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Chemistry > Energetics and cells > Flashcards

Energetics and cells Flashcards

1
Q

What is the principle of energy conservation in chemical reactions?

A

Energy is conserved in chemical reactions; products must have less energy than reactants if energy is transferred to surroundings.

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2
Q

What is chemical energy?

A

Energy that’s stored in chemical bonds.

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3
Q

What defines exothermic reactions?

A

Reactions that transfer energy to the surroundings.

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4
Q

Give examples of exothermic reactions.

A
  • Combustion (burning)
  • Many oxidation reactions
  • Neutralisation
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5
Q

What happens to the temperature in exothermic reactions?

A

The reaction mixture and surroundings become hotter.

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6
Q

How is the energy change represented in exothermic reactions?

A

The energy change is negative.

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7
Q

What defines endothermic reactions?

A

Reactions that take in energy from the surroundings.

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8
Q

Give examples of endothermic reactions.

A
  • Electrolysis
  • Thermal decompositions
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9
Q

What happens to the temperature in endothermic reactions?

A

The reaction mixture and surroundings become colder.

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10
Q

How is the energy change represented in endothermic reactions?

A

The energy change is positive.

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11
Q

What is a common use for endothermic reactions?

A

Used in some sports injury packs.

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12
Q

What is activation energy?

A

The minimum amount of energy that particles must have to react.

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13
Q

What should you do when answering exam questions about reaction profiles?

A

Replace the reactants and products with the actual reactants and products.

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14
Q

In a reaction profile, what does the increase in energy to the top of the hill represent?

A

Energy being put in to break the bonds.

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15
Q

Fill in the blank: In exothermic reactions, the overall energy change is ______.

A

negative

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16
Q

Fill in the blank: In endothermic reactions, the overall energy change is ______.

A

positive

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17
Q

What is the energy change during bond breaking?

A

Energy is required to break the bonds in the reactants, therefore bond breaking is endothermic.

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18
Q

What happens during bond making?

A

New bonds are formed which gives out energy, making bond making exothermic.

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19
Q

How do you calculate overall energy change in a reaction?

A

Energy Change = Energy of bonds broken - Energy of bonds made (BREAK - MAKE)

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20
Q

When is a reaction considered endothermic?

A

The reaction is overall endothermic if the energy needed to break the bonds in the reactants is more than the energy given out when new bonds form.

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21
Q

When is a reaction considered exothermic?

A

The reaction is overall exothermic if the energy given out when new bonds form is more than the energy needed to break the bonds in the reactants.

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22
Q

What are the steps for bond energy calculations?

A
  1. Calculate bonds broken.
  2. Calculate bonds made.
  3. Cross off the bonds as you add them up.
  4. Do break - make.
  5. Explain if it’s exothermic or endothermic.
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23
Q

What indicates an exothermic reaction in terms of bond energies?

A

More energy is released making bonds than needed to break bonds.

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24
Q

What indicates an endothermic reaction in terms of bond energies?

A

Less energy is released making bonds than needed to break bonds.

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25
Q

What is the simplest design of a cell?

A

Two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by a wire.

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26
Q

What role does the more reactive metal play in a cell?

A

The more reactive metal forms ions more easily with the electrolyte, releases ions, which gives it a negative charge and sets up a charge difference.

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27
Q

What occurs as electrons flow in a cell?

A

Electrons flow around the circuit to the now more positive electrode.

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28
Q

Fill in the blank: A cell contains ______ that react to produce electricity.

A

[chemicals]

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29
Q

What is the purpose of using a thermometer in a practical energy change experiment?

A

To measure the temperature change during the reaction.

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30
Q

What do you plot to analyze temperature changes during a reaction?

A

Plot results and two lines of best fit - one when it’s increasing and one when the temperature is decreasing.

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31
Q

Where does the maximum temperature occur in a temperature change graph?

A

Where the lines intersect.

32
Q

What happens during the energy release after the top is reached in a reaction?

A

Energy is being released whilst making the bonds.

33
Q

What is the practical method to measure temperature change in a reaction?

A

Place a beaker in a polystyrene cup and use a thermometer.

34
Q

What should you do after adding sodium hydroxide into hydrochloric acid?

A

Measure the temperature change.

35
Q

How should you plot the results of a temperature change experiment?

A

Plot two lines of best fit - one when it’s increasing and one when the temperature is decreasing.

36
Q

What does the intersection of the two lines in a temperature change graph represent?

A

The maximum temperature.

37
Q

What type of process is bond breaking?

A

Endothermic.

38
Q

What type of process is bond making?

A

Exothermic.

39
Q

What is the formula for calculating energy change in a reaction?

A

Energy Change = Energy of bonds broken - Energy of bonds made.

40
Q

Under what condition is a reaction overall endothermic?

A

If the energy needed to break the bonds in the reactants is more than the energy given out when new bonds form.

41
Q

Under what condition is a reaction overall exothermic?

A

If the energy given out when new bonds form is more than the energy needed to break the bonds in the reactants.

42
Q

What is the first step in bond energy calculations?

