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GCSE Chemistry > C7 - Energy Changes > Flashcards

C7 - Energy Changes Flashcards

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1
Q

What does the Law of Conservation of Energy state?

A

Energy is conserved in chemical reactions. It is neither created or destroyed.

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2
Q

What is an Exothermic Reaction?

A

A reaction in which energy is transferred from the reacting substances TO the surroundings.

*Think about energy EXITING the substances to the surroundings.

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3
Q

What is an Endothermic Reaction?

A

A reaction in which energy is transferred to the reacting substances FROM the surroundings.

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4
Q

What is the easiest way to summarise Exothermic and Endothermic Reactions?

A

Exothermic = heat energy given out (to surroundings)

Endothermic = heat taken in (from the surroundings)

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5
Q

When measuring the temperature of a solution that is either undergoing an exothermic or endothermic reaction, what is the trend for temperature?

A

If the temperature goes up, it is exothermic. Heat is being given to the surroundings (solution) from the substances.

If the temperature goes down, it is endothermic. Heat is being given to the substances by the surroundings (solution).

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6
Q

What percentage of natural reactions are exothermic and endothermic?

A

90% are exothermic.

10% are endothermic.

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7
Q

What are 2 examples of exothermic reaction?

A

1) Combustion - e.g combustion of methane gas:

CH4 (g) + 2O2 (g) ———> CO2 (g) + 2H2O (l)

2) Respiration

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8
Q

What is a common use of exothermic reactions in sport?

A

Heat Pack (snaps and the chemicals mix)

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9
Q

What are 2 examples of an endothermic reaction?

A

1) Photosynthesis (sunlight to leaf)
2) Thermal decomposition reactions. E.g Calcium Carbonate thermal decomposition.

CaCO3 (s) ———> CaO (s) + CO2 (g)

Thermal decomposition rule = if one reactant gives two products, it is a thermal decompostion.

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10
Q

What is a common use of endothermic reactions in sport? What endothermic reactions is in sweets?

A

1) Artificial Ice Pack

2) Sherbet Sweets (the sherbet cools when it reacts with the water (saliva) in the mouth, causing a cooling effect.

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11
Q

In terms of energy content, how do the energy contents of the reactants and products of an EXOTHERMIC reaction compare?

A

The products have a lower energy content than the reactants.

This is because energy has been transferred from the substances to the surroundings.

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12
Q

In terms of energy content, how do the energy contents of the reactants and products of an ENDOTHERMIC reaction compare?

A

The products have a higher energy content than the reactants.

This is because energy has been transferred from the surroundings to the substances.

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13
Q

What is the reaction used in self-heating cans and hand warmers?

A

Exothermic

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14
Q

What is the reaction used in instant cold packs and drink coolers?

A

Endothermic

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15
Q

What specific reaction is used in non-reusable hand warmers? How long do non-reusable hand warmers last for?

A

Iron oxide is reacted with water with a sodium chloride catalyst - Exothermic

Iron Oxide + Water ———> Hydrated Iron (III) Oxide

Lasts: Non-reusable hand warmers can last for hours.

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16
Q

What specific reaction is used in reusable hand warmers? How can they be re-used?

A

The crystallisation of Sodium Ethanoate - Exothermic

Reuse:

  • Add the pack to hot water and then let it cool. It will then be able to recrystallise.
  • Reusable hand warmers can only work for 30 minutes before they need to be reset.
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17
Q

What reaction is usually used in self-heating cans? How does the mechanism of the can use the reaction?

A

Calcium Oxide + Water ———> Calcium Hydroxide - Exothermic

Mechanism:

  • Chemicals are at the bottom of the can.
  • Button pressed in the base of the can - breaks a seal and mixes water with calcium oxide.
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18
Q

What specific reaction is used in chemical cold packs? How long do they last for?

A

Ammonium nitrate + water

Lasts: 20 minutes

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19
Q

What do reaction profiles show?

