VCE Chemistry Question Thread

[dream chaser]

Quick question. What is the difference between the origin and formation of fossil fuels or are they the same thing?

Also, what would the environmental impacts of Coal be for instance. I don't get what they say in my textbook.

And what does it mean in relation to about chapter 1 in the study design saying "(in reference to fossil fuels and biofuels) environmental impacts related to sourcing and combustion". For instance, would Electricity of coal be one and if so which one would it fall under(sourcing or combustion of environmental impacts?)

Thanks

[Yertle the Turtle]

Quick question. What is the difference between the origin and formation of fossil fuels or are they the same thing?

Also, what would the environmental impacts of Coal be for instance. I don't get what they say in my textbook.

And what does it mean in relation to about chapter 1 in the study design saying "(in reference to fossil fuels and biofuels) environmental impacts related to sourcing and combustion". For instance, would Electricity of coal be one and if so which one would it fall unde(sourcing or combustion of environmental impacts?)

Thanks

The origin of fossil fuels relates to their formation, but it is just talking about which fossil fuels you use (such as coal or crude oil, etc), whereas the formation is to do with how these fuels are created in the earth.

The main environmental impacts of the use of coal are to do with your next question, so I will answer them together. The sourcing of fuels relates to how these fuels are obtained, does it remove food sources if it is a biofuel and does it damage the environment with something like an open-cut mine for coal or potential oil-spills in the case of crude oil. The combustion then refers to how much the waste products (such as CO2) damage the environment when the fuel is combusted, so basically how much energy can you get from a certain amount of a) biofuels and b) fossil fuels. Thus when talking about coal you can talk about the methods of extracting coal and whether an open-cut mine damages the environment, and also about the amount of CO2 released to add to the Greenhouse Effect.
Hope this all helps.

[dream chaser]

The origin of fossil fuels relates to their formation, but it is just talking about which fossil fuels you use (such as coal or crude oil, etc), whereas the formation is to do with how these fuels are created in the earth.

The main environmental impacts of the use of coal are to do with your next question, so I will answer them together. The sourcing of fuels relates to how these fuels are obtained, does it remove food sources if it is a biofuel and does it damage the environment with something like an open-cut mine for coal or potential oil-spills in the case of crude oil. The combustion then refers to how much the waste products (such as CO2) damage the environment when the fuel is combusted, so basically how much energy can you get from a certain amount of a) biofuels and b) fossil fuels. Thus when talking about coal you can talk about the methods of extracting coal and whether an open-cut mine damages the environment, and also about the amount of CO2 released to add to the Greenhouse Effect.
Hope this all helps.

Okay, thanks for the reply.

So therefore, I don't need to know about how it is formed then? Also, I still don't get as to what you mean about 'origin'. And how do we get the environmental impacts relating to source and combustion for each fuel. Just by searching it up on the internet? Because I can't find any in the book about it.

[Lear]

Chemistry is a real life thing. You can definitely google up impacts of sourcing and combustion. Often textbooks do skip out on things.

[dream chaser]

Chemistry is a real life thing. You can definitely google up impacts of sourcing and combustion. Often textbooks do skip out on things.

Okay thanks. Would I need to know about the formation of fossil fuels in general and the formation of each specific fossil fuel? On the study design, it says they want us to only know about its renewability and origin. Also, what would the 'origin' of fossil fuels be? I'm confused by the term. And when I say "On the study design, it says they want us to only know about its renewability and origin", does that correspond to fossil fuels as a whole or about each specific fossil fuel.

This is just what I think. No idea if I am right or not with this.

I think the origin of fossil fuels correspond to 'what are the fossil fuels?' (as what Yertle the Turtle said). I think this because on the Study Design, they talk(I think) about Fossil Fuels as a whole when referring to origin. As different fossil fuels have slightly different ways of being formed, i.e some due to the decaying of plants and others using fractional distillation, I think 'origin' just means what are the fossil fuels. Also, when they do specify about particular fossil fuels, they don't mention about origin. Could someone please clarify this with me.

