VCE Chemistry Question Thread

[lzxnl]

I have another question:

Is this definition for a fuel okay? or is there anything that should be added, such as "it stores chemical energy which can be easily released"

A fuel is a substance that produces energy as a result of a chemical or nuclear reaction.


Thanks 

Sounds fine. I mean, nuclear fuels don't store chemical energy for starters. Although it might help to write it as 'producing energy as a result of a combustion reaction'.

[Butterflygirl]

Why is it that all atoms of a certain element don't have the same kinetic energy in a container (temperature and pressure constant)?

Thanks!

[lzxnl]

Why is it that all atoms of a certain element don't have the same kinetic energy in a container (temperature and pressure constant)?

Thanks!

Temperature is a measure of average kinetic energy. In real life, particles you see around you have a distribution of energies as they collide randomly with each other.

[peanut]

Does brown coal have a carbon content of 70% or 80%? My textbook says 70% but my teacher says 80%.

[Sine]

Does brown coal have a carbon content of 70% or 80%? My textbook says 70% but my teacher says 80%.

Yeah I also learned 70%.
However go with what your teacher says for any Sacs

[shxre]

Hi guys, for some reason i can't seem to figure how to solve limiting and and excess reagent problems, so it'd be great if you could help me out as i'm just confusing myself even more from the number of attempts i've had at this question 

4.40 g of P₄O₆ and 3.00g of I₂ are mixed and allowed to react according to the equation;

5P₄O₆ (s) + 8I₂ (g) -------> 4P₂I₄ (s) + 3P₄O₁₀ (s)

a) Which reactant is in excess and by how many grams
b) What mass of P₂I₄ is formed?
c) What mass of P₄O₁₀ is formed?
d) What is the total mass of all the products?

Your help is appreciated, and also any tips or advice when solving these problems are also very much welcome!   

[sweetcheeks]

Hi guys, for some reason i can't seem to figure how to solve limiting and and excess reagent problems, so it'd be great if you could help me out as i'm just confusing myself even more from the number of attempts i've had at this question 

4.40 g of P₄O₆ and 3.00g of I₂ are mixed and allowed to react according to the equation;

5P₄O₆ (s) + 8I₂ (g) -------> 4P₂I₄ (s) + 3P₄O₁₀ (s)

a) Which reactant is in excess and by how many grams
b) What mass of P₂I₄ is formed?
c) What mass of P₄O₁₀ is formed?
d) What is the total mass of all the products?

Your help is appreciated, and also any tips or advice when solving these problems are also very much welcome!   

a) the first step to any problem like this is to find the moles of each known substance.
nP4O6=m/Mw  4.40g/220.0gmol. n=0.00200.
nI2=m/Mw  3.00g/253.8gmol. n=0.0118.

To find which agent is in excess we need to ratio the coefficients of the reactants. If we have 5 mole of P4O6 we need 8 mole of I2 to react. This means that the number of mole of I2 required will be 8/5 moles of the amount of P4O6. If we have 0.00200 mol of P4O6 we require 8/5x0.00200 mol I2. This means we need 0.0032 mole of I2, however we have much more I2 than that (0.0086 mole more), this means that I2 is in excess. Now convert the excess moles to mass to find the grams excess.

Seeing that the P4O6 is the limiting reagent, use that value in all the calculations.

EDIT: See my reply below, I have mistakingly used 0.00200 as the amount of P4O6 when it is actually 0.0200, making my calculations incorrect.

[shxre]

a) the first step to any problem like this is to find the moles of each known substance.
nP4O6=m/Mw  4.40g/220.0gmol. n=0.00200.

