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Aqa a level chemistry > Topic 4: Energetics > Flashcards

Topic 4: Energetics Flashcards

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1
Q

enthalpy change

A

Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.

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2
Q

standard enthalphy

A

Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature

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3
Q

Alkenes

A

double carbon bond

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4
Q

Steps to calculating enthalphy change

A

1) write balanced equation

2) work out how many of each bond is broken and total energy required

3) work out how many of each bond is made and total energy released

4) break - make

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5
Q

exothermic

A

a reaction that releases energy from the system in the form of heat

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6
Q

endothermic

A

a reaction that the system absorbs energy from its surroundings in the form of heat

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7
Q

system
surroundings

A

-System–> energy stored in the chemicals that are being studied (reactants and products)

-Surroundings –> everything around the system e.g the air in the lab etc

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8
Q

energy

A

symbol = Q
measured in joules

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9
Q

enthalphy

A

symbol = H
measured in KJ/mol

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10
Q

what is enthalphy change

A

Enthalphy change of a process is the heat energy that is transferred between the system and the surroundings at constant pressure

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11
Q

types of enthalphy change

A

exothermic
endothermic

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12
Q

exothermic and endothermic

A

In an exothermic reaction products have less energy than the reactants

In an endothermic reaction products have more energy than the reactants

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13
Q

what happens to enthalphy in exothermic system

A

decreases

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14
Q

what happens to enthalphy in an endothermic system

A

increases

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15
Q

bond enthalphy equation

A

Molecule + bond enthalpy –> molecular fragments

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16
Q

what state must molecules be in

A

gaseous

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17
Q

define bond energy

A

Bond energy –> the energy needed to break one mole of a particular covalent bond

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18
Q

define bond enthalphy

A

Bond enthalpy –> the enthalpy change when one mole of a bond in the gaseous state is broken

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19
Q

why is enthalphy change always positive

A

Enthalpy change is always positive as the energy required to break the strong covalent bond attractions between a shared pair of electrons and nuclei

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20
Q

mean bond enthalphy

A

-When bonds are in different compounds there are slightly different bond enthalpies

-bond enthalpy decreases as there are less elements

-mean bond enthalpy is the enthalphy change when one mole of a bond averaged at over many different gaseous molecules is broken

CH4 (g) –> C(g) + 4H(g)

Mean bond enthalpy –> +Q /4 –> ¼ CH4 (g) –> ¼ C (g) + H (g)

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21
Q

bond energy calculations

A

Negative = exothermic

Positive = endothermic

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22
Q

types of exothermic reactions

A

combustion
oxidation
neutralisation

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23
Q

types of endothermic reactions

A

thermal decomposition

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24
Q

standard conditions

A

100KPa and 298K

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25
Q

types of enthalphy change

A

standard enthalphy change of reaction

standard enthalphy change of formation

standard enthalpy change of combustion

standard enthalphy change of neutralisation

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26
Q

equation for enthalpy change

A

q = m c ∆ T

q = heat loss/gain (j)

m = mass of water (g)

c = specific heat capacity of water (4.18 J/g/K)

∆T = temperature change of water

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27
Q

calculate how much heat would be given out when 8g of methane is burnt completely

A

mass / mr = moles (8/16 = 0.5)

moles x KJ = ans

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28
Q

how does pressure affect the amount of heat energy given out by reactions that involve gases

A

greater atmospheric pressure = more energy needed to push gas away
this means that less energy remains to be given out as heat by the reaction

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29
Q

∆fH (formation)

A

enthalphy change when one mole of substance is formed from consistent elements under standard conditions

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30
Q

∆cH

A

enthalphy change when one mole of a fuel is completely burnt in excess oxygen under standard conditions with elements in their standard states

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31
Q

what physical property should be kept constant when measuring an enthalpy change

A

pressure

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32
Q

standard enthalpy change

A

ΔH°

Standard enthalpy changes are measured at a standard constant pressure of 100 kPa and a temperature of 298K

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33
Q

definition of standard enthalphy changes

A

Standard enthalpy change of reaction is the enthalpy change measured at 100kPa and 298K when the number of moles of substances as written in the equation react

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34
Q

what would happen if the number of moles half

A

If the number of moles of an equation half the enthalpy change will also half

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35
Q

Standard enthalpy change of formation: ΔH°f

A

-the enthalpy change measured at 100kpa and 298K when one mole of a substance is formed from its elements in their standard states

-standard enthalpy of formation of an element in its standard state must be zero

-elements must be in their standard state e.g F2(g)

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36
Q

equation for enthalphy change of formation

A

-only ever 1 mole of products

-Methane –> C(s) + 2H2 (g) –> CH4

37
Q

endothermic and exothermic

A

Bond breaking is an endothermic process, because it requires energy.