Also, how do you know what pollutants are produced in combustion of fuels. For instance, coal.

And is LPG the same as Crude oil because I can't find the amount of carbon dioxide produced from combustion of crude oil?

And what does "combustion of fuels as exothermic reactions with the reference to the use of energy transformations". For instance, if you take Electricity of coal(where the combustion of coal takes place) in coal-fired power stations, do you have to have an indepth knowledge of what is happening. Or if you can just explicitly tell the transformations that is enough(eg. chemical energy in coal --> thermal energy of burning coal --> etc..)?

All responses will be much appreciated. Thanks 

[persistent_insomniac]

Is the change in enthalpy the same thing as the heat of combustion because don’t they both have the same signs (delta H)?

[huity]

Is the change in enthalpy the same thing as the heat of combustion because don’t they both have the same signs (delta H)?

Hey there

Definitely not the same thing (although they're similar). The key difference is that change in enthalpy can be a negative or positive value. However, the heat of combustion is always a positive value.

Take a look at 11. Heat of combustion of common fuels in your data book 

[parchmentpaper]

100mL of ethane gas was mixed with 500mL oxygen gas at 20C and 1 atmosphere pressure and sparked. The reaction shown by the following equation occurred
2C2H6(g) + 7O2(g) -> 4CO2(g) +6H2O(l)
What would be the total volume of gas present when the reaction had cooled to the original temperature and pressure?

The method I used was to find the amount of mol of the ethane and oxygen, then multiply the ethane mol amount by 3.5 to find the mol of oxygen that is being reacted, then multiply that number by the molar mass of oxygen to find the mass of oxygen reacting.  My answer was 373.33 mL. The worked solutions say to just do (100*7)/2=350mL. Can someone please explain why my method is wrong and what they're doing in the worked solution?

[sweetcheeks]

100mL of ethane gas was mixed with 500mL oxygen gas at 20C and 1 atmosphere pressure and sparked. The reaction shown by the following equation occurred
2C2H6(g) + 7O2(g) -> 4CO2(g) +6H2O(l)
What would be the total volume of gas present when the reaction had cooled to the original temperature and pressure?

The method I used was to find the amount of mol of the ethane and oxygen, then multiply the ethane mol amount by 3.5 to find the mol of oxygen that is being reacted, then multiply that number by the molar mass of oxygen to find the mass of oxygen reacting.  My answer was 373.33 mL. The worked solutions say to just do (100*7)/2=350mL. Can someone please explain why my method is wrong and what they're doing in the worked solution?

Without seeing your working out I cannot comment on why your answer was different to the solutions. It may be a matter of incorrect unit conversion or rounding errors.

An important property of ideal gases (any gas you deal with in VCE) is that under the same conditions (temperature, pressure), the volume of a gas will be proportional to the amount of gas present. In your question you have 100 mL of ethane and 500 mL oxygen. Given that they are at the same pressure and temperature, there is 5 times the amount (mol, molecules etc.) oxygen then there is ethane.

Given this proportionality, you can do stoichiometry using volumes in the same way you do it with moles. So the ratio between oxygen and ethane is 7:2 or 3.5:1. For every 1 mL of ethane you would require 3.5 mL of oxygen. For 100 mL of ethane, you would require 350 mL of oxygen (100 mL * 7/2) = 350 mL

[dream chaser]

Hi Guys,

Just got a question regarding exothermic and endothermic reactions. I don't understand by what they mean when energy is released to the surroundings, it is an exothermic reaction and energy that is absorbed by the reaction is callled an endothermic reaction. Could someone please explain how this works to me properly

Thanks. All help will be much appreciated.

[DBA-144]

Hi Guys,

Just got a question regarding exothermic and endothermic reactions. I don't understand by what they mean when energy is released to the surroundings, it is an exothermic reaction and energy that is absorbed by the reaction is callled an endothermic reaction. Could someone please explain how this works to me properly

Thanks. All help will be much appreciated.