A

Do two separate calculations for bonds broken and bonds made.

43
Q

What should you do after adding the bonds in energy calculations?

A

Cross off the bonds as you add them up.

44
Q

What does it mean if a reaction is exothermic?

A

More energy is released making bonds than needed to break bonds.

45
Q

What does it mean if a reaction is endothermic?

A

Less energy is released making bonds than needed to break bonds.

46
Q

What is contained in a cell that produces electricity?

A

Chemicals that react.

47
Q

What is the simplest design of a cell?

A

Two electrodes made from metals of different reactivity immersed in an electrolyte.

48
Q

What role does the more reactive metal play in a cell?

A

It forms ions more easily with the electrolyte and releases ions, giving it a negative charge.

49
Q

What happens to electrons in a cell?

A

They flow around the circuit to the now more positive electrode.

50
Q

What causes a voltage to be produced in an electrochemical cell?

A

The difference in the ability of the electrodes to release electrons

This difference in reactivity between the electrodes is crucial for voltage generation.

51
Q

What is the role of the negative electrode in an electrochemical cell?

A

It is the more reactive metal, loses electrons, and is oxidised

The negative electrode is often referred to as the anode.

52
Q

What is the role of the positive electrode in an electrochemical cell?

A

It is the less reactive metal, gains electrons, and undergoes reduction

The positive electrode is commonly known as the cathode.

53
Q

What factors affect the voltage produced in an electrochemical cell?

A

Electrodes and electrolyte

Specific factors include the difference in reactivity, surface area of the electrode, distance between them, type, concentration, and temperature of the electrolyte.

54
Q

How does the difference in reactivity relate to voltage?

A

The greater the difference in reactivity, the greater the voltage

This principle is fundamental to the design of batteries and electrochemical cells.

55
Q

What is a battery?

A

Two or more cells connected in series

Batteries can be classified into rechargeable and non-rechargeable types.

56
Q

What happens to non-rechargeable batteries over time?

A

The electrodes degrade and the reactions are non-reversible

This degradation leads to the eventual death of the battery once reactants are consumed.

57
Q

What are the advantages of non-rechargeable batteries?

A

They are cheap

However, they pose disposal challenges due to toxic chemicals.

58
Q

What are the disadvantages of non-rechargeable batteries?

A

Disposal issues and non-reversible reactions

The toxic chemicals in used cells require careful handling.

59
Q

What defines rechargeable batteries?

A

The reactions are reversible and can be recharged

This feature allows them to be used multiple times.

60
Q

What are the advantages of rechargeable batteries?

A

Can be used many times and are cheaper than fuel cells

Their longevity makes them a preferred choice for many applications.

61
Q

What are the disadvantages of rechargeable batteries?

A

Takes time to recharge and are more expensive than non-rechargeable cells

This can affect their usability in certain situations.

62
Q

What is a fuel cell?

A

A cell supplied by an external source of fuel and oxygen or air

Fuel cells convert chemical energy directly into electrical energy.

63
Q

How does a fuel cell operate?

A

Fuel donates electrons at one electrode while oxygen gains electrons at the other

This electrochemical process generates electricity.

64
Q

What occurs as the fuel enters the fuel cell?

A

It becomes oxidised, setting up a voltage within the cell

Oxidation of the fuel is essential for energy release.

65
Q

What is produced in a hydrogen-oxygen fuel cell?

A

Energy and water

This reaction highlights the efficiency and environmental benefits of fuel cells.

66
Q

What is the negative electrode in hydrogen-oxygen fuel cells?

A

H2 → 2e + 2H*

The negative electrode is also known as the anode.

67
Q

What is the positive electrode reaction in hydrogen-oxygen fuel cells?

A

O2 + 4H* + 4e → 2H2O

The positive electrode is also known as the cathode.

68
Q

What is the overall reaction in hydrogen-oxygen fuel cells?

A

O2 + 2H2 → 2H2O

69
Q

List three advantages of hydrogen-oxygen fuel cells.

A
  • They do not produce any pollution.
  • They produce more energy per kilogram than either petrol or diesel.
  • No power is lost in transmission as there are no moving parts.
70
Q

What is a disadvantage of hydrogen-oxygen fuel cells related to materials?

A

Materials used in producing fuel cells are expensive.

71
Q

True or False: Hydrogen-oxygen fuel cells are affected by low temperatures.

A

True

72
Q

Fill in the blank: Hydrogen is _______ to produce and store.

A

expensive

73
Q

What is the environmental benefit of hydrogen-oxygen fuel cells regarding waste?

A

No batteries to dispose of which is better for environment + no toxic chemicals.

74
Q

What type of process do hydrogen-oxygen fuel cells use for energy production?

A

Continuous process

75
Q

What are high-pressure tanks needed for in hydrogen-oxygen fuel cells?

A

To store oxygen and hydrogen

76
Q

List two reasons why hydrogen-oxygen fuel cells can be considered dangerous.

A
  • High pressure tanks needed to store oxygen and hydrogen
  • Hydrogen is expensive to produce and store

Chemistry (16 decks)

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