Chemistry - Test Your Understanding

[Evolio]

Chemistry - Test Your Understanding
Hey everyone!  Following on from the other threads created, J_Rho suggested we make a Chemistry thread as well!

Every week on Monday, a handful of questions on a topic will be posted to help you revise, consolidate knowledge and help each other out! You can post your answers for feedback, as well as having the opportunity to give others feedback! (If you want to post anon you can PM me or any other member to post them for you)

This week we are going to be testing our knowledge of 'Obtaining Energy from Fuels', which includes the first 6 dot points of the 3 4 study design!

Week 1: Obtaining Energy From Fuels

Q1. Which one of the following statements best defines a renewable energy resource?
A. An energy resource that will not be consumed within our lifetime.
B: An energy resource that does not produce greenhouse gases when consumed.
C: An energy resource derived from plants that are grown for the production of liquid biofuels
D: An energy resource that can be replaced by natural processes within a relatively short time.
[VCAA 2015, Q5]

Q2. Describe and explain the energy transformations involved in the production of electricity from coal [3 marks]

Q3. Write the balanced thermochemical equations for the complete and incomplete combustion of ethanol [2 marks]

Q4. What is the definition of gas pressure? [1 mark]

Q5. VCAA 2014, Q 3a

Q6. VCAA 2014, Q 3 c ii

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Note: I probably won't be able to do this every week so if other peeps doing Chemistry would like to contribute questions, please feel free to do so! We could take turns providing questions. PM me and I can send you the template J_Rho sent me.

Solutions to Week 1

1. D. A renewable resource can be replaced at a faster rate than it it used up
2. Coal is combusted where the chemical energy in the coal is converted to thermal (heat) energy. This heat energy is used to boil water, producing steam. This steam rises and moves the turbine which spins, converting thermal energy to mechanical energy. The turbine then drives the generator, converting mechanical energy to electrical energy, thus producing electricity.
3. Complete: C2H5OH (l) + 3O2 (g) --> 2CO2 (g) + 3H2O (l) delta H= -1360 KJ/mol
Incomplete: C2H5OH (l) + 2O2 (g) --> 2CO (g) + 3H2O (l) delta H= -1360 KJ/mol
4. Force exerted on a unit area of the surface as the gas particles collide with the surface
5. Taken from VCAA 2014 Examiners Report
 
6. The heat content is measured in k/g since fuels such as canola oil and biodiesel are impure compounds. This means that we do not know how much of each element is in the compound and so we don't have a specific formula for the molar mass. Since we don't have the molar mass, we cannot divide it by the mass to figure out the number of mol.

[Geoo]

Ohh, this is also going to be a good revision tool!

Spoiler

Q1. Which one of the following statements best defines a renewable energy resource?
A. An energy resource that will not be consumed within our lifetime.
B: An energy resource that does not produce greenhouse gases when consumed.
C: An energy resource derived from plants that are grown for the production of liquid biofuels
D: An energy resource derived from plants that are grown for the production of liquid biofuels.
[VCAA 2015, Q5]

Q2. Describe and explain the energy transformations involved in the production of electricity from coal [3 marks]
Chemical energy is transformed into thermal energy, when coal is combusted, and the thermal energy from the reaction is used to heat up water. That water then undergoes a phase change from a liquid to steam, and said steam turns a turbine. So, the thermal energy is then transformed into mechanical energy, as the steam turns the turbine. The final transformation is from mechanical energy to electrical energy, as the turbine spins, it moves electrons (via magnetic field, i'm not good at physics don't come for me), thus generating electricity.

Q3. Write the balanced thermochemical equations for the complete and incomplete combustion of ethanol [2 marks]
Complete:
C₂H₅OH (l) + 3O₂ (g) --> 2CO₂ (g) + 3H₂O (g)
Incomplete:
C₂H₅OH (l) + 2O₂ (g) --> 2CO (g) + 3H₂O (g)

Q4. What is the definition of gas pressure? [1 mark]
The amount of force excreted by a gas against it's confines.

Q6. VCAA 2014, Q 3 c ii
(ooo, this is a bit of a harder one). Since one mole of a substance can have a different mass depending on what the substance is, it would be harder to make a comparison on the fuel content. So, it would be fairer to measure in grams, as it allows for a better comparison on the efficiency of fuels.

Kind of rushed answers but here they are.

[Evolio]

Hey Geoo!

Spoiler

1. I am so sorry, this is my fault! I wrote the D answer twice! D was supposed to be 'an energy resource that can be replaced by natural processes within a relatively short time, making D for the correct answer. ' Please let me know when I make a mistake so I can fix it. 😥
2. Your answer is very detailed and clearly explains each of the energy transformations. Oh also, the turbine's movement drives the generator, thus producing electrical energy. It's just a minor detail I would add in, along with the electrons part. I would give you full marks!
3. Since it's a thermochemical equation, you need to write the enthalpy change denoted by the delta H. So, at the end of each equation, you would write delta H= =-1360 KJ/mol. I got this value from the data booklet by the way.
4. I think you meant 'exerted'. You could be more specific by saying 'force exerted on a unit area of the surface as the gas particles collide with the surface' as pressure increases depending on the frequency of collisions against the container that the gas particles are held in. However, I really like how you specified 'gas'.
5. I'll fix the image.
6.  This is a good try however I believe it is incorrect, referring to the examiner's reports. They measured it in KJ/g rather than KJ/mol because canola oil and biodiesel are mixtures. They are not pure compounds, meaning that they do not have a specific molar mass (as they do not have a specific formula as we don't know how much of each element is in the mixture), so without us knowing the correct molar mass, we wouldn't be able to express KJ in mol so g is more meaningful.