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reactivity 1 Flashcards

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1
Q

What is a chemical reaction?

A

reaction involving a transfer of energy between the system and the surroundings with total energy being conserved

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2
Q

What is the difference between heat and temperature?

A

heat - process of energy transfer due to a temperature difference
temperature - measure of the average kinetic energy of particles

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3
Q

What is energy?

A

measure of the ability to do work

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4
Q

What is the system and the surroundings?

A

system - reaction mixture
surroundings - everything else

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5
Q

What is enthalpy?

A

(H) is a measure of the amount of heat energy contained in a substance stored in chemical bonds and intermolecular forces as potential energy

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6
Q

What are the characteristics of endothermic reactions?

A
  • ΔH is positive
    surroundings –> system
  • bond breaking
  • reactants more stable than products
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7
Q

What are the characteristics of exothermic reactions?

A
  • ΔH is negative
    system –> surroundings
  • bond making
  • products are more stable than reactants
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8
Q

What is an open system?

A

system where both energy and matter can be exchanged

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9
Q

What is a closed system?

A

system where energy can be exchanged but not matter

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10
Q

What is activation energy?

A

minimum amount of energy particles need for a reaction to occur

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11
Q

What is an isolated system?

A

system where neither energy or matter can be exchanged

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12
Q

What is standard enthalpy change?

A

heat transferred at constant pressure under standard conditions and states determined by a change of temperature in a pure substance - measured in kJ mol-1

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13
Q

What are examples of thermochemical reactions?

A

CH4 (g) + 2O2 (g) –> CO2 (g) + H2O (g) - ΔH= -890kJ mol-1
6CO2 (g) + 6H2O (l) –> C6H12O6 (aq) + 6O2 (g)
- ΔH= +2802.5kJ mol-1

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14
Q

What is specific heat capacity?

A

amount of energy needed to raise 1kg of a substance by 1K

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15
Q

How do you calculate specific heat capacity?

A

Q=mcΔT

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16
Q

What are 3 assumptions about enthalpy change?

A
  1. heat is lost as soon as zinc is added and the reaction starts
  2. all heat generated was absorbed by the water
  3. volume of copper sulfate = volume of water (1g cm3 density)
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17
Q

What is a covalent bond?

A

electrostatic attraction between the shared pair of negative electrons and the positive nuclei of a bonded atom

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18
Q

What is bond making?

A

exothermic reaction that gives off energy

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19
Q

Why might enthalpy change be less than expected?

A
  • not investigated under standard conditions
  • heat lost to surrounding / calorimeter
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20
Q

What is bond breaking?

A

endothermic reaction which takes in energy

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21
Q

What is bond enthalpy?

A

energy needed to break 1 mole of bonds of gaseous molecules under standard conditions

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22
Q

What type of reaction is bond enthalpy?

A

Endothermic

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23
Q

What type of reaction is ionization enthalpy?

A

Endothermic as it involves pulling an electron away from the electrostatic attraction force due to positively charged nucleus

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24
Q

What type of reaction is enthalpy of atomization?

A

Endothermic

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25
Q

What is average bond enthalpy?

A

energy needed to break 1 mole of bonds of gaseous molecules under standard conditions averaged over similar compounds

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26
Q

What is the relationship between bond enthalpy, bond length, amount of bonds and polarity?

A

shorter bond = higher bond enthalpy
more bonds = higher bond enthalpy
polar bond = higher bond enthalpy

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27
Q

How can you calculate bond enthalpy?

A

ΔH = Σ bond energies broken - Σ bond energies made

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28
Q

What is Hess’s Law?

A

states that enthalpy change for a reaction is independent of it’s pathway between initial and final states

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29
Q

What is standard enthalpy change of formation?

A

enthalpy change that occurs when one mole of the substance is formed from it’s elements in their standard states when they are stable (293K and 1.0 x 10^5Pa)

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30
Q

What is a born haber cycle?

A

Born-Haber cycle - specific application of Hess’s Law for ionic compounds and enables us to calculate lattice enthalpy, which cannot be found by experiment by drawing a diagram in which energy increases going up the cycle and energy decreases going down the cycle.
- cycle shows all the steps needed to turn atoms into gaseous ions and from gaseous ions into the ionic lattice
- starts from the enthalpy of formation with elements in their standard states

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31
Q

Why are experimental lattice enthalpies always larger than theoretical lattice enthalpies?

A
  • theoretical values are based on an ionic model
  • doesn’t take into account additional covalent contributions to the bonding
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32
Q

What happens as ionic radius increases?

A

more covalent character as it is more easily polarized
- there is a larger difference between theoretical and experimental values
- electronegativity decreases

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33
Q

How do you calculate % difference?

A

( difference/mean ) / 100

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34
Q

What is standard enthalpy of combustion?

A

enthalpy change that occurs when one mole of the substance burns completely under standard conditions

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35
Q

What are the equations for enthalpy change?