A

The relative difference in energy of the reactants and products.

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20
Q

What does an exothermic reaction look like on a reaction profile and why?

90% of reactions occur like this.

A

The reactants have more energy (in kj/mol) than the products. This is because exothermic reactions give out energy to the surroundigs, meaning the products will have a lower energy level.

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21
Q

What does an endothermic reaction look like on a reaction profile and why?

10% of reactions occur like this.

A

The products have more energy than the reactants. This is because energy is transferred from the surroundings, meaning that the products end up having more energy.

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22
Q

What is Activation Energy? What type of reaction requires no activation energy

A

The minimum amount of energy needed before colliding particles of reactants have sufficient energy to start a reaction.

  • Activation energy could be by applying heat or mixing.
  • Displacement reactions require no activation energy.
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23
Q

How do you show Activation energy on a reaction profile?

A

Draw an arrow in the space before the curve drops or increases.

Arrow must touch the peak of the profile.

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24
Q

What type of reaction is Bond breaking?

Endothermic or Exothermic

A

Endothermic

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25
Q

What type of reaction is Bond Making?

Endothermic or Exothermic

A

Exothermic

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26
Q

Why is bond breaking endothermic? Why is bond making exothermic?

A

Bond breaking is an endothermic process as energy has to go IN to break the strong bonds.

Bond making is an exothermic reaction as energy needs to be released OUT to stabilise the bond.

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27
Q

Energy must be supplied to allow for Bond _______.

A

BREAKING

Bond Breaking is Endothermic as energy is supplied to the bonds.

28
Q

Energy is released when new bonds are formed in Bond ______.

A

MAKING

Bond Making is Exothermic as energy is released.

29
Q

In exothermic reactions (90% of reactions), is the energy released when new bonds are formed (as products) more or less than the energy needed to break the bonds in the reactants.

A

In an exothermic reaction, the energy released when new bonds are formed, is GREATER than the energy when bonds are broken.

More Energy (kj/mol) needed to form the bonds in the products as you are making STRONGER BONDS than the bonds that were broken.

30
Q

In endothermic reactions (10% of reactions), Is the energy released when new bonds are formed (as products) more or less than the energy needed to break the bonds in the reactants.

A

In an endothermic reaction, the energy released when new bonds are formed is LESS than the energy absorbed when bonds are broken.

More Energy (kj/mol) needed to break the bonds in the reactants as you are breaking STRONGER BONDS than the bonds that are being formed.

31
Q

What is the energy supplied to break the bonds between atoms called?

A

Bond energy

Measured in kj/mol

32
Q

What are common bond energies used for?

A

Common bond energies are used for calculating the energy change for chemical calculations.

By doing this, you can work out whether the reaction is exothermic or endothermic.

33
Q

How are common bond energies arranged?

A

They are arranged in a table (provided in exam) which states the amount of energy (kj/mol) that is needed to BREAK BONDS. When you want to know the amount of energu required for making new bonds, the amount of energy required is the same.

E.g C-C = 347kj/mol to break bonds. This means energy released forming a C-C bond is also 347kj/mol.

34
Q

What type of number will the energy change be if the chemical reaction is exothermic?

A

A negative number - energy lost to surroundings

35
Q

What type of number will the energy change be if the chemical reaction is endothermic?

A

Positive

36
Q

How do you calculate energy change using bond energies?

A

Energy Change = Energy of bonds broken (reactants) - Energy released when bonds made (products)

37
Q

Calculate the energy required to break the bonds in the reactants and make the bonds in the products for:

H—H + Br-Br ———> 2 H-Br

A 
H-H = 436kj/mol
Br-Br = 193kj/mol
H-Br = 366kj/mol
Reactants = 436+193 = 629kj/mol
Products = 2 x 366 = 732kj/mol

Energy change = energy supplied to break bonds - energy released to make bonds

= 629 - 732 = -103kj/mol

It’s exothermic (negative number) - more energy in bond making

38
Q

What is an electrochemcial cell?

