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chemistry igcse > chapter 9 - energy changes and reversible reactions > Flashcards

chapter 9 - energy changes and reversible reactions Flashcards

1
Q

what always happens during any chemical reaction?

A

there is always an energy change.
energy is given in or taken out.
the energy is usually in the form of heat (or light and sound)
total energy is the same on each side of the arrow

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2
Q

what is the difference between an exothermic reaction and an endothermic reaction?

A

exothermic reactions give out energy—> temp rise

endothermic reactions take in energy—>temp falls

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3
Q

describe an exothermic reaction

A

reactant —-> products + energy

products have lower energy than the reactants.

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4
Q

what unit is used to measure energy

A

kilojoule (kJ)

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5
Q

what would be the energy change in an exothermic reaction?

A

-__kJ

the minus sign shows that energy is given out

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6
Q

what are examples of exothermic reactions?

A
  • neutralization
  • combustion
  • respiration
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7
Q

describe an endothermic reaction

A

reactant + energy —-> products

products have higher energy than the reactants

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8
Q

what would be the energy change in an endothermic reaction?

A

+__kJ

the plus sign shows that energy is taken in.

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9
Q

what are examples of endothermic reactions?

A
  • reactions that take place during cooking

- photosynthesis

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10
Q

what is the difference between breaking bonds and making bonds? (in terms of energy)

A

breaking bonds takes in energy

making bonds releases energy

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11
Q

define bond energy

A

the energy needed to break bonds, or released when these bonds form. it is given in kJ/ mole

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12
Q

describe exothermic reactions in terms of bond energies

A

energy taken in to break the bonds is less than the energy released in making bonds.

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13
Q

describe endothermic reactions in terms of bond energies

A

energy taken in to break bonds is greater than the energy releases in making bonds.

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14
Q

how do we calculate energy change?

A

=energy in- energy out

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15
Q

why do some exothermic reactions need heat?

A

to just start bonds breaking

then the energy given out by the reaction breaks further bonds.

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16
Q

what is a fuel?

A

any substance we use to provide energy

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17
Q

what are the main fuels around the world?

A

fossil fuels: coal, petroleum and natural gas.

we burn them to release heat.

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18
Q

how are fossil fuels used in power stations?

A

they are burnt to heat water to make steam. a jet of steam drives the turbines that generate electricity.

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19
Q

how are fossil fuels used in factories?

A

they are burnt to heat furnaces.

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20
Q

how are fossil fuels used in cars?

A

petrol and diesel are burned in engines, to give the hit gas that moves the pistons, which make the wheels turn.

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21
Q

what makes a good fuel?

A
  • how much heat does it give out
  • does it cause pollution
  • is it easily available
  • is it safe and easy to store and transport
  • how much does it cost
22
Q

what new fuels have been growing in importance?

A

ethanol, which can be made from any plant material, and used in car engines on its own or mixed with petrol.
hydrogen, which burns explosively with oxygen and gives out a it of energy. it is used to fuel space rockets. also used in fuel cells, which gives energy in the form of electricity without burning.

23
Q

what fuel provides the largest amount of energy?

A

hydrogen
ethanol is next
natural gas gives the least energy

24
Q

are nuclear fuels burned?

A

no.

25
Q

how do nuclear fuels give out energy?

A

they contain unstable atoms called radioisotopes, which break down naturally over time into new atoms, giving out radiation and a lot of energy.

26
Q

what happens in nuclear power stations?

A

radioisotopes are forced to break down, by shooting neutrons at them.
the energy given out is used to heat water, to make steam. jets of steam are then used to drive the turbines for generating electricity.
uranium-235 is often used.

27
Q

what are the advantages of using nuclear fuel?

A
  • gives out a huge amount of energy.

- no carbon dioxide or any polluting gases.

28
Q

what are the disadvantages of using nuclear fuel?

A
  • an explosion in a nuclear power station could spread radioactive material over a huge area.
  • waste material produced in a nuclear power station is also radioactive, and may remain dangerous for hundreds of years.
  • finding a safe place to store it safely is difficult.
29
Q

what is electricity?

A

a current of electrons.

it is a form of energy.

30
Q

how do we construct a simple cell?

A

using metal strips, wire, and a beaker of an ionic solution.

31
Q

give an example if a simple cell

A

metal strips: magnesium and copper

solution: dilute sodium chloride
- magnesium is more reactive than copper so it gives up electrons. it is the negative pole.
- copper is the positive pole.

32
Q

how would we make the cell produce the largest voltage?

A

using metals that have a bigger difference in reactivity

33
Q

what is the difference between electrolysis and simple cells?

A

in electrolysis, a current brings about a reaction.

in simple cells, reactions produce a current.

34
Q

describe a hydrogen fuel cell

A

both poles are made of carbon.
the negative pole is surrounded by hydrogen.
the positive pole is surrounded by oxygen.
the electrolyte contains OH- ions (ex: potassium hydroxide).

35
Q

what reactions take place at the pole of a hydrogen fuel cell?

A

negative pole: hydrogen loses e- to the OH- ions. it is oxidized.
2H2(g) + 4OH-(aq) —-> 4H2O(l) +4e-
positive pole: the e- are accepted by oxygen molecules. oxygen is reduced to OH- ions.
O2(g) + 2H2O(l) + 4e- —-> 4OH-(aq)
full equation? 2H2(l) + O2(g) –> 2H2O(l)

36
Q

what are the advantages of a hydrogen fuel cell?

A
  • only water is formed. no pollutants.
  • gives out plenty of energy.
  • we will not run out of hydrogen.
37
Q

what are the disadvantages of a hydrogen fuel cell?

A

hydrogen is very flammable, so it must be stored safely.

38
Q

what is a reversible reaction?

A

a reaction that can go both ways.

39
Q

are reversible reactions endo or exo?

A

they are endothermic in one direction, and exothermic in the other. the same amount of energy is transferred in each time.

40
Q

what are some important reversible reactions?

A

nitrogen + hydrogen —> ammonia
sulfur dioxide + oxygen —-> sulfur trioxide
calcium carbonate —-> calcium oxide + carbon dioxide

41
Q

what does equilibrium mean?

A

there is no overall change

42
Q

what does dynamic mean?

A

there is continual change

43
Q

what is dynamic equilibrium?

A

in a closed system, a reversible reaction reaches the state of dynamic equilibrium, where the forward and backward reactions take place at the same rate, so there is no overall change.

44
Q

what happens in a closed system?

A

the concentrations of reactants and products stays constant.

45
Q

what is Le Chatelier’s principle?

A

when a reversible reaction is in equilibrium and you make a change, the system acts to oppose the change, and restore the equilibrium. a new equilibrium mixture forms.

46
Q

What changes can you make to shift equilibrium?

A
  • changing the temperature
  • changing the pressure
  • removing the product
  • adding a catalyst
47
Q

what effect does changing the temperature have on the position of equilibrium?

A

heating speeds up both reactions (reaction will reach equilibrium faster.)
but the equilibrium will shift to oppose the change.
endothermic reaction is favored.

48
Q

what effect does changing the pressure have on the position of equilibrium?

A

a higher pressure increases the rate of both reactions.
the more molecules present the higher the pressure.
when you increase the pressure, equilibrium shifts towards the side with fewer gas molecules.

49
Q

what happens when you remove the product?

A

more product will form, to restore the balance.

50
Q

what does a catalyst do?

A

it just speeds up the reaction.

it doesn’t produce more product.

51
Q

how do we get the best yield of ammonia?

A
  • high pressure
  • remove the ammonia produced
  • moderate temperature
  • iron catalyst

chemistry igcse (6 decks)

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