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A level Chemistry > Chapter 9 - Enthalpy > Flashcards

Chapter 9 - Enthalpy Flashcards

1
Q

conservation of energy?

A
  • chemical bonds are forces of attraction that bind atoms together
  • chemical energy lies between these chemical bonds
  • it’s a form of potential energy
  • No energy is lost in a chemical reaction. It’s simply CONVERTED from 1 form to another
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2
Q

what is enthalpy?

A
  • H

- the amount of thermal energy that is stored in a system

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3
Q

what CAN we measure?

A
  • we can’ measure the direct enthalpy of the reactants and products
  • instead, we CAN measure the amount of energy that is absorbed or released to the surroundings
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4
Q

we can measure the change in energy by looking at?

A

the change in thermal energy

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5
Q

temp ⬆ =

A

heat gain of surroundings

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6
Q

temp ⬇ =

A

heat loss of surroundings

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7
Q

enthalpy change =

A

delta H

the heat energy change at a constant pressure

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8
Q

delta (triangle) means

A

change

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9
Q

standard states?

A

the states substances are in under standard conditions

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10
Q

system?

A

the chemical reaction (atoms & bonds)

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11
Q

surroundings?

A

everything else

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12
Q

enthalpy change EQUATION?

A

Hproducts - H reactants

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13
Q

Exothermic reactions ?

A

release heat so deltaH is -

- enthalpy of products is smaller than enthalpy of reactants

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14
Q

endothermic reactions?

A
  • reaction absorbs heat
  • product enthalpy > reactant enthalpy
  • DeltaH is +
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15
Q

activation energy?

A
  • input of energy required to break the initial bonds & start the reaction
  • once the AE has been overcome, the energy output of the reaction provides enough energy to sustain the reaction - the reaction becomes SELF - SUSTAINING
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16
Q

SHC?

A

the energy required to raise the temp of 1g of a substance by 1K

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17
Q

calorimetry - delta H remember?

A

REMEMBER SIGN

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18
Q

what is calorimetry?

A

quantitative study of chemical energy in a chemical reaction

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19
Q

2 types of calorimetry?

A

1 - polystyrene cup

2- spirit burner

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20
Q

1 - polystyrene cup?

A

lid = insulates

  • thermometer
  • styrofoam cup - which could be held in another cup/ beaker
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21
Q

calorimetry: error - evaporation?

A

some of the alcohol/water might evaporate

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22
Q

what is average bond enthalpy?

A

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

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23
Q

what does the bond enthalpy provide us info about?

A

how much energy is needed to BREAK a bond

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24
Q

bond enthalpy mnemonic?

A 
Breaking bonds is
E
N dothermic
D
O 
M aking bonds is
E
Xothermic
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25
Q

in an endothermic reaction (bonds)?

A

more energy is needed to break the bonds in the reactants than is released when the bonds in the products are formed

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26
Q

in an exothermic reaction (bonds)?

A

more energy is given out when the bonds in the product are formed than is taken in when the bonds in the reactant are broken

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27
Q

delta H =

A

(bond enthalpy of reactants) - ( bond enthalpy of products)

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28
Q

some bonds occur in one envir but some occur in

A

multiple

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29
Q

bond strength varies based on?

A

the environment the bond is found in

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30
Q

actual bond enthalpy?

A

is specific to each individual molecule

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31
Q

use of enthalpy cycles?

A
  • the enthalpy change of a reaction cannot always be directly measured
  • we can use enthalpy cycles to help us indirectly calculate the enthalpy change
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32
Q

what is an enthalpy cycle?

A

pictorial representation showing the alternative routes of reaction between reactants and products

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33
Q

enthalpy cycles: rules?

A
  • are triangular
  • in each corner, there should be the same no. of each atom
  • like energy, matter is also conserved
  • the sides of the triangles r arrows
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34
Q

DIRECTION OF ARROW!!!!!

A

IF OTHER WAY ROUND MINUS INSTEAD OF PLUS

35
Q

MOLES!!!!

A

MULTIPLY THE BOND ENTHALPY X MOLES

36
Q

suggest why the enthalpy change of formation cannot be determined directly?

A
  • rate too slow, Ea too high, other products may be formed instead (named)
37
Q

the heat energy stored in a chemical system is called?

38
Q

⭐ enthalpy equation?

A

Delta H = H(products) - H(reactants)

39
Q

exothermic?

A
  • if there is less enthalpy in the products than in the reactants, then Delta H is negative. The energy lost from the reaction is given out to the surroundings and the reaction is described as exothermic
40
Q

endothermic reaction?

A
  • if there is more enthalpy in the products than in the reactants, then delta H is positive. The energy gained by the reaction is taken in from the surroundings and the reaction is described as endothermic
41
Q

{endo vs exo} heat is….

A

endo vs exo

taken in from the surrounding vs given out to the surroundings

42
Q

{endo vs exo} the temp?

A

endo vs exo

decreases vs increases

43
Q

{endo vs exo} enthalpy of products is…

A

endo vs exo

higher vs lower

44
Q

{endo vs exo} delta h is

A

endo vs exo

positive vs Negative

45
Q

e.g. of an exothermic reaction?

