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chemistry > energetics > Flashcards

energetics Flashcards

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1
Q

system

A

the chemicals being studied (reactants +products)
- we measure the energy change of the system

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2
Q

surrounding

A

everything around th esystem (container, air, water)
- we cant measure the temp of the system - so we measure the surrounding temp

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3
Q

exothermic

A

energy of system decreases
- heat lost to surroundings
temp of surroundings increases
-ΔH

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4
Q

endothermic

A

energy of system increases
- heat taken in from surroundings
temp of surroundings decreases
+ΔH

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5
Q

energy level diagram

A

energy on y axis
reaction progress on x
reactants +/-ΔH then products

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6
Q

reaction profile diagram

A

energy level diagram with activation evergy

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7
Q

why reaction cant react when acid is solid

A

acids need to be aq to react so ions can move and can release H+ ions

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8
Q

why use steel wool not a nail

A

larger suraface area increases rate of reaction

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9
Q

what factors affect temperature change measured

A
  • heat loss to surroundings
  • stir - removes hot spots
  • distance between flame and calorimeter
  • vol of solution - less = faster = less heat loss
  • initial water temp - hot = more heat loss
  • incomplete combustion
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10
Q

calorimeter

A

any apparatus used to measure ΔT

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11
Q

Q (heat energy transferred )equation

A

Q = mcΔT
(mass) x (specific heat capacity) x (temp chnage)

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12
Q

specific heat capacity

A

4.18
amount of heat energy needed to raise the temp of 1g of a substance by 1degree

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13
Q

assumptions in calorimetry

A
  • all heat is absorbed/released by the water present
  • any aq mixture has a c = to water
  • any aq mixture has a density = to water (vol = mass)
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14
Q

mass in calorimetry

A

vol = mass
add mass of 2 solutions
ignore mass of solids

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15
Q

enthalpy change

A

molar energy change (ΔH)

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16
Q

ΔH equation

A

ΔH = (Q/1000)/n
(energy transfered KJ)/ (amount of LIMITING reactant)

17
Q

ΔH sign

A

-ΔH = exo
+ΔH = endo

18
Q

%error between experimental and true values

A

= (difference between experimental +true /true) x 100

19
Q

calculated vs true values

A

ΔH was less exo/endothermic than true values - lots of heat loss to surroundings/not transfered to calorimeter

20
Q

sources of error in experiment

A
  • incomplete combustion - not as much energy release lower ΔT
  • heat loss to surroundings lower ΔT
  • lid not put on quickly - fuel evaporates - larger mass and n so less exo ΔH
  • assumption all heat is absorbed by water not equiptment - lower ΔT
21
Q

modifications to make final values more acurate

A
  • add lid - insulation
  • polestyrene cup - insulation
  • insulation to sides /draught sheild
  • stir - minimise hotspots
  • make flame closer to water
  • add O2 supply to base to encourage complete combustion
22
Q

neutralisation reaction

A

EXOTHERMIC
reaction in solution
eg HCl +NaOH -> NaCl + H2O
- record intial temp of both solutions
- add base
- add acid
- stir and record max temp reached

23
Q

displacment reaction

A

EXOTHERMIC
reaction in solution
eg Zn + CuSO4 -> ZnSO4 +Cu
- add CuSO4 solution to polestyrene cup
- record intial temp
- add Zn
- stir and record max temp reached

24
Q

combustion reaction

A

EXOTHERMIC - ethanol
- put water in calorimeter and record initial temp
- record initial mass of fuel
- light fuel under water
- record max temp reached
- record final mass of fuel

25
Q

salts dissolving in water

A
  • record initial temp of water and salt
  • put water in cup
  • put salt in water
  • record max temp reached
26
Q

see pics for calorimeters, energy level diagrams

A

-

27
Q

bond breaking is

A

endothermic - takes in energy from surroundings

28
Q

bond making is

A

exothermic - gives out energy to surroundings

29
Q

so if bond breaking energy is more then bond making energy

A

if the energy taken in to break bonds is more than the energy given out when making bonds it is endothermic

30
Q

so if bond breaking energy is less then bond making energy

A

if the energy taken in to break bonds is less than the energy given out when making bonds it is exothermic

31
Q

activation energy

A

the minimum energy needed to start a reaction
used to break bonds so a reaction can happen

32
Q

change in H in terms of bond making/ breaking

A

bonds broken - bonds made

33
Q

mean bond energy

A

the average amount of energy needed to break 1 mole of a particular type of covalent bond

34
Q

why is mean bond energy sometimes inaccurate

A

bond energy is an average from a range of compounds - strength of bonds can vary depending on atoms in the molecule

chemistry (17 decks)

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