A

A device capable of generating electrical energy from chemical reactions.

39
Q

How can an electrochemical cell be made?

A

By taking two different metals and placing them in an electrolyte, we can produce electricity.

For example, you could take copper and zinc as the electrodes , and place them in an electrolyte of lemon juice.

40
Q

What is an electrolyte?

A

A solution that can conduct electricity (e.g an ionic compound).

41
Q

How can a cell be set up? What causes electricity to be produced?

*Use copper and magnesium as the 2 metals, and an un-named electrolyte.

A

1) Strip of copper and strip of magnesium placed in an electrolyte.
2) Connect the 2 metals to a voltmeter - can see a potential difference between them.
3) Electric current will flow through the wire.

A chemical reaction takes place on the surface of the 2 metals.

42
Q

How long can an electrochemical cell produce electricity? When does it stop?

A

It can only produce electricity for a certain amount of time.

Until the chemicals in the cell run out and the reaction stops.

43
Q

Can a cell produce electricity if the 2 metals were the same?

A

No - no potential difference.

Cells can ONLY produce electricity if the metals are of different reactivities.

44
Q

How does the reactivity series impact the metals chosen for an electrochemical cell?

A

The greater the difference in reactivity, the greater the potential difference produced by the cell, and the more electricity produced.

E.g a cell containing magnesium and copper will have a large potential difference. However, a cell containing zinc and tin will have a lower potential difference.

45
Q

Does the electrolyte effect the potential difference of 2 metals?

A

Yes.

Don’t need to know anymore details at GCSE

46
Q

In a table showing the potential difference between Metal 1 and Metal 2, what does a positive and negative number mean?

A

If the voltage measured is POSITIVE, metal 1 is LESS reactive than metal 2.

If the voltage measured is NEGATIVE, metal 1 is MORE reactive than metal 2.

47
Q

What is a Battery?

A

Two or more cells connected in series to produce a greater voltage.

E.g If you have a cell that produces 2.5V, if you connect 2 of this cell in series, you will produce 5V.

48
Q

When does a non-rechargeable cell run out of battery?

A

When the reactants in the cell run out.

49
Q

What are some common examples of non-rechargeable cells?

A

1) Modern alkaline batteries
2) Carbon Zinc Batteries

*basically regular batteries in everyday appliances

50
Q

What are the advantages and disadvantages of non-rechargeable cells?

A

Advantages: Convenient. Cheap (short term).

Disadvantages: Finite life. Needs to be disposed of in a recycling centre. Costs more (long term).

51
Q

What makes a rechargeable cell rechargeable?

A

The chemical reactions can be REVERSED, when an electrical current is applied, to regenerate the original reactants.

52
Q

What are some common examples of rechargeable cells?

A

The batteries in iPads, phones and golf carts.

53
Q

What are the advantages and disadvantages of rechargeable cells?

A

Advantages: Can be used repeatedly. No emissions.

Disadvantages: More expensive. Can become weak and unpredictable with age. Takes time to recharge. Finite life.

54
Q

What happens to metals in terms of electrons when they react?

A

Metals tend to lose electrons and form positive ions.

55
Q

How does the reactivity of metals impact how readily they give up electrons?

A

The more reactive a metal, the more readily it gives off its electrons.

56
Q

What happens in terms of electrons, electrodes and half-equations to set up a cell of Zinc and Copper, in an electrolyte of aqueous Sodium Chloride?

A

Zinc is more reactive than copper. It is the more reactive metal in the cell. Therefore, the electrons will travel from zinc, through a wire and end up at the copper electrode.

Zn (s) ———> Zn2+ (aq) + 2e-
-Here we can see the half equation for the zinc electrode where the zinc electrode is losing electrons.

The copper electroce cannot except the electrons but the copper 2+ ions can except them.

