Thermochemistry Flashcards

1
Q

what is thermochemistry?

A

deals with the change in energy that occurs during a chemical reaction

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2
Q

chemical equations v thermochemical equations

A
  • they are different

- thermochemical must have enthalpy value

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3
Q

what is an enthalpy value?

A

tell chemist about the reaction participants and about energy change. enthalpy is defined as the amount of energy change per set amount of substance

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4
Q

to use enthalpy in mol what must be present

A

chemical formula

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5
Q

incomplete combustion

A
  • limited O2
  • products CO and H2O
  • all product gas
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6
Q

complete combustion

A
  • O2
  • products CO2 and H2O
  • products gas
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7
Q

how to balance combustion reactions normally?

A

CHO

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8
Q

how to balance when oxygen is present in the fuel or reactant?

A

need to subtract O atom total in the products by the number of O atoms in the chemical reactant (not O2) before you balance O2

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9
Q

if you have to halve or double the stoichiometry you must…. the enthalpy

A

halve or double

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10
Q

how do you reverse a thermochemical equation?

A

stoich is the same but enthalpy value becomes positive

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11
Q

how does the state of water impact the enthalpy value?

A

H2O to liquid releases heat

H2O to gas needs heat

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12
Q

what are the thermochemical reaction types?

A

endothermic or exothermic

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13
Q

endothermic?

A
  • absorb thermal energy from surrounding environment

- prac will indicate a decrease in temp

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14
Q

exothermic?

A
  • release thermal energy to environment

- prac indicates an increase in temp

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15
Q

what is activation energy needed for?

A

to break bonds

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16
Q

what happens in an endothermic energy profile?

A
  • reactants have lower energy than products

- energy is absorbed

17
Q

what happens in an exothermic energy profile?

A
  • reactants have higher energy content than products

- there is some energy absorption for bonds to break

18
Q

what happens at the transition state?

A

atoms begin to be rearranged

19
Q

how is enthalpy calculated from an energy profile?

A

H(product)-H(reactant)

  • positive in endo
  • negative in exo
20
Q

how is energy for a set thermochemical equation calculated? do you need a negative or positive sign?

A
  • get enthalpy from data book
  • multiply by number of mole of the fuel
  • no sign because not saying whether exo or endo
  • if in grams, convert to mol
21
Q

how to calculate the amount of substance from energy?

A

(the number of mol x the energy)/ enthalpy that correlates to mol

22
Q

what is specific head capacity?

A

ability to hold onto heat energy

23
Q

what is the density of water?

A

1ml is 0.997g

24
Q

what is the SHC equation?

A

energy= SHC x mass x change in temp

25
Q

how do you use an experiment to calculate enthalpy?

A

use water’s capacity to hold onto energy to determine the chemical’s energy

26
Q

how to find energy produced by burnt fuel?

A

E = (SCH of water) x mass of water x change in temp of water

this gives energy in J which should be converted to KJ

27
Q

how is the enthalpy of the burnt fuel calculated?

A

energy/amount (mol or mass)

28
Q

calculate enthalpy with mass

A

KJ of energy/mass

  • KJ from SHC eqn
  • mass from initial-final mass of fuel
29
Q

calculate enthalpy with mole

A
  • calculate mol with mass and molar mass

- divide energy KJ by mol

30
Q

calculate % efficiency

A

experimental enthalpy/ theoretical enthalpy x 100

31
Q

calculate heat loss

A

100%-% efficiency

32
Q

calculate mass lost

A

mass x heat loss

33
Q

calculate amount of fuel needed to reach 100%

A

100%/ your efficiency x quantity or mass of fuel or chemical