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Chemistry > Energetics > Flashcards

Energetics Flashcards

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1
Q

in what state are bond enthalpies

A

gaseous state (always)

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2
Q

Properties of endothermic reactions

A
  • heat is absorbed from the surroundings
  • the temperature of the surroundings decrease
  • the temperature of the system increases
  • reactants are more stable
    ΔH>0
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3
Q

Properties of exothermic reactions

A
  • heat is released to the surroundings
  • the temperature pf the surroundings increase
  • the temperature of the system decrease
  • products are more stable
    ΔH<0
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4
Q

Examples of endothermic reactions

A

bond breaking, photosynthesis, solid→liquid

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5
Q

Examples of exothermic reactions

A

bond making, combustion, neutralisation, gas→liquid

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6
Q

How to find the enthalpy change and activation energy on an energy profile

A

enthalpy change: from reactants to products
activation energy: from reactants to the hump

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7
Q

Define the standard enthalpy of formation

A

the energy change when of one mole of a compound is formed from its elements under STP

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8
Q

Define standard enthalpy of combustion

A

the energy change when one mole of a compound is burnt in excess oxygen under STP

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9
Q

Define standard enthalpy of neutralisation

A

the energy change when on mole of water is produced by an acid and base reacting under STP

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10
Q

How to calculate the enthalpy change of formation

A

enthalpy change of formation of the products - enthalpy change of formation of the reactants

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11
Q

What enthalpy change of formation do elements have

A

0

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12
Q

How to calculate the enthalpy change of combustion

A

enthalpy change of combustion of the reactants - enthalpy change of combustion of the products

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13
Q

How to calculate the enthalpy change of a reaction

A

q = m x c x ΔT
m = mass of water or solution (g)
c = specific heat capacity of water (assumption)
ΔT = temperature change
in Joules

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14
Q

How to calculate the enthalpy change

A

ΔH = q/mols
- if the temperature of the substance has increased, ΔH is negative
- mols = limiting reactant

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15
Q

Assumption being made when calculating the ΔH

A
  • no heat losses
  • complete combustion
  • the specific heat capacity of the solution is the same as pure water
  • the density of the solution is the same as pure water
  • occurs under standard conditions
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16
Q

How to make a temperature correction graph

A

1) Make a temperature reading before adding the reactants
2) Add the second reactant and continue recording the temperature and time
3) Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added

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17
Q

What assumption is being made with graph

A

An assumption made here is that the rate of cooling is constant

18
Q

Define average bond enthalpy

A

the energy needed to break one mole of gaseous molecules averaged ver similar compounds

19
Q

Why are average bond enthalpies not very accurate

A

they are average values and do not take into account situations in which the bond strength is different from the average

20
Q

How to calculate the enthalpy change of reaction

A

bond broken - bonds made

21
Q

Why are different wavelengths needed to break oxygen and ozone

A

oxygen = bond order of 2
ozone = bond order of 1.5

22
Q

What are the units in the formula for energy

A

q = in joules
m = grms
enthalpy change = kilojoules

23
Q

Define enthalpy of atomisation

A

the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard states

24
Q

What is the formula for the enthalpy of atomisation

A

X (s) –> X (g) (endothermic)

25
Q

Define lattice enthalpy

A

the enthalpy change when 1 mole of a solid ionic compound is separated into its gaseous atoms (STP)

26
Q

What is the formula for lattice enthalpy

A

XY (s) –> X- (g) + Y+ (g)

27
Q

Define electron affinity

A

the enthalpy change when 1 mole of electrons are added to 1 mole of gaseous atoms

28
Q

What is the formula for electron affinity

A

X(n-1)- + e- –> X(n)-

29
Q

Define the enthalpy of hydration

A

the enthalpy change when one mole of gaseous ions is dissolved in water to give one mole of aqueous ions and a solution of infinite dilution

30
Q

What is the formula for enthalpy of hydration

A

X- (g) –> X- (aq)

31
Q

Define bond-dissociation enthalpy

A

the enthalpy change when 1 mole of bonds are broken into gaseous atoms

32
Q

What is the formula for bond-dissociation enthalpy

A

X2 (g) –> 2X (g)

33
Q

What are the factors that affect lattice enthalpy

A
  • ionic charge
  • ionic radius
34
Q

What is the effect of ionic charge on the lattice enthalpy

A

the greater the ionic charge, the greater the force of attraction and the higher the lattice enthalpy (more exothermic)

35
Q

What is the effect of ionic radius on the lattice enthalpy

A

the smaller the ionic radius, the greater the force of attraction and the higher the lattice enthalpy (more exothermic)

36
Q

Outline/explain how enthalpy of hydration works

A

when solid ionic dissolves in water, ion-dipole forces are formed (cus water is polar)
- oxygen is attracted to positive ions
- hydrogen is attracted to negative ions

37
Q

Outline the dissolution cycle

A

ΔH solution: solid ionic→ hydrated ions
ΔH hydration: gas ions → hydrated ions
ΔH lattice: solid ionic → gas ions

38
Q

Outline the steps in the Born-Haber cycle

A

1) enthalpy of formation
2) enthalpy of atomisation
3) ionisation energy
4) electron affinity
5) lattice enthalpy

39
Q

What are the factors that affect entropy

A
  • changes in temperatures
  • changes in state
  • dissolving
  • changes in amounts of gaseous reactants or products
40
Q

What is the unit for entropy

A

Joules

41
Q

Outline the relationship between entropy and Gibbs energy

A

+ΔH, -ΔS: never spontaneous
+ΔH, +ΔS: spontaneous at high temp.
-ΔH, -ΔS: spontaneous at low temp.
-ΔH, +ΔS: always spontaneous

Chemistry (12 decks)

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