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chemistry > 4 energetics > Flashcards

4 energetics Flashcards

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1
Q

what is an exothermic process?

A

where energy is released from the chemical to the surroundings
temp of surrounding increases

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2
Q

what is an endothermic process?

A

energy is taken in by the chemical from the surroundings during the process
temp of surroundings decreases

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3
Q

define enthalpy change

A

change in heat energy at constant pressure

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4
Q

is the enthalpy change for exothermic processes positive or negative?

A

negative

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5
Q

is the enthalpy change for endothermic processes positive or negative?

A

positive

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6
Q

what are standard conditions?

A

pressure: 100 kPa
temp: 298K
conc: 1.0 mol dm^-3

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7
Q

is breaking bonds an exo or endothermic process?

A

endothermic

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8
Q

is making bonds an exo or endothermic process?

A

exothermic

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9
Q

define activation energy

A

the minimum energy needed to start a reaction

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10
Q

define mean bond enthalpy

A

the energy required to break one mole of a covalent bond into gaseous atoms averaged over a range of different compounds

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11
Q

are mean bond enthalpies exo or endothermic?

A

endothermic

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12
Q

define standard enthalpy of formation

A

the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions with all reactants and products in standard states

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13
Q

why is the enthalpy of formation of Na zero?

A

Na is an element

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14
Q

why is the enthalpy of formation of liquid Na not zero?

A

Na is not liquid in its standard state

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15
Q

define standard enthalpy of combustion

A

the enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions all reactants and products in standard states

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16
Q

why may the enthalpy of combustion of a compound be difficult to measure?

A

incomplete combustion may occur

17
Q

what is an enthalpy of reaction?

A

any reaction that doesn’t match one of the other definitions

18
Q

what does hess’s law state?

A

the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products

19
Q

which way do arrows always point in hess cycles for enthalpies of formation?

A

up

20
Q

what is at the top of the hess cycle in all three bond enthalpies?

A

reactants to products

21
Q

what is at the bottom of the hess cycle for all three bond enthalpies?

A

bond enthalpy: gaseous atoms of elements
combustion: combustion products
formation: elements in standard states

22
Q

what is the equation for calorimetry?

A

Q = mc x change in temp

23
Q

what do the letters in the equation for calorimetry stand for?

A

Q= energy (J)
m= mass of water (g)
c= specific heat capacity (4.18)
T= change in temp (K or °C)

24
Q

how is enthalpy change calculated?

A

Q/n

25
Q

example exam question:
Give one reason why the bond enthalpy that you calculated above is different from the mean bond enthalpy quoted in a data book

A

The data book value is averaged over a range of different compounds

26
Q

what following equipment would be needed to measure an enthalpy change of combustion?

A

thermometer
calorimeter
water
combusting fuel

27
Q

how do you calculate mean bond enthalpies?

A

energy to break bonds - energy to form bonds

28
Q

what is an assumption made in calorimetry?

A

the energy transferred to the water from the chemicals is equal to the energy released by the reaction

29
Q

what are some sources of error in calorimetry?

A
  • heat loss to surroundings
  • incomplete combustion
  • heat energy transferred to metal calorimeter
  • some fuel evaporates
30
Q

how can you reduce heat loss in calorimetry?

A
  • add a lid
  • insulate sides of calorimeter
  • reduce distance between flames and beaker
31
Q

how do you measure enthalpy using a cooling curve?

A
  • record temperature for 3 minutes before adding reactants to establish an accurate initial temperature
  • mix reactants and record temperature every minute until a trend
  • plot a graph
  • extrapolate the cooling curve back to the point of addition
  • to establish a theoretical temperature change accounting for heat loss
32
Q

what do you use a styrofoam cup for?

A

measuring enthalpy change when:
- two solutions added together
- a solid is added to a solution

33
Q

what is a source of error for the source calorimetry?

A

heat loss to surroundings

chemistry (34 decks)

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