Energy & Fuels: Chemical energy to mechanical movement

Question 1

What is an exothermic reaction?

Answer 1

A reaction where heat is released from the system

Question 2

What is an endothermic reaction?

Answer 2

A reaction where heat is absorbed into the system

Question 3

What is internal energy? What is the symbol for it?

Answer 3

• It is the total energy of the molecules from their kinetic energy (e.g. vibration energy…) and potential energy (e.g. bonds…)
• symbol = U

Question 4

If the internal energy decreases, is this exo or endothermic? Why is this?

Answer 4

When internal energy decreases this indicates that it is leaving the system making it exothermic

Question 5

How can we measure U? What other methods are there to measure internal energy?

Answer 5

• We can’t measure U

- We can measure the change in ∆U (change in internal energy)

Question 6

What is internal energy (U) a property of?

Answer 6

The pressure, temperature and volume of the of the system

Question 7

What happens to the energy when U decreases?

Answer 7

It is released as Q and W (heat and work)

Question 8

What is enthalpy?

Answer 8

The potential energy that can be obtain as heat

Question 9

What is ∆H a property of?

Answer 9

P, T and V

Question 10

What is the enthalpy when ∆H is - and when it is +?

Answer 10

- = exothermic
\+ = endothermic

Question 11

What is the SI unit of enthalpy?

Answer 11

kJ/mol

Question 12

What is specific enthalpy (aka energy density)?

Answer 12

The heat of combustion in kJ/mass (i.e. how much energy released when combusted)

Question 13

How can the heat of combustion be measured? How does it do this?

Answer 13

• In a calorimeter
• Reaction occurs in insulated container so no (theoretical) heat loss, causes temperature of water to increases and this can be used to calculate the total energy released

Question 14

What are the 4 motions of a four stroke engine?

Answer 14

1 - Intake stroke
2- compression stroke
3 - Power stroke (this can be considered as the spark ignition [3a] and then the power stroke [3b])
4 - Exhaust stroke

Question 15

Explain what happens in the first motion

Answer 15

The intake port opens and gasoline and air are mixed in the combustion chamber as the piston goes down sucking it into the chamber

Question 16

Explain what happens in the second motion

Answer 16

Intake and exhaust valves are closed and the gasoline/air mixture are compressed as the piston goes up

Question 17

Explain what happens in the third motion

Answer 17

The spark plug fires and the compressed gas and air ignite forcing the piston down

Question 18

Explain what happens in the fourth motion

Answer 18

The exhaust port opens and the combustion products are pushed out of the cylinder as the piston goes up

Question 19

During the first and second motion what is happening to the system? What is remaining constant? What is this process called and why is it this one?

Answer 19

• The volume increases
• Pressure and temperature are constant
• This is expansion as the temperature and pressure are constant as the volume increases

Question 20

During the second to third motion (spark ignition) what is happening to the system? What is remaining constant? What is this process called and why is it this one?

Answer 20

• Pressure and temperature are increasing as volume decreases
• The entire system is changing
• This is adiabatic compression as temperature is changing as the volume decreases

Question 21

During the third motion what is happening to the system? What is remaining constant? What is this process called and why is it this one?

Answer 21

• Pressure and temperature both increases
• Volume is constant
• Combustion as pressure increases while volume is constant

Question 22

During the third motion (power stroke) to fourth motion what is happening to the system? What is remaining constant? What is this process called and why is it this one?

Answer 22

• Pressure is decreasing, volume increases and temperature decreasing
• There are no constants
• Adiabatic expansion as temperature is changing as the volume increases

Question 23

What happens after the fourth motion?

Answer 23

The cycle goes back to the second then first motion (compression then intake) to repeat the cycle again

Question 24

How do you figure out the net work from the four stroke cycle?

Answer 24

Calculate the energy change in the compression + energy change in the expansion

Question 25

Is it possible for all energy to be converted into work? Why?

Answer 25

No, Law of thermodynamics make it impossible for all energy to be converted into work