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Chemistry > Chemical Energetics, Rates of Reaction, Equlibria and Nitrogen > Flashcards

Chemical Energetics, Rates of Reaction, Equlibria and Nitrogen Flashcards

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1
Q

What does an exothermic reaction do

A

Produce energy so the temperature of the reaction mixture and surroundings goes up

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2
Q

What reactions are exothermic

A

all combustion

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3
Q

What does an endothermic reaction need

A

An energy input

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4
Q

How is an energy input created for endothermic reactions

A

By heating the reaction mixture

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5
Q

Give an example of heating the reaction mixture for endothermic

A

Thermal decomposition of carbonates

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6
Q

What type of reaction is neutralisation

A

exothermic

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7
Q

What type of reaction is respiration

A

exothermic

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8
Q

What type of reaction is photosynthesis

A

endothermic

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9
Q

Endothermic reactions have…

A

A negative ^H value

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10
Q

Exothermic reactions have…

A

A positive ^H value

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11
Q

Where does the energy produced during a reaction come from

A

the potential energy stored in the bonds

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12
Q

reactions involve a combination of

A

bond breaking and bond making

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13
Q

Breaking bonds requires

A

energy to be put in (so is ENDOTHERMIC)

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14
Q

making bonds gives out

A

energy (so is EXOTHERMIC)

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15
Q

exothermic reaction and bonds energy

A

more energy is released when product bonds form, than us needed to break bonds in reactants

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16
Q

endothermic reaction and bonds energy

A

more energy is needed to break bonds in reactants than is released when product bonds form

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17
Q

what is the activation energy

A

the minimum energy required by particles if they are going to react on collision.

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18
Q

what is required for activation energy

A

an energy input is needed in all reactions to overcome the activation energy barrier

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19
Q

how will a reaction be exothermic?

A

more heat energy is given out making bonds than is required to break bonds

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20
Q

how will a reaction be endothermic?

A

less energy is given out making bonds than is required to break bonds

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21
Q

what is the collision theory

A

a chemical reaction can only occur between particles when they collide. particles may be atoms, ions or molecukes

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22
Q

what happens if colliding particles have less than the activation energy

A

they bounce off eachother and no reaction occurs. the activation energy works as a barrier to reactions happening

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23
Q

the faster the particles:

A

the more energy they have. they are more likely to react when they collide

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24
Q

why are fast moving particles more likely to react when colliding

A

a higher proportion will have an amount of energy equal to or greater than the activation energy

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25
Q

how to give particles more energy

A

heat them up / raise temperature

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26
Q

how fast a reaction happens depends on 5 things:

A

surface area , concentration , pressure, temperature and presence of a catalyst

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27
Q

experiment for particle size if solids

A

marble chip experiment (CaCO3) using different sized chips

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28
Q

how to ensure marble chip test is fair

A

ensure temp, total mass if chips, concentration and volume of HCl are kept constant

29
Q

By having the same mass of smaller particles instead of larger particles:

A
  • surface area of solid is increased
  • more H+ ions can colide with CaCO3 particles per second -> more frequent collisions
  • rate of reaction is faster
30
Q

why are catalysts usually used in the form of powders

A

has the largest surface area so will have the fastest reaction rate

31
Q

the rate of reaction always decreases with time because:

A

the marble chips are reacting away (total surface area decreasing) and so is the acid (decrease in conc)

32
Q

experiment for concentration

A

iodine clock - time taken for the blue black solution to form is measured. concentration of one of the reactants is varied

33
Q

how to ensure fair test iodine clock

A

ensure temp and total volume of solutions kept constant

34
Q

increasing the concentration of a substance in solution means

A
  • more particles of substance of dm3 of the solution
  • particles are closer together
  • more frequent collisions
  • rate of the reaction increases
35
Q

what does iodine clock experiment show

A

rate of reaction increases w concentration

36
Q

what is rate of reaction to concentration

A

directly proportional

37
Q

if the concentration of a reactant is doubled

A

there is twice as many of the particles present, so twice as many collisions in a given time so rate of reaction will double

38
Q

in order to increase the pressure

A

number of particles in a given volume must be increased

39
Q

increasing pressure means

A
  • particles closer togethter
  • more frequent collisions
  • rate of reaction increases
40
Q

experiment for temperature

A

sulphur cross - amount of time taken for a cross to disappear when a precipitate is formed by the reaction of two solutions. experiment is repeated at different temps

41
Q

fair test for sulphur cross

A

same concentrations of solutions, total volume of solution

42
Q

result of sulphur cross

A

reaction is faster when temp is higher.