A
  • Mg + Hcl

- combustion of Mg ribbon

46
Q

example of endo reaction?

A
  • NaHCO3 + Hcl

- photosynthesis

47
Q

⭐enthalpy profile diagrams?

A

SINGLE HEADED ARROWS

48
Q

what is activation energy?

A
  • Ea

- min energy a particle must have to take part in a successful collision

49
Q

why are endothermic reactions less likely to occur than exothermic reactions?

A

endothermic reactions have a much larger Ea

see reaction profile diagram

50
Q

what is the value for standard pressure?

51
Q

standard temperature?

52
Q

standard conc?

A

1moldm(^-3)

53
Q

standard states?

A

the standard state of a substance is its most stable state under standard conditions e.g. H2O - liquid

54
Q

⭐standard enthalpy change of reaction DEFINITION?

A

The enthalpy change when amounts of reactants AS SHOWN IN THE REACTION EQUATION react together under standard conditions to give products in their standard states

55
Q

⭐standard enthalpy change of formation DEFINITION?

A
  • The enthalpy change when ONE MOLE OF A COMPOUND IS FORMED from its elements under standard conditions. Both the compound and elements are in their standard states - 298K, 100kPa
56
Q

⭐ standard enthalpy change of neutralisation DEFINITION?

A

The enthalpy change when 1 MOLE OF WATER IS FORMED from the reaction of an acid with a base under standard conditions. both the reactants and products are in their standard states

57
Q

⭐in definitions,?

A

always quote standard temp and pressure

58
Q

⭐quantity of energy equation?

A

q = mcΔT

- Q is in J

59
Q

ALWAYS DIVIDE Q BY?

A

1000 TO GET ANSWER IN kJ

60
Q

how to calculate enthalpy change from experimental data?

A
  • q(in kJ)/ Moles
61
Q

⭐ calculating enthalpies from experimental data remember?

A
  • ADD THE SIGN

- if the reaction is exothermic (temp increases) then ΔH is - and if the reaction is endo (temp decreases) then ΔH is +

62
Q

q:?

A
  • Quantity of energy
  • Joules
  • ^/1000 to get kJ
63
Q

m:?

A

mass of water heated

64
Q

c:?

A

SHC of WATER

4.18Jg(^-1)K(^-1)

65
Q

ΔT:?

A
  • temp change

- K (but change is the same in degrees to)

66
Q

explain why the ΔcH for the experiment is less exothermic than the published data book values?

A
  • less exo bc:
  • 1 - not under standard conditions
    2 - heat loss to surroundings and apparatus (look if exo or endo)
    3 - incomplete combustion
67
Q

as the number of carbon atoms in the alcohol ⬆,

A

ΔcH gets more exothermic

68
Q

bond breaking is an ______ process?

A

endothermic

69
Q

bond forming is an ______ process?

A

exothermic

70
Q

⭐ average bond enthalpy definition?

A

the average enthalpy change that takes place when breaking 1 mole of given bonds in molecules in the gaseous phase forming gasesous atoms.
e,g. CL2(g) -> 2CL(g)

71
Q

why might the exact value of a bond enthalpy be different from the average bond enthalpy?

A

the exact value of a bond enthalpy depends upon the paticular compound in which the bond is found

72
Q

The values for bond enthalpies are always ? and why

A

+ as the value is given for bond breaking which is an endo process

73
Q

whic state are reactants and products always in for bond enthalpeis?

74
Q

to calculate energy change using bond enthalpies, must:

A

1 - write out the balanced equation using DISPLAYED FORMULA

2- use the equation ΔrH = (bond enthalpies of reactants) - bond enthalpies of products

75
Q

enthalpy cycles - conservation of energy?

A

energy is neither created nor destroyed in a chemical reaction

76
Q

what does Hess’s Law state?

A

“the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place provided the initial and final conditions are the same”

77
Q

what do energy cycles allow us to do?

A
  • calculate the enthalpy change for reactions that do not occur directly or cannot be found by experiment (e.g. too dangerous or too slow)
78
Q

⭐using hess’s cycles to calculate ΔHr⦵ RULES?

A

1 - Look at the enthalpy data given in the question

2 - Draw the appropriate enthalpy cycle

79
Q

when ΔfH⦵ data is given?

A
  • in the circle, add elements of the reactants and products
  • Formation = BUILD IT UP
  • both arrows go up
80
Q

when the arrow goes in the opposite direction in a cycle?

A

change the sign

81
Q

when ΔcH⦵ is given?

A
  • oxides of the elements in our reactants and products are in circle
  • this is usually CO2 + H2O
  • BURN IT DOWN - both arrows go down (from equation)
82
Q

when going from reactants to circle to proucts?

A

the arrow goes down

then up

83
Q

SIGN - when do u add and when don’t u add?

A
  • Hess’ law - no sign added!!! sign already there

- calorimetry - add sign!! depending on if endo or exo

84
Q

⭐standard enthalpy change of combustion definition?

A

the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions

A level Chemistry (39 decks)

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