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Chemistry > C5 - Energy Changes > Flashcards

C5 - Energy Changes Flashcards

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1
Q

What is an exothermic reaction?

A
  • Release energy to surroundings
  • Indicated by a rise in temp
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2
Q

What are some examples of exothermic reactions?

A
  • Combustion
  • Neutralisation
  • Most oxidation reactions
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3
Q

What is an endothermic reaction?

A
  • Absorbs heat energy from surroundings
  • Indicated by a fall in temp
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4
Q

What is activation energy?

A
  • Minimum amount of energy
  • particles require to collide with each other
  • and react
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5
Q

If reactant molecules in a reaction have more energy in their chemical bonds than the product molecules, will energy be absorbed or released overall?

A

Released

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6
Q

What three things happen during a chemical reaction?

A
  1. Bonds of reactant molecules are broken
  2. Atoms rearrange themselves
  3. Bonds of product molecules are formed
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7
Q

What do bond energies refer to?

A

Amount of energy required to break one mole of a particular covalent bond.

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8
Q

What kind of process is bond-breaking?

A

Endothermic - requires energy

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9
Q

What kind of process is bond-making?

A

Exothermic - releases energy

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10
Q

How do you calculate the overall energy change of a reaction?

A

Energy required to break the bonds of the reactants
-
Energy released by forming bonds of the product.

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11
Q

What is a cell?

A

Use chemical reactions to produce electricity

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12
Q

How is a cell made?

A
  • Connecting two different electrodes with a wire
  • Placing them in contact with an electrolyte.
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13
Q

What are the factors that affect the voltage of a cell?

A
  1. Greater the difference in reactivity of the two electrode metals, greater the voltage will be.
  2. Type + concentration of the electrolyte used.
  3. Conditions like temperature
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14
Q

What are rechargeable cells?

A
  • Can be recharged because the chemical reactions are reversed when external electrical current is supplied.
  • Batteries used in laptops and mobile phones
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15
Q

What are non-rechargeable cells?

A
  • Chemical reactions stop when one of the reactants has been used up.
  • Alkaline batteries are non-rechargeable.
  • Usually used in smoke alarms and TV remotes.
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16
Q

Fuel cells are electrochemical cells. What does this mean?

A

Converts energy between electrical and chemical forms

17
Q

What does a hydrogen-oxygen fuel cell do?

A

Uses hydrogen and oxygen to form water and generates a lot of electrical energy.

18
Q

Describe the structure of a fuel cell:

A
  • Centre: electrolyte (potassium hydroxide)
  • On either side of centre: electrodes
  • Positive cathode (right) + negative anode (left)
  • Wire connects electrodes
  • Anode compartment (left): hydrogen enters inlet
  • Cathode compartment (right): oxygen enters inlet + water and heat leave outlet
19
Q

In fuel cells what are the electrodes made of ?

A
  • Porous carbon (tiny holes)
  • Catalysts to speed up reaction
20
Q

How does a fuel cell actually work?

A
  1. Hydrogen enters anode compartment through inlet
  2. Hydrogen oxidised by anode
  3. Hydrogen loses electrons
  4. Electrons move up through wire to cathode whilst H+ move through electrolyte to cathode
  5. Electrons and H+ react with oxygen which enters cathode compartment through inlet
  6. All combine to make water
  7. Water leaves cell
21
Q

What are the half equations for the reaction which occurs in fuel cells?

A

H2 –> 2H+ + 2e-

O2 + 4H+ + 4e –> 2H2O

22
Q

Key points to remember about fuel cells for exams?

A
  • As fuel (hydrogen) enters the cell
  • It becomes oxidised (loses electrons)
  • Sets up a potential difference across the cell (diff in charge of electrodes)
  • Drives electrons around circuit
  • Generates electricity
23
Q

What are the advantages of using hydrogen-oxygen fuel cells?

A
  • Only require hydrogen and oxygen
  • Don’t produce waste
  • Don’t produce carbon dioxide or other pollutants
  • Last longer than batteries
  • Less polluting to dispose of
24
Q

What are the disadvantages of using hydrogen-oxygen fuel cells?

A
  • Hydrogen takes up too much space to store
  • Hydrogen is explosive when mixed with air
  • Making hydrogen requires energy

Chemistry (8 decks)

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