Bond forming is an exothermic process, because it releases energy.

38
Q

if pressure is constant then…

A

Q = enthalphy change

39
Q

if temperature increases then…

A

it is an exothermic reaction (forming bonds)

40
Q

true or false - incomplete combustion is less exothermic

A

true

41
Q

elements in their standard states

A

= zero

42
Q

suggest two disadvantages of using a glass beaker on a tripod and gauze

A

-glass beaker is a poorer conductor
-reduced heat transfer to tripod and gauze

43
Q

suggest two reasons why the value of enthalphy of combustion is less exothermic than data book value

A

incomplete combustion

heat loss to surroundings

44
Q

suggest one addition to this apparatus that would improve the accuracy of the enthalphy value obtained

A

use wind shield to reduce heat loss

45
Q

outline a simple practical experiment that the engineer woulc use in order to determine the enthalphy of combustion for one of the unknown alcohols

A

-weigh alcohol before and after combustion
-place water in calorimeter
-measure mass of water
-burn alcohol to heat the water
-measure temperature change in water

46
Q

Calorimetry sources of error

A

-heat loss e.g not all heat used to heat the water
-evaporation of the fuel
-incomplete combustion of the fuel e.g soot under beaker
-neglecting specific heat capacity of calorimeter
-non-standard conditions e.g water vapour is produced

47
Q

enthalphy change of combustion

A

-The enthalpy change measured at 100kPa and 298K, when one mole of a fuel (hydrocarbons/alcohols) is completely burned in excess oxygen with all substances in their standard states

48
Q

requirements for standard enthalphy of combustion

A

-the standard enthalphy of combustion is always negative (temperature increases)

-should only be 1 mol in products and water produced should be a liquid

49
Q

Measuring enthalphy change of combustion (Calorimetry)

A

1) measure initial mass of the fuel

2) determine the volume and initial temp of water in the copper can

3) heat water with fuel and record temperature every minute

4) measure final temperature

5) reweigh the fuel and determine the moles that have been combusted

50
Q

mean bond enthalpies

A

different covalent bonds require different amounts of energy to be broken

values can be found experimentally using calorimetry methods

51
Q

why is a copper cup better to use than a glass beaker

A

Copper cup is better to use than a glass beaker because the heat will transfer directly to the solution being heated.

Glass is a poorer conductor and less heat would reach tripod+gauze

52
Q

errors in enthalphy of combustion

A

-heat loss

-incomplete combustion

-experiment was not completed under standard conditions

-evaporation of fuel

53
Q

3 factors that affect the amount of thermal energy of a substance

A

-temperature

-mass

-material it is made from

54
Q

specific heat capacity

A

-Water has the highest specific heat capacity

Specific heat capacity is the amount of energy needed to increase the temperature of 1kg of a substance by 1 degree

55
Q

4.18

A

1g of water 1 degree hotter = 4.18J (energy required)

1 of water 2 degree hotter = 4.18 x 2 = 8.36J

56
Q

equation for energy

A

Q = m c Δ T

Q = energy transferred in joules

M = mass of substance being heated (water) in grams

C = specific heat capacity of water in J/K/g

ΔT = change of temperature in kelvin

Volume/density = mass

57
Q

calculating enthalphy change

A

1) Q = m c Δ T

2) Q / mol = enthalpy change

58
Q

calorimetry sources of error

A

-heat loss e.g not all heat used to heat the water

-evaporation of the fuel

-incomplete combustion of the fuel e.g soot under beaker

-neglecting specific heat capacity of calorimeter

-non-standard conditions e.g water vapour is produced

59
Q

what is calorimetry

A

experimental method for finding enthalphy change by measuring temperature over time

60
Q

what is measured change in temperature proportional to

A

energy change

61
Q

what is the state symbol for carbon

A

solid

62
Q

Hess’s law

A

-the enthalphy change accompanying a chemical change (reaction) is independent of the route by which the chemical change occurs

-the enthalphy change will always be the same but the route may be different

63
Q

why is it difficult to measure enthalphy change directly

A

-when carbon is burned biproducts are formed such as soot

-it would require a significant amount of thermal energy to cause reactants to react

-hard to distinguish between the amount of energy required and the amount of energy released

64
Q

hess’s law

A

-overall enthalphy change for a reaction is independent of the route it takes as energy can only be transferred and change its form

