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chem spec > energy changes > Flashcards

energy changes Flashcards

1
Q

define an exothermic reaction

A

a reaction that transfers energy to the surroundings, so the temperature of the surroundings increases. therefore, the product molecules have less energy than the reactants

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2
Q

examples of exothermic reactions

A
  • combustion
  • many oxidation reactions
  • neutralisation
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3
Q

describe reaction profile of exothermic reactions

A
  • reactants are at a higher energy than the products because energy has been transferred from the molecules to the surroundings (energy is lost to the surroundings)
  • negative energy change
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4
Q

everyday uses of exothermic reactions

A
  • self-heating cans
  • hand warmers
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5
Q

define an endothermic reaction

A

a reaction that takes in energy from the surrounds so the temperature of the surroundings decrease. therefore, the reactant molecules have less energy than the products

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6
Q

examples of endothermic reactions

A
  • thermal decompositions
  • reaction of citric acid and sodium hydrogencarbonate
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7
Q

describe reaction profile of endothermic reactions

A
  • reactants are at a lower energy than the products because energy is taken in from the surroundings to the molecules (temperature of surroundings decrease)
  • positive energy change
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8
Q

everyday uses of endothermic reactions

A
  • sports injury packs (cold packs)
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9
Q

define activation energy

A

the minimum amount of energy that particles must have to react (for a reaction to occur)

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10
Q

what for reaction profiles show

A
  • the relative energies of reactants and products
  • activation energy
  • if its exothermic or if its endothermic
  • overall energy change
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11
Q

when is energy supplied and released during a chemical reaction

A
  • energy must be supplied to break bonds in the reactants
  • energy is released when bonds in the products are formed
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12
Q

what is bond energy

A

it’s the energy needed to break bonds and the energy released when bonds are formed

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13
Q

how do you calculate overall energy change

A
  • calculate the bond energies for the reactants and for the products
    energy change = energy of bond breaking - energy of bond making
    i.e. bond energy of reactants minus bond energy of products
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14
Q

what are the units for overall energy change

A

kJ/mol

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15
Q

define overall energy change

A

the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed

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16
Q

describe making and breaking bonds in an exothermic reaction

A

the energy released from forming new bonds is greater than the energy needed to break existing bonds i.e. overall energy change will be negative

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17
Q

describe making and breaking bonds in an endothermic reaction

A

the energy needed to break existing bonds is greater than the energy released from forming new bonds i.e. overall energy change will be positive

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18
Q

define an electrolyte

A

a solution that can conduct electricity e.g. an ionic compound solution

19
Q

how can you produce electricity using an electrolyte

A

by taking two different metals and placing them into an electrolyte

20
Q

what do cells contain

A

chemicals which react to produce electricity

21
Q

how is a simple cell made

A

it can be made by connecting two different metals in contact with an electrolyte

22
Q

what is the potential difference produced by a cell dependent on

A
  • type of electrode
  • type of electrolyte
  • concentration; the greater the concentration of ions in the electrolyte, the greater the potential difference produced
23
Q

how do you provide a greater voltage

A

by connecting two or more cells together in series (batteries do this)

24
Q

when will cells produce electricity

A

when there is a difference in reactivity between the two metals e.g., if they are the same metal, there will be no difference in reactivity, so no current will flow

25
Q

how does difference in reactivity relate to potential difference

A

the greater the difference in reactivity between the two metals, the greater the potential difference produced by the cell. e.g., using magnesium and copper will produce a greater potential difference than using iron and copper

26
Q

what happens in non-rechargeable cells and batteries

A

the reaction is irreversible, so the chemical reactions stop when one of the reactants has been used up (i.e. the limiting reactant), meaning no more electricity is produced

27
Q

how do rechargeable cells and batteries work

A

the reaction is reversible, so when an external electrical current is supplied, the chemical reaction is reversed and the reactants are produced again

28
Q

why does the more reactive metal transfer electrons to a less reactive metal. use the example of zinc and copper electrodes

A

because more reactive metals have a higher tendency to lose electrons to form positive ions e.g., the zinc electrode will lose electrons (oxidise) to the copper electrode which gains these electrons (reduced). because the zinc is the source of the electrons, the more reactive metal is the negative electrode, and the less reactive metal is the positive electrode

29
Q

example of a non-rechargeable battery

A

an alkaline battery

30
Q

what are fuel cells supplied by

A

an external source of fuel (e.g. hydrogen) and oxygen or air

31
Q

how do fuel cells produce a potential difference

A

the fuel is oxidised electrochemically within the fuel cell to produce a potential difference

32
Q

function of a hydrogen fuel cel

A

the overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water

33
Q

what do hydrogen fuel cells offer

A

a potential alternative to rechargeable cells and batteries

34
Q

cons of rechargeable batteries and cells in relation to fuel cells

A
  • rechargeable batteries store less electricity the more charging cycles they go through, so eventually need to be replaced, however hydrogen fuel cells do not get less efficient the longer they run
  • batteries store less energy than fuel cells, so need to be recharged more often
  • recharging batteries takes a long time, whereas fuel cells take a very short amount of time to refill with hydrogen
35
Q

pros of rechargeable cells and batteries in relation to fuel cells

A
  • fuel cells run on hydrogen, which is an explosive gas if exposed to a flame, and is very difficult to store safely. no dangerous fuels are required with rechargeable batteries that are manufactured correctly
  • hydrogen fuel cells produce a relatively low potential difference so several are needed together, whereas rechargeable batteries produce a greater potential difference
  • hydrogen is a gas so takes up way more space to store than rechargeable batteries
36
Q

fuel cell reaction of hydrogen and oxygen; what does this reaction do

A

2H₂ + O₂ → 2H₂O
it releases energy in the form of an electrical current and the only waste product it releases is water

37
Q

what happens at the negative electrode of a fuel cell; mention the half equation

A

hydrogen molecules lose electrons (are oxidised) to form H⁺ ions; we can represent this using
2H₂ → 4H⁺ + 4e⁻

38
Q

what happens to the electrons lost from the hydrogen molecules at the negative electrode of a fuel cell, and what happens to the ions formed

A

the lost electrons pass through the wire to form the electrical current. the H⁺ ions produced from the oxidation of hydrogen molecules are then passed through the electrolyte and to the cathode where they react with oxygen to form water

39
Q

what does the electrolyte not/let through it

A

it doesn’t let hydrogen or electrons through, it only allows H⁺ ions (protons) through it

40
Q

what happens at the positive electrode of a fuel cell; mention the half equation

A

oxygen molecules gain electrons (reduced) and react with the H⁺ ions to form water; we can represent this using
O₂ + 4H⁺ + 4e⁻ → 2H₂O

41
Q

how are hydrogen fuel cells different from electrolysis, and how are they the same

A

DIFFERENCE: in electrolysis, the negative electrode is the cathode and the positive electrode is the anode. in hydrogen fuel cells, it is the opposite

SIMILARITY: the anode is always the site of oxidation and the cathode is always the site of reduction

42
Q

where does each thing come from in a hydrogen fuel cell

A
  • the H⁺ ions come from the oxidation of hydrogen at the anode and have travelled through the electrolyte
  • the electrons have come from the electrical circuit
43
Q

what is oxidised, what is reduced and where in a hydrogen fuel cell

A
  • hydrogen is oxidised at the anode (the negative electrode)
  • oxygen is reduced at the cathode (the positive electrode)

chem spec (12 decks)

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