Here is my understanding of this. I believe you are struggling to wrap your head around what these reactions actually are.

Endothermic reactions are those that absorb energy from their environment. That is, that when this sort of reaction occurs, the products have more energy than the reactants (energy of products greater than that of reactants). Note that this is potential energy, which is stored in the bonds of the product molecules. What happens here is that reactant molecules absorb energy from their environment, and when the product forms, it has this energy stored as potential energy in its bonds.

On the other hand, exothermic reactions are those whose products have less energy than their reactants. What occurs in this instance is that the reactants have released the potential energy stored in their bonds, to their external enviroment.

Hope this helps. If I said something wrong, or something doesnt make sense, please let me know.

[dream chaser]

Here is my understanding of this. I believe you are struggling to wrap your head around what these reactions actually are.

Endothermic reactions are those that absorb energy from their environment. That is, that when this sort of reaction occurs, the products have more energy than the reactants (energy of products greater than that of reactants). Note that this is potential energy, which is stored in the bonds of the product molecules. What happens here is that reactant molecules absorb energy from their environment, and when the product forms, it has this energy stored as potential energy in its bonds.

On the other hand, exothermic reactions are those whose products have less energy than their reactants. What occurs in this instance is that the reactants have released the potential energy stored in their bonds, to their external enviroment.

Hope this helps. If I said something wrong, or something doesnt make sense, please let me know.

Thanks for the reply. Say for instance in a combustion reaction, you would use the flame to cause the combustion reaction to occur. In this case, would the flame be the surrounding? Also, I don't get how when the reaction is getting absorbed, the surrounding gets cooler and vice versa.

[DBA-144]

Thanks for the reply. Say for instance in a combustion reaction, you would use the flame to cause the combustion reaction to occur. In this case, would the flame be the surrounding? Also, I don't get how when the reaction is getting absorbed, the surrounding gets cooler and vice versa.

I will assume you mean energy is being absorbed, not reaction being absorbed.

Think about what the flame is doing. Its providing heat energy to the molecules and giving them energy. So, in a way, you could consider the flame to be the surrounding, but I dont think you need to worry about what specifically is the surrounding.

Kinetic energy is heat. Heat is the prescence of kinetic energy. Cold is the absence of kinetic energy. Now, think about what happens when the prodcuts absorb energy from their surrounding. It sort of works like this:

Products absorb energy from surroundings stored as potential energy --> surroundings lose energy --> as heat is prescence of kinetic energy, surroundings have lost energy so are colder.

Hope this helps. Happy to be corrected, if you need help, please ask!

[dream chaser]

I will assume you mean energy is being absorbed, not reaction being absorbed.

Think about what the flame is doing. Its providing heat energy to the molecules and giving them energy. So, in a way, you could consider the flame to be the surrounding, but I dont think you need to worry about what specifically is the surrounding.

Kinetic energy is heat. Heat is the prescence of kinetic energy. Cold is the absence of kinetic energy. Now, think about what happens when the prodcuts absorb energy from their surrounding. It sort of works like this:

Products absorb energy from surroundings stored as potential energy --> surroundings lose energy --> as heat is prescence of kinetic energy, surroundings have lost energy so are colder.

Hope this helps. Happy to be corrected, if you need help, please ask!

Thank you DBA-144. I'll pm you if I have any other problems in regards to this if that is okay with you.

[yplee0926]

Hi guys,
The question is about an electrolytic cell with electrodes of iron and magnesium, in a solution of magnesium sulfate. The electricity is flowing towards iron electrode.
My question is:
Because there is no spontaneous reaction occurring in the system anyway, would the electrolytic cell still work should the electrons be pumped towards the magnesium electrode?

However, one thing I realise is that Fe2+ ions will be free floating in the solution and possibly (would it?) be reduced to Fe(s) on magnesium electrode instead of magnesium. This, if true, I believe would mean that there would be deposits of both Fe and Mg on Mg electrode. Can someone clarify these two questions?

Thanks in advance