-There is a direct route e.g A–>B

-there are indirect routes x 2 e.g A—C—B or A—D—B

So ΔH1 = ΔH2 + ΔH3

65
Q

direction of the arrow and sign of the value for formation

A

-the direction of the arrow determines the sign of the value

e.g if the arrow points up but you need to go down the value will be the opposite sign

Arrow goes from elements to products for enthalphy change of formation

66
Q

steps for calculating the hess cycle (formation)

A

1) write equation for reactants —> products (ignore single elements e.g O2 as their value is zero)

2) Work out elements required to form the molecules and ensure it is balanced with the state symbols in standard states

3) Place arrows in correct directions with ΔH labels –> ensure if there is more than 1 product formed multiply by 2

4) two arrows can be drawn if more than 2 products

5) Find ΔH using Hess’s law e.g (+239.1) + (-393.5) + 2(-285.8) = -726 KJ/mol

67
Q

which direction do the arrow’s point for combustion

A

down

68
Q

steps for hess cycle of combustion reactions

A

1) State products of combustion

2) Add oxygen to both the reactants and products in the main reaction

3) Write out equation for each reaction to help you

4) check arrow direction (should be downwards)

69
Q

enthalpies of combustion

A

Answer should be endothermic (positive) –> enthalpies of combustion

70
Q

what should you do if there are two fuels in the reactants

A

multiply by 2

71
Q

Explain how the experimental method and use of apparatus can be improved to provide more accurate data

Describe how this data from the improved method can be used to determine an accurate value for the temperature change

A

-use burette instead of measuring cylinder
-use a polystrene cup instead of a beaker to insulate it
-use powdered solid and reweigh watch glass
-measure initial and final temperature and measure temp at regular intervals
-plot graph with temp and time and determine overall change in time

72
Q

how would u calculate Q

A

mol x enthalphy change

73
Q

suggest how to reduce percentage uncertainty in the temperature change

A

increase the concentrations of the solution

74
Q

state why the heat change calculated from the bomb calorimeter experiment is not an enthalphy change

A

pressure is not constant

75
Q

Give one reason why the bond enthalphy calculated is different from the mean bond enthalphy in the data book

A

data book value derived from different compounds

76
Q

suggest the most likely reason for the large difference between the students experimental value and the data book value

A

heat loss = less exothermic

77
Q

suggest how the students method and analysis of results could be improved to determine a more accurate enthalphy change

A

-insulate beaker/use polystyrene cup to reduce heat loss
-record initial temp and record temp at regular intervals
-exbalish maximum temperature on graph

78
Q

suggest why the value for the standard enthaply of formation of liquid Sb is not zero

A

liquid is not Sb’s standard state

79
Q

reaction profiles

A

exothermic = products less than reactants

endothermic = products more than reactants

80
Q

calorimetry experiment

A

washes the equipment (cup and pipettes etc) with the solutions to be used
 dry the cup after washing
 put polystyrene cup in a beaker for insulation and support  Measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup
 clamp thermometer into place making sure the thermometer bulb is immersed in solution
 measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
 At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance.  If using a solid reagent then use ‘before and after’ weighing method
 stirs mixture (ensures that all of the solution is at the same temperature)  Record temperature every minute after addition for several minutes

81
Q

mean bond enthalphy

A

The mean bond energy is the enthalpy needed to
break the covalent bond into gaseous atoms, averaged over
different molecules.

82
Q

combustion

A

negative final value

83
Q

improve the apparatus

A

-polystyrene cup
-calculate mass by difference
-powdered solid
-burette instead of measuring cylinder

84
Q

enthalphy of formation

A

products - reactants

85
Q

errors in calorimetry experiments

A
  • energy transfer from surroundings (usually loss)
  • approximation in specific heat capacity of solution. The method assumes all
    solutions have the heat capacity of water.
  • neglecting the specific heat capacity of the calorimeter- we ignore any
    energy absorbed by the apparatus. * reaction or dissolving may be incomplete or slow.
  • density of solution is taken to be the same as water.
86
Q

errors in combustion

A
  • Energy losses from calorimeter * Incomplete combustion of fuel * Incomplete transfer of energy
  • Evaporation of fuel after weighing
  • Heat capacity of calorimeter not included
  • Measurements not carried out under standard conditions as
    H2O is gas, not liquid, in this experiment
87
Q

combustion vs formation

A

combustion = reactants - products (only combustion if it says changes from combustion)

formation = products - reactants

88
Q

as carbon atoms increases

A

enthalphy of combustion increases

Aqa a level chemistry (19 decks)

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