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C3 > Enthalpy changes > Flashcards

Enthalpy changes Flashcards

1
Q

define enthalpy (H)

A

heat content of reactions

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2
Q

define enthalpy change, ΔH

A

a change in the heat energy of a system, measured under constant pressure

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3
Q

what are exothermic reactions

A
  • reactions where energy is given to the surroundings (-ve enthalpy change)
  • less energy taken in to break bonds than released when making bonds
  • enthalpy of products is smaller that that of the reactants
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4
Q

what are endothermic reactions

A
  • reactions where energy is taken in from the surroundings (+ve enthalpy change)
  • more energy taken in to break bonds than released when making bonds
  • enthalpy of products is greater than that of reactants
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5
Q

what is activation energy Ea

A

the minimum energy required for a reaction to take place

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6
Q

pressure under standard conditions

A

100kPa

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7
Q

temperature under standard conditions

A

298K = 25C

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8
Q

concentration under standard conditions

A

1 moldm-3

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9
Q

define standard state

A

physical states under standard conditions

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10
Q

define standard enthalpy change of reaction

A
  • enthalpy change when 1 mole of a substance is reacted under standard conditions
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11
Q

example of equation for standard enthalpy change of reaction ( Mg and O2)

A

Mg(s) + 1/2O2(g) –> MgO(S)

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12
Q

define standard enthalpy change of formation

A
  • enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions
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13
Q

example of equation for standard enthalpy change of formation (H2 and O2)

A
  • H2(g) + 1/2O2 —> H2O (l)
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14
Q

Define standard enthalpy change of combustion, ΔH°c

A
  • enthalpy change when 1 mole of a substance combusts completely with oxygen under standard conditions
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15
Q

example of equation for standard enthalpy change of combustion, ΔH°c
(C4H10 and O2)

A

C4H10(g) + 6.5O2(g) —> 4CO2(g) + 5H2O(l)

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16
Q

Define standard enthalpy change of neutralisation

A
  • enthalpy change when an acid and a base react together to form 1 mole of liquid water under standard conditions
17
Q

example of equation for standard enthalpy change of neutralisation (HCl and NaOH)

A

HCl (aq) + NaOH (aq) —> NaCl (aq) + H2O (l)

18
Q

define average bond enthalpy

A

average energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission

19
Q

example of equation for average bond enthalpy (CF4)

A

1/4CF4(g) —> 1/4 C (g) + F (g)

20
Q

do all reactions consist of an endothermic and an exothermic part

A

YES

21
Q

how do you work out overall enthalpy change

A

Bb - Bf = O (overall)

22
Q

equation for calculating heat change

A

q = mcΔT
q = heat change (J)
m = mass of solution, ignore solid (g)
c = specific heat capacity (Jg-1K-1)
ΔT = temperature change (K)
- then q/n

23
Q

what’s coffee cup calorimetry used for

A

to directly determine the enthalpy change of reaction that involves 2 chemicals in a solution when mixed together

24
Q

why is a polystyrene cup used in coffee cup calorimetry

A

insulating material - reduces heat loss from reaction to the surroundings

25
Q

since density of water is 100gdm-3, 1cm3 is equal to

A

1g

26
Q

what’s copper calorimetry used for

A
  • used to indirectly determine the enthalpy change of combustion
  • involves burning a fuel. The heat transferred from this reaction determined via measuring temp increase of a known mass of water
27
Q

enthalpy change of neutralisation is always
sum(r) - sum(p)

A

-ve

28
Q

enthalpy change of combustion is always
sum(r) - sum(p)

A

-ve

29
Q

enthalpy change of formation can be
sum(p) - sum(r)

A

+ve or -ve

30
Q

enthalpy change of reaction can be
sum(p)-sum(r)

A

+ve or -ve

31
Q

for enthalpy change of formation (and reaction) what goes at the bottom of the Hess cycle

A

elements in standard states (arrows point away from these)

32
Q

for enthalpy change of combustion (or neutralisation) what goes at the bottom of the Hess cycle

A
  • CO₂(g) + H₂O(l) (arrows point towards these)
  • for neutralisation you can also have salt and water
33
Q

bond breaking is…

A

endothermic

34
Q

bond breaking is…

A

exothermic

35
Q

bond enthalpy values are always

A

+ve

36
Q

which species don’t need arrows pointing towards or away from them

A

elements

37
Q

why do calculated enthalpy values different from those in databases

A

….

C3 (7 decks)

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