C5-Energy changes

Question 1

Can energy be created and destroyed?

Answer 1

Energy is conserved in chemical reactions so is neither created or destroyed

Question 2

What are reactions called that transfer energy from the reacting substances to their surroundings?

Answer 2

Exothermic

Question 3

What are reactions called that transfer energy to the reacting substances from their surroundings?

Answer 3

Endothermic

Question 4

How can exothermic and endothermic reactions be used?

Answer 4

Exothermic-hand warmers, self heating cans
Endothermic-instant cold packs for sport injuries

Question 5

What is the minimum amount of energy needed before colliding particles of reactants have suffficient energy to cause a reaction?

Answer 5

Activation energy

Question 6

During bond breaking and bond making which is exothermic and which is endothermic?

Answer 6

Bond breaking-endothermic (requires energy from the surroundings)
Bond making-exothermic (releases energy to the surroundings)

Question 7

Fill in the blanks:
In an exothermic reaction, the energy released when new bonds are formed is …………. than the energy absorbed when bonds are broken

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

Answer 7

• Greater
• Less

Question 8

What happens when two metals are dipped in a salt solution and joined by a wire?

Answer 8

The more reactive metal will donate electrons to the less reactive metal-this forms a simple electrical cell.

Question 9

What affects the voltage produced by a cell?

Answer 9

The greater the difference in reactivity between 2 metals, the higher the voltage produced by the cell

Question 10

How can hydrogen fuel cells be used as a fuel?

Answer 10

1) Hydrogen gas is supplied as a fuel to the negative electrode-it diffuses through the graphite electrode through the electrolyte and reacts with oxygen ions to form water and provides a source of electrons to an external circuit.
2) Oxygen gas is supplied to the positive electrode-it diffuses through the graphite and reacts to form water, accepting electons from the external circuit.

Question 11

What is the half equation of hydrogen fuel cells at the negative electrode?

Answer 11

H₂ + –> 2H⁺ + 2e⁻

Question 12

What is the half equation of hydrogen fuel cells at the positive electrode?

Answer 12

O₂ + 4H⁺ + 4e⁻ –>2H₂O

Question 13

What are advantages of hydrogen fuel cells?

Answer 13

• Do not need to be electrically recharged-quicker
• No pollutants are produced
• Can be a range of sizes for different uses

Question 14

What are disadvantages of hydrogen fuel cells?

Answer 14

• Hydrogen is highly flammable
• Hydrogen is sometimes produced for the cell by non-renewable sources
• Hydrogen is difficult to store

Question 15

What charge is the anode and cathode in fuel cells?

Answer 15

• Negative
• Positive
  This is the opposite of what they are called in electrolysis