DF - energetics *03 Flashcards

1
Q

Thermodynamics first law?

A

energy can be neither created nor destroyed but it can be converted from one form to another

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2
Q

What is enthalpy

A

a measure of the heat content of a substance at constant pressure

enthalpy can’t be measured directly

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3
Q

What are the standard conditions?

A

Pressure = 100kPa (1atm)
Temperature = 298K (25 degrees)
All solutions have a concentration of 1 moldm-3

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4
Q

Enthalpy change of reaction

A

enthalpy change of a stated reaction

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5
Q

enthalpy change of combustion

A

enthalpy change when one mole of a substance undergoes complete combustion

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6
Q

enthalpy change of formation

A

enthalpy change when one mole of a substance is formed from its elements

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7
Q

enthalpy of neutralisation

A

the enthalpy change when one mole of water is formed form the reaction of an acid and alkali

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8
Q

what is the enthalpy of elements in their standard conditions

A

zero

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9
Q

bond dissociation enthalpy

A

energy required to break one mole of gaseous bond to form one mole of gaseous atoms

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10
Q

why are mean values used

A

because bond strength depends on environment

- the stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length.

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11
Q

what is the bond enthalpy for diatomic gases

A

twice the enthalpy of atomisation

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12
Q

what does higher bond enthalpy mean?

A

stronger bonds

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13
Q

Hess’s Law

A

enthalpy change is independent of the path taken

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14
Q

what is hess’s law used for

A

To calculate changes that cant be measured directly.

e.g combustion. formation or lattice enthalpy

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15
Q

exothermic reaction

A

a reaction that releases more energy from making bonds than the energy required (taken in) to break bonds

e. g combustion - CH4 (g) + 2O2 (g) –> CO2 (g) + 2H2O (l)
e. g respiration

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16
Q

endothermic reaction

A

a reaction that requires more energy to break bonds than the energy released when bonds are made

e. g photosynthesis
e. g thermal decomposition - CaCO3 (s) –> CaO (s) + CO2 (g)

17
Q

enthalpy of reaction

formation reactants and products

A

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

18
Q

enthalpy of reaction

combustion of reactants and products

A

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

19
Q

enthalpy change of a reaction ( when given average bond enthalpies)

A

total energy absorbed to break bonds - total energy released in making bonds

20
Q

is bond making exo or endo

A

exothermic

21
Q

is bond breaking exo or endo

A

endo

22
Q

enthalpy level diagram of an endothermic reaction

A
23
Q

enthalpy level diagram of an exothermic reaction

A
24
Q

techniques and procedures for measuring energy transferred when reactions occur in solution

A
  1. add a known vol. of acid to an insulated container
  2. then add known vol. of alkali
  3. record temp. rise (stir solution)
  4. can work out heat needed to raise temp. using q=mcT
  5. assume all solutions have the same density of water
25
Q

techniques and procedures for measuring energy transferred when reactions occur when flammable liquids burn

A

calorimetry

  1. as fuel burns it heats the water
  2. the temp. change and SHC of water can be used to work out the heat absorbed by the water (using q=mcT)
26
Q

how do you calculate enthalpy changes from experimental results?

A

q = m x c x T

q - heat lost or gained in J
m - mass of water in g
c - specific heat capacity of water (4.18 J g-1 K-1)
T - change in temperature of water in Kelvin (+273)

27
Q

PAG 3

A