Cu2+ (aq) + 2e- ———> Cu(s)

Summary:
-The most reactive metal loses electrons (acts as the negative electrode) and the least reactive metal gains the electrons.

Cell reaction (add 2 half-equations together): Zn(s) + Cu2+(aq) + 2e- ———> Zn2+(aq) + Cu(s) + 2e-:
-cancel 2e- on both sides
Overall cell reaction: Zn(s) + Cu2+(aq) ———> Zn2+(aq) + Cu(s)

  • This is a redox reaction because the zinc has been oxidised (losing electrons), and the copper 2+ has been reduced (gaining electrons).
  • KEY RULE* - Electrons flow from negative to positive so that means the zinc electrode is the negative electrode, and the copper electrode is the positive electrode.
57
Q

What metal becomes the negative electrode in a cell and why? Why do the electrode names switch?

A

The most reactive metal becomes the negative electrode.

  • Although oxidation takes place, it’s because the negative electrode is the anode (-) in a cell.
  • This is counter-intuitive due to electrolysis unit.

Electrode names (why?):

  • Electrons always flow from negative to positive. So, as oxidation is happening (loss), the zinc electrode is called the anode.
  • Electrodes are not named for their charge - it is whether they allow oxidation or reduction.
58
Q

What are the differences between electrochemical cells and electrolysis?

A

Electrolysis:
Anode (+) ——> Oxidation
Cathode (-) ——> Reduction

Electrochemical cell:
Anode (-) ——> Oxidation ——> Most reactive metal
Cathode (+) ——> Reduction ——> Least Reactive Metal

Remember, cells produce electricity. In electrolysis, electricity is supplied.

59
Q

How can you remember what happens at each electrode in an electrochemical cell?

A

At the Anode (-) it’s ‘OXANA’ - Oxidation at the Anode

At the Cathode (+) it’s ‘REDCAT’ - Reduction at the Cathode

60
Q

What would happen for a cell of zinc and silver?

A

Zinc is more reactive than silver - becomes the anode (-)

Zn (s) ——> Zn2+(aq) + 2e- OXIDATION at the anode

Ag+(aq) e- ——> Ag(s) REDUCTION at the cathode

Overall equation:

Need to multiply silver equation by 2 before adding the half-equations.

Zn(s) + 2Ag+(aq) + 2e- ——> Zn2+(aq) + 2Ag(s) + 2e-

Zn(s) + 2Ag+(aq) ——> Zn2+(aq) + 2Ag(s)

61
Q

What is a hydrogen fuel cell?

A

A cell where hydrogen fuel is reacted with either oxygen in the air - causing a chemical reaction to take place and an electric current to be generated.

62
Q

What is the word and chemical equation for the reaction that occurs in a hydrogen fuel cell?

A

Hydrogen + Oxygen ———> Water

2H2 + O2 ———> 2H2O

63
Q

What are the half-equations for the reactions at the negative and positive electrode in a hydrogen fuel cell?

*Higher Tier

A

Negative Electrode (Anode) - Oxidation:

2H2 ———> 4H+ + 4e- These 4 electrons form the current and pass through the wire.

Positive Electrode (Cathode) - Reduction:

O2 + 4H+ + 4e- ———> 2H2O

Here the oxygen molecules combine with the electrons and the hydrogen ions produced at the negative electrode.

64
Q

What happens to the hydrogen in terms of oxidation and reduction in a hydrogen fuel cell?

A

The hydrogen at the anode (-) / negative electrode is oxidised.

65
Q

What is the overall equation for the reaction in a hydrogen fuel cell?

A

2H2 + O2 ———> 2H2O

*The ions and electrons cancel from the half equations.

66
Q

What are the advantages and disadvantages of hydrogen fuel cells?

A

Advantages: No pollutants produced. Only waste product is water. Don’t need to be electrically recharged.

Disadvantages: Highly flammable. Difficult to store. Expensive.

GCSE Chemistry (17 decks)

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