A

ΔHf = Σ bond energies of products - Σ bond energies of reactants
ΔHc = Σ bond energies of reactants - Σ bond energies of products

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36
Q

What is ionization energy?

A

energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms to form a positive ion OR energy needed to form a positive ion from a gaseous atom
Na (g) –> Na+ (g) + e- ΔH = +496kJ mol-1

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37
Q

What is electron affinity?

A

energy released when 1 mole of electrons is added to 1 mole of gaseous atoms to form a negative ion; a tendency to accept electrons OR enthalpy change when 1 mole of gaseous atoms attracts 1 mole of electrons
Cl (g) + e- –> Cl- (g) ΔH = -394kJ mol-1

38
Q

What is lattice enthalpy?

A

enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions
NaCl (s) –> Na + (g) + Cl- (g)

39
Q

What type of reaction is lattice enthalpy?

A

endothermic reaction - all values are positive

40
Q

What is the trend between ionic charge and lattice enthalpy?

A
  • the greater the charge, the greater the lattice enthalpy
  • lattice enthalpy is proportional t the product of charges of positive and negative ions?
41
Q

What is the trend between ionic radius and lattice enthalpy?

A
  • the larger the ionic radius, the smaller the lattice enthalpy
  • lattice enthalpy is inversely proportional to the sum of ionic radii of positive and negative ions
42
Q

What is enthalpy change of atomization?

A

enthalpy change that occurs when 1 mole of gaseous atoms is formed from the element in it’s standard state
0.5 Cl2 (g) –> Cl (g)

43
Q

What is the difference between enthalpy of formation and atomization?

A

atomization - forms atoms
formation - forms compounds

44
Q

What is the difference between enthalpy of atomization and bond enthalpy?

A

bond enthalpy - starts with 1 mole
enthalpy of atomization - makes 1 mole
bond enthalpy = 2x enthalpy of atomization
enthalpy of atomization = 0.5x bond enthalpy

45
Q

What values are 1st electron affinities?

A

first electron affinities are almost always exothermic (negative)

46
Q

Why is second electron affinity endothermic?

A

adding an extra electron to an already negatively charged atom causes repulsion between 2 negative charges, so energy is needed to force them together, so it is an endothermic reaction with a positive enthalpy change

47
Q

What does combustion of organic compounds do?

A

breaks the carbon chain
- doesn’t react spontaneously due to the high activation energy
- safe to transport and store

48
Q

What is combustion?

A

reaction in which an element or compound burns in oxygen

49
Q

What are equations of combustion with C3H8 and C2H5OH?

A

C3H8 (g) + 5O2 (g) –> 3CO2 (g) + 4H2O (l)
C2H5OH (g) + 3O2 (g) –> 2CO2 (g) + 3H2O (l)

50
Q

What is made during incomplete combustion?

A

carbon monoxide, carbon and water

51
Q

Why is incomplete combustion dangerous?

A

CO binds to haemoglobin in blood and prevents O2 being transported

52
Q

What are fossil fuels and how are they formed?

A

non-renewable sources of energy such as crude oil, natural gas that are formed from the reduction of biological compounds

53
Q

What is an ideal fuel?

A

a fuel that releases significant amount of energy at a reasonable rate with minimal pollution

54
Q

What is crude oil and how is it formed?

A

a valuable fuel and chemical feed stock that is a mixture of alkanes, cyclo-alkanes that have been formed from the remains of marine animals subject to high pressure and temperature which caused the bacteria to decay the organic matter

55
Q

What are the advantages and disadvantages of crude oil?

A

advantages - easily transported, volatile, high enthalpy density, impurities removable
disadvantages - photochemical smog, CO, acid rain, contributes to global warming

56
Q

What is natural gas and how is it formed?

A

mainly methane and nitrogen capped by impermeable rock and subject to high heat and pressure with bacteria decaying organic matter - explosive in air

57
Q

What is coal and how is it formed?

A

most plentiful fossil fuel formed from the remains of plant life subject to geological heat and pressure turning it into a combustible sedimentary rock

58
Q

What are the advantages and disadvantages of natural gas?

A

advantages - cleanest fossil fuel, easily transported, no acid rain, low carbon content, high specific energy
disadvantages - limited supply, risk of explosion, contributes to global warming

59
Q

What are the advantages and disadvantages of coal?

A

advantages - cheap, safer than nuclear, ash used in roads, longest life span, easy to use
disadvantages - acid rain, contributes to global warming, particulates, mining is hard

60
Q

What is a carbon footprint?

A

measure of all the greenhouse gases we individually produce in our day-to-day activities though the use of fossil fuels

61
Q

What is the relationship between carbon content and specific energy?

A
  • higher carbon content means more CO2 is released and less energy is released
  • lower carbon content means less CO2 is released and more energy is released
62
Q

How does carbon content link to combustion?

