3b.2 - Energy

Question 1

What is an exothermic reaction? (2)

Answer 1

• Gives out energy

- Shows with rise in temperature

Question 2

What is an endothermic reaction? (2)

Answer 2

• Takes in energy

- Shows with decline in temerature

Question 3

What happens during a chemical reaction?

Answer 3

Old bonds are broken and new bonds are made

Question 4

Is bond breaking exothermic or endothermic?

Answer 4

Endothermic

Question 5

Why is endothermic reactions bond breaking?

Answer 5

Energy must be supplied to break existing bonds

Question 6

Is bond making exothermic or endothermic?

Answer 6

Exothermic

Question 7

Why is bond making exothermic?

Answer 7

Energy is released when bonds are made

Question 8

What makes an endothermic reaction an endothermic reaction?

Answer 8

The energy required to break old bonds is greater than the energy released when new bonds are formed

Question 9

What makes an exothermic reaction an exothermic reaction?

Answer 9

The energy released in bond formation is greater than the energy used in breaking old bonds

Question 10

How can energy transfer be measured? (3)

Answer 10

• Taking the temperature of the reagents and making sue they’re the same
• Mixing them in a polystyrene cup
• Measuring the temperature of the solution at the end

Question 11

What is the biggest problem with measuring energy?

Answer 11

Energy is lost to the surroundings

Question 12

How can you reduce the amount of energy lost to surroundings when measuring energy transfer? (2)

Answer 12

• Insulating the polystyrene cup

- Put a lid on it to stop evaporation

Question 13

What is fuel energy energy calculated using?

Answer 13

Calorimetry

Question 14

What is the method of calculating fuel energy? (6)

Answer 14

• Put 50g water in the copper can and record it’s temperature
• Weigh the spirit burner and lid
• Put spirit burner underneath the can, and light the wick
• Stir the water, stirring constantly until it reaches about 50C
• Put out the flame using the butner lid and measure the final temperature of the water
• Weigh the spirit burner and lid again

Question 15

What is the equation for the amount of energy transferred?

Answer 15

Energy transferred (J)= Mass of water (g) x specific heat capacity (C) x change in temperature (degrees)

Question 16

What type of reaction has a negative energy change?

Answer 16

Exothermic

Question 17

What does the energy level diagram look like for an exothermic reaction?

Answer 17

• Straight line at top of graph
• Curve up a little
• Falls
• Line

Question 18

What the initial rise of the line represent on an energy level diagram?

Answer 18

The activation energy

Question 19

What type of reaction has a positive energy change?

Answer 19

Endothermic

Question 20

What does the energy level diagram look like for an endothermic reaction?

Answer 20

• Straight line at bottom of graph
• Curve up
• Decreases a little
• Line

Question 21

What does the difference of height represent on an endothermic energy level diagram?

Answer 21

Energy taken in per mole

Question 22

What is the activation energy?

Answer 22

Minimum amount of energy needed to break the bonds between reacting particles

Question 23

Using known bond energies calculate the energy change for this reaction:
H---H + Cl---Cl -> 2H---Cl
H2 + Cl2 -> 2HCl
Bond energies:
H---H: +436 kJ/mol
Cl---Cl: +242 kJ/mol
H---Cl: +431 kJ/mol

Answer 23

1) Calculate how much energy to break bonds:
436 + 242 = 678 kJ

2) Calculate how much energy to make bonds:
2 x 431 = 862 kJ

3) Overall energy change:
Energy to break - energy to make
678 - 862 = -184 kJ/mol

Question 24

Why is hydrogen used to power vehicles?

Answer 24

• Burns to form water

- Exothermic reaction so gives out energy

Question 25

What is the pro of hydrogen gas to power vehicles?

Answer 25

• Produces water which is clean

Question 26

What are the cons of hydrogen gas to power vehicles? (3)

Answer 26

• You need a special and expensive combustion engine
• You need to use energy to make it
• Hydrogen is hard to store safely

Question 27

What is a fuel cell?

Answer 27

An electrical cell that’s supplied with fuel and oxygen and uses energy from the reaction between them to generate electricity

Question 28

What are the advantages of using fuel cells in the car industry? (3)

Answer 28

• Doesn’t produce conventional pollutants
• Makes countries less dependent on crude oil
• Only products are water and heat

Question 29

What are the disadvantages of of using fuel cells in the car industry? (3)

Answer 29

• Takes up more space to store than a liquid
• Very explosive
• Made form hydrocarbons or the electrolysis of water