Shouldn't n(P₄O₆) = 0.0200 not 0.00200??
Which means that I₂ is actually limiting?? Aah im so confused haha

[sweetcheeks]

Shouldn't n(P₄O₆) = 0.02000 not 0.00200??
Which means that I₂ is actually limiting?? Aah im so confused haha

Sorry about that, very easy to accidentally add an extra zero. If we have 0.0200 mol of P4O6, we will require 8/5x0.0200 I2 (0.0320mol), however we only have 0.0118. That means P4O6 is in excess. To find how much excess of P4O6 there is, we need to find out how much will react with the I2. If we have 0.0118mol I2, 5/8x0.0118 mol of P4O6 is required to react (0.00736 mol) meaning that (0.0200-0.00736=0.0126mol of P4O6 is in excess)

[shxre]

Sorry about that, very easy to accidentally add an extra zero. If we have 0.0200 mol of P4O6, we will require 8/5x0.0200 I2 (0.0320mol), however we only have 0.0118. That means P4O6 is in excess. To find how much excess of P4O6 there is, we need to find out how much will react with the I2. If we have 0.0118mol I2, 5/8x0.0118 mol of P4O6 is required to react (0.00736 mol) meaning that (0.0200-0.00736=0.0126mol of P4O6 is in excess)

Got it! That makes so much more sense. Thanks for your help!!!

[Gogo14]

1. what does the "1-" in 1-butanol mean?
2. We are doing an experiment on combustion. When writing the equation, is water in liquid state or gaseous? Still cant get my head around it. The question says "write balanced chemical equations for the combustion of ....." but in the previous page we were asked for the molar heat of combustion. So confused

[Shadowxo]

1. what does the "1-" in 1-butanol mean?
2. We are doing an experiment on combustion. When writing the equation, is water in liquid state or gaseous? Still cant get my head around it. The question says "write balanced chemical equations for the combustion of ....." but in the previous page we were asked for the molar heat of combustion. So confused

1. 1-butanol can also be written as butan-1-ol. The 1 shows where the -OH is located, in this case the first carbon.
2. With combustion, usually the water produced is gaseous, as the temperature is high. Molar heat of combustion = energy released through the reaction (∆H), balanced chemical equation = the equation for that reaction.

Does this answer your questions?

[Sine]

Yes as Shadowxo said water produced is in a gaseous state. However at year 12 level (VCAA exam) you aren't penalized if you "forget" and put down liquid. (Sacs you may lose a mark)

[sweetcheeks]

1. what does the "1-" in 1-butanol mean?
2. We are doing an experiment on combustion. When writing the equation, is water in liquid state or gaseous? Still cant get my head around it. The question says "write balanced chemical equations for the combustion of ....." but in the previous page we were asked for the molar heat of combustion. So confused

The one refers to the hydroxyl (OH) group being on the first carbon.

Generally, VCAA accept either liquid or gas. Generally when combustion occurs, the temperature is high enough to have steam form rather than liquid water. For example, if you have a gas stove you may notice that when you first light it, condensed water droplets form on the outside of the pot. Eventually the water will disappear. The condensation is due to the temperature of the combustion not being hot enough to keep the water in a gaseous state, however it eventually reaches a point where the water can stay as steam.

Molar heat of combustion refers to how much energy is released when 1 mole of a substance is burned (at 25C and 1atm). This can be different to the thermochemical equation.

For example octane C8H18 may have a molar enthalpy/heat of combustion of -5000kJ (I'm just guessing). That means when 1 mole of octane is combusted under standard conditions 5000kJ of energy will be released.

The balanced equation for the combustion of octane is 2C8H18 + 25O2 --> 16CO2 + 18H2O. The delta h of this reaction will be -10000kJ/mol, due to there being a coefficient of 2 next to the octane. The thermochemical reaction is read as the combustion of 2 moles of octane in the presence of 25 moles of oxygen gas, resulting in the formation of 16 moles of carbon dioxide and 18 moles of water will release 10000kJ of energy.

[Gogo14]

Sorry forgot to mention its for a SAC. In the experiment, we are not boiling water just heating it by 30 degrees. So should I put the state as liquid?
Also the aim of the experiment is :"measuring energy released in combustion of alcohols and evalute their fuel efficiency"
In the experiment we are measuring the change in mass and temperature (although its meant to be kept constant at 30degrees) of the alcohols after burning. Is the dependent variable the energy released or the change in mass ?