A

lower carbon content - complete combustion
higher carbon content - incomplete combustion

63
Q

What is the greenhouse effect?

A

greenhouse gases allow shortwave radiation to pass through the atmosphere, but absorbs longer wave radiation (infrared), which is re-radiated back to Earth, warming it up

64
Q

What is a biofuel and how is it formed?

A

fuels that have formed from the biological fixation of carbon over a period of time through photosynthesis

65
Q

What are some of the impacts of greenhouse effect?

A

rising sea levels, melting ice caps, extreme weather, extinction of species

66
Q

What is ethanol and how can it be used?

A

liquid biofuel made from the fermentation of plants high in sugar and starch at 37 degrees in anaerobic conditions
- lower specific energy than octane as the C-O and O-H bonds are already present

67
Q

What is photosynthesis?

A

6CO2 (g) + 6H2O (l) –> C6H12O6 (s) + 6O2 (g) - turns light energy into chemical energy and is an endothermic process

68
Q

What are the disadvantages of photosynthesis?

A

not an efficient process as only a small percentage (10%) of solar energy is captured

69
Q

What is wood and how can it be used?

A
  • mainly cellulose, a polymer made out of glucose molecules
  • has a higher oxygen % and a lower carbon %, leading to a lower specific energy than coal
  • water vapor and oxygen are lost when exposed to high pressures
70
Q

What is methane and how can it be used?

A

produced by bacterial breakdown of plant matter and produces carbon dioxide
C6H12O6 (s) –> 3CO2 (g) + 3CH4 (g) = 50% of methane in carbohydrates
2C15H31COOH (s) + 14H2O (l) –> 9CO2 (g) + 23CH4 (g) = 72% of methane in fats

71
Q

What are the disadvantages of biofuels?

A

uses lots of land, high transport costs, uses lots of fertilizers, lower specific energy

72
Q

What are the advantages of biofuels?

A

cheap, readily available, renewable and sustainable, produces less pollution

73
Q

What is a fuel cell?

A

used to convert chemical energy from a fuel directly to electrical energy

74
Q

How does a hydrogen fuel cell work?

A
  1. H2 goes into the cathode and donates electrons
  2. O2 goes into the anode and gains the electrons
  3. water comes out of the bottom
    - works with an alkaline electrolyte
75
Q

What is the overall equation of a hydrogen fuel cell?

A

2H2 (g) + O2 (g) –> 2H2O (l) ΔH = -286kJ mol-1

76
Q

What happens at the anode of a hydrogen fuel cell?

A

hydrogen is oxidised in the presence of OH- ions to form water and electrons
H2 (g) + 2OH- (aq) –> 2H2O (lO + 2e-

77
Q

What happens at the cathode of a hydrogen fuel cell?

A

oxygen is reduced by gaining the electrons and reacting with water to form OH- ions
O2 (g) + 4e- +2H2O (l) –> 4OH- (aq)

78
Q

How can methanol be used in a fuel cell?

A

methanol is oxidized using catalysts in the fuel cell to form carbon dioxide; water is consumed at the anode and produced at the cathode
CH3OH (g) + 1.5O2 (g) –> CO2 (g) + 2H2O (l)

79
Q

What are the half equations of methanol in a fuel cell?

A

methanol is oxidized at the anode
CH3OH (l) + H2O (l) –> CO2 (g) + 6e- + 6H+
oxygen is reduced at the cathode
O2 (g) + 4e- + 4H+ –> 2H2O (l)

80
Q

What is the full equation of methanol in a fuel cell?

A

CH3OH (l)+ H2O (l) –>CO2 (g) +6H+ (aq) +6e−

81
Q

What is standard enthalpy change of combustion?

A

enthalpy change when 1 mole of a substance burns completely under standard conditions
- must be a negative value

82
Q

What is standard enthalpy change of formation?

A

enthalpy change when 1 mole of a substance is formed from its elements in their standard states

83
Q

What are standard states?

A

the form of an element at standard conditions - usually the most stable

84
Q

What are the standard conditions?

A

298K and 1.0 x 10-5 Pa

85
Q

What is the enthalpy change of formation for an element?

A

0

86
Q

In what direction does Hess’s triangle face for enthalpies of combustion and formation?

A

combustion - downwards from reactants and products to the combustion products
formation - upwards from elements to reactants and products

87
Q

What is the equations for enthalpy change of formation?

A

enthalpy change for products - enthalpy change for reactants

88
Q

What is the equations for enthalpy change of combustion?

A

enthalpy change for reactants - enthalpy change for products

89
Q

What is 1st ionisation enthalpy?

A

minimum energy required to remove 1 mole of electrons from 1 mole of gaseous atoms

90
Q

What is bond enthalpy?

A

energy needed to break 1 mole of bonds in a gaseous molecule under standard conditions

91
Q

What is en

A

chemistry (28 decks)

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