43
Q

at higher temperatures particles:

A
  • move faster so collide more frequently
  • have more energy
  • more collisions will have energy greater than or wqual to activation energy
  • rate of reaction increases
44
Q

what is a catalyst

A

a substance that increases the rate of a reaction and can be recovered chemically unchanged at the end of a reaction

45
Q

what do catalysts do

A

lower the activation energy reducing the amount of energy needed for a collision to result in a reaction

46
Q

what are enzymes

A

biological catalysts

47
Q

when will enzymes work

A

when the temperature is below 40°c otherwise they are denatured

48
Q

enzymes in yeast used in fermentation

A

C6H12O6 (aq) -> 2C2H5OH + 2CO2

conditions: yeast, 25-35°c, no O2

49
Q

what is enzymes in fermentation used for

A

to make beer and wine and in baking to make bread rise

50
Q

catalytic converter contains:

A

alloy of platinum, rhodium and other metals

51
Q

what structure does the catalytic converter have

A

honeycomb to give as large of a surface area as possible

52
Q

why are large surface area and high temperature needed

A

for the reactions to occur at a fast enough rate to remove most of the pollutants

53
Q

Nitrogen N2

A
  • diatomic molecule

* has a triple covalent bond which is VERY strong and hard to break

54
Q

why is nitrogen very unreactive

A

large amount of energy needed to break bond

55
Q

the haber process

A

nitrogen reacts with hydrogen to give ammonia. the reaction doesnt go to completion and us reversible - dynamic equilibrium

56
Q

what is dynamic equilibrium

A

reached when the rate of the forward reaction equals the rate of the reverse reaction

57
Q

how is n2 gas obtained

A

fractional distillation of air

58
Q

how is h2 gas obtained

A

natural gas

59
Q

Le Chatelier’s Principle

A

“If a change is imposed on a system in equilibrium the position of the equilibrium will shift in order to minimise the effect of that change”

60
Q

temperature

A

• exothermic forward reaction
• low temp gives high yield
HOWEVER
• low temp gives a very slow rate so it would take a long time to produce a sufficient amount of ammonia
COMPROMISE
• intermediate temp of 450°c is used to give reasonable yield and a high rate

61
Q

pressure

A

• the smaller the number of moles in a given volume, the lower the pressure
• if we increase the pressure, Le Chatilier states that the system will shift the equilibrium to reduce the pressure and so increase the forward reaction, which produces fewer moles
• High pressure gives high rate of forward reaction, producing NH3
HOWEVER
• high pressure is expensive to remain and can be dangerous
COMPROMISE
• 200 atm used

62
Q

catalyst

A
  • does not alter the equlibrium position so does not affect yield
  • does increase rate of reaction (equilibrium is reached but quicker)
  • BUT it speeds up the rate of the forward and backwards equally
  • means a lower temp can be used to achieve the same rate of reaction
  • finely divided iron catalyst
63
Q

Properties of ammonia

A
  • alkaline gas, turns pink litmus blue
  • very water soluble - NH3 + H20 -> NH4 + OH
    reaction doesnt go to completion, so NH3 solution is a weak alkaline
  • distinctive, pungent smell
  • reacts with acids to form salts
64
Q

uses of ammonia

A
  1. making fertilisers
  2. making nylon and other polymers
  3. making explosives e.g TNT
  4. used in household cleaning products
65
Q

advantages of artificial fertilisers

A
  • contains high proportion of N to aid plant growth
  • solid so easily transported
  • water soluble so can be applied by spraying
  • easily absorbed by plants
66
Q

disadvantages of artificial fertilisers

A
  • very soluble so easily washed out into lakes and rivers (leading to eutrophication)
  • nitrates in water supplies are dangerous, especially to babies
  • reduced crop variation and biodiversity as only crops that respond well are grown
  • expensive for farmers (so not feasible in developing countries)
67
Q

advantages of organic fertilisers

A
  • better for the environment, encourages biodiversity
  • improves structure of soil
  • resulting crops contain no artificial chemicals
  • cheaper than artificial
  • better for developing countries as they are then not dependent on countries or foreign aid
68
Q

disadvantages of organic fertilisers

A
  • slower / less effective so less crop produced and food therefore more expensive
  • food produced may not appear as pleasing
  • more labour intensive i.e collecting and spreading the fertiliser

Chemistry (17 decks)

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