
Q

enthalpy change of hydration

A

as ionic radius decreases, the enthalpy change of hydration becomes more negative
hydration depends on the ability of an ion to attract and bond with water. smaller molecules exert more attraction

24
Q

what is the relation of lattice enthalpy, enthalpy change of solution and enthalpy change of hydration

A

enthalpy change of solution is more affected by lattice enthalpy than enthalpy change of hydration
if lattice enthalpy is smaller than hydration, the enthalpy of solution is exothermic
if lattice enthalpy is greater than hydration, the enthalpy of solution is endothermic

25
Q

entropy

A

measure of the dispersal of energy in a system, which is greater the more disordered the system
has an irregular arrangement of particles and so has a high entropy.
has a regular arrangement of particles and so has a low entropy
symbol is S
units is JK-1mol-1

26
Q

what is an idea of entropy

A

nature tends to move from an ordered state to one of disorder
a system becomes energetically more stable when it becomes less ordered

27
Q

what are factors that affect entropy

A
  • when a solid becomes a gas
  • when a solid lattice dissolves
  • when the number of gaseous molecules increases
28
Q

explanation- when a solid becomes a gas

A

solids wobble around a fixed point

gas has high randomness

29
Q

explanation- solid lattice dissolves

A

dissolving increases entropy

moves freely

30
Q

explanation- more gas

A

more particles means there’s more ways they and their energy can be arranged

31
Q

what is the equations for entropy

A

entropy= sum of products- sum of reactants

32
Q

spontaneous changes

A

occurs in 1 particular direction and takes place without the need for work
exothermic reactions are usually spontaneous

33
Q

what is free energy

A

a reaction is only spontaneous if it can do work and must generate a free potential
- free energy (△G) indicates a reaction capable of proceeding of its own accord

34
Q

free energy and spontaneity

A

if free energy is < 0, it is negative and a spontaneous reaction
if free energy is = 0, the system is in equilibrium
if free energy is > 0, it is positive and a non-spontaneous reaction

35
Q

what is the 2nd law of thermodynamics

A

based on entropy
entropy tends to a maximum
infers that all chemical and physical changes involve an overall increase in entropy

36
Q

how to increase entropy

A 
solids melt
liquids boil 
ionic substances dissolve in water 
number of gas molecules increases 
temperature increases
37
Q

what is the free energy equation

A

△G= △H-T△S
free energy = enthalpy change - (entropy X temperature)
if free energy in a reversible reaction at equilibrium is 0,
△S= △H/T or △H=T△S

chemistry (10 decks)

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