Energy & Fuels: Chemical energy to mechanical movement Flashcards
What is an exothermic reaction?
A reaction where heat is released from the system
What is an endothermic reaction?
A reaction where heat is absorbed into the system
What is internal energy? What is the symbol for it?
- It is the total energy of the molecules from their kinetic energy (e.g. vibration energy…) and potential energy (e.g. bonds…)
- symbol = U
If the internal energy decreases, is this exo or endothermic? Why is this?
When internal energy decreases this indicates that it is leaving the system making it exothermic
How can we measure U? What other methods are there to measure internal energy?
- We can’t measure U
- We can measure the change in ∆U (change in internal energy)
What is internal energy (U) a property of?
The pressure, temperature and volume of the of the system
What happens to the energy when U decreases?
It is released as Q and W (heat and work)
What is enthalpy?
The potential energy that can be obtain as heat
What is ∆H a property of?
P, T and V
What is the enthalpy when ∆H is - and when it is +?
- = exothermic \+ = endothermic
What is the SI unit of enthalpy?
kJ/mol
What is specific enthalpy (aka energy density)?
The heat of combustion in kJ/mass (i.e. how much energy released when combusted)
How can the heat of combustion be measured? How does it do this?
- 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
What are the 4 motions of a four stroke engine?
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
Explain what happens in the first motion
The intake port opens and gasoline and air are mixed in the combustion chamber as the piston goes down sucking it into the chamber
Explain what happens in the second motion
Intake and exhaust valves are closed and the gasoline/air mixture are compressed as the piston goes up
Explain what happens in the third motion
The spark plug fires and the compressed gas and air ignite forcing the piston down
Explain what happens in the fourth motion
The exhaust port opens and the combustion products are pushed out of the cylinder as the piston goes up
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?
- The volume increases
- Pressure and temperature are constant
- This is expansion as the temperature and pressure are constant as the volume increases
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?
- 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
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?
- Pressure and temperature both increases
- Volume is constant
- Combustion as pressure increases while volume is constant
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?
- Pressure is decreasing, volume increases and temperature decreasing
- There are no constants
- Adiabatic expansion as temperature is changing as the volume increases
What happens after the fourth motion?
The cycle goes back to the second then first motion (compression then intake) to repeat the cycle again
How do you figure out the net work from the four stroke cycle?
Calculate the energy change in the compression + energy change in the expansion
Is it possible for all energy to be converted into work? Why?
No, Law of thermodynamics make it impossible for all energy to be converted into work
