VCE Chemistry Question Thread

[Bri MT]

Thank you all

Sorry - I'm still uncertain about the whole equilibrium K value stuff with regards to changes in temperature and pressure.

So how (and under what circumstances) is it that ΔTemperature or ΔPressure affects K? Is it only whilst the ΔT or ΔP is occurring, or is it after having changed the temp/pressure already?
Example:
For aA + bB <=> cC + dD with given values of the concentrations and coefficients, if K is measured at 8*C, and then the temperature is changed to 50*C, will a different value of K be measured (not that I'm NOT meaning measuring it whilst K is changing; only measure it after the temperature change has been performed)

Note sure if I'm explaining my confusion correctly, but I'd greatly appreciate if anyone can try to decipher and answer my babble haha

Temperature is the only thing that will change the K value.
With pressure, it changes the relative concentrations of reactants (only if there's a greater amount of particles on one side of the equation) but because of how the equilibrium constant equation works, you get the same K value if the pressure is different as long as the temp is constant.

[Vaike]

*snip*

Just to add onto what miniturtle mentioned, a ΔP will never result in a change in K, but a ΔT will result in a change in K.

Spoiler

Why this is so is beyond the scope of VCE chemistry (you don't need to know this), but iirc, it is because a ΔT will result in a value called the rate constant changing.

I believe this change occurs whilst temperature change is occurring (it's a dynamic shift), so you would observe a range of K values as you heated a system from 8 degrees to 50 degrees. Once the system reached 50 degrees, its equilibrium constant would remain, ugh, constant, until it undergoes a temperature change.

[sweetcheeks]

Both Miniturtle and Vaike have provided excellent explanations, but if you still don't fully understand why temperature affects the equilibrium constant but pressure doesn't, I can explain the theory behind it, but it is a bit complicated to understand. Understanding why can help greatly in chemistry, but I don't want to overload anyone if I don't need to.

[TheAspiringDoc]

Both Miniturtle and Vaike have provided excellent explanations, but if you still don't fully understand why temperature affects the equilibrium constant but pressure doesn't, I can explain the theory behind it, but it is a bit complicated to understand. Understanding why can help greatly in chemistry, but I don't want to overload anyone if I don't need to.

Yes please - I'd love to understand what is causing it.
THankyou vaike and mini turtle, I'm starting to get more of a picture.

[sweetcheeks]

I found this to be quite difficult to explain and some of this information goes beyond even first year university

Lets take a look at equilibrium. The reaction A + C ⇔ D + E. We can easily write the equilibrium constant as k = [D][E]/[A][C], but what exactly does k mean?

K is the ratio between two constants, known as rate constants. Rate constants are used like any other constant in physics. The rate of the forward reaction is (rate forward = Kf[A][C]) and the backwards (rate backward = Kr[D][E]). Kf and Kr are the rate constants (which I will show below how they are derived). As you can see the rate of reaction is proportional to the concentration of reactants multiplied by their rate constant.

At equilibrium the rate of the forward reaction must be the same as the backwards reaction. This means that Kf[A][C] = Kr[D][E]. We can rearrange the equation to be Kf/Kr = [D][E]/[A][C]. Now, Kf/Kr is also a constant (a constant divided by a constant is constant), we can write it as Keq, which we know as the equilibrium constant.

How is the rate constant derived? There is an important equation, called the Arrhenius equation (attached). It shows that the rate constant is related to A (collision factor, it is related to the proportion of collisions where molecules collide with the correct orientation), activation energy and temperature. The collision factor, A, is related to temperature, and also temperature is a variable in the equation.

When you change temperature, some of the values in the Arrhenius equation change. However, the amounts that Kf and Kb change differ (not by the same factor) and the ratio between them also changes. This can be seen when the temperature is increased. The endothermic reaction will be more favored, because the rate constant for it increases in a higher amount compared to the exothermic reaction.

Pressure has no effect on the values of the rate constants. The reason that pressure effects a system when there is a difference in the amount of gases in the equation (e.g. A(g) <=> C(g) + D(g)) is because the rate of forward and backward reaction don't change by the same proportion. Rate of forward = Kf[A] and Rate of Backward = Kr[C][D]. If you double the concentrations (by doubling pressure), the forward reaction will increase by 2x and the backwards reaction will increase by 4x, because Kf[2A] = 2x Kf[A] and Kr[2C][2D] = 4x Kr[C][D].

[Vaike]

I found this to be quite difficult to explain and some of this information goes beyond even first year university

This is an amazing explanation sweetcheeks! Very clear, I'm gonna save this and link it to anyone else who asks this question!

[rainbowsparkles15]

What is the difference between esterification and transesterification?

[sweetcheeks]

What is the difference between esterification and transesterification?

Esterification is the formation of an ester from an alcohol and an acid. Transesterification is where you already have an ester but you are substituting the alcohol for another. The reaction between ethanoic acid and ethanol is an esterification, forming ethyl ethanoate. The reaction between methanol and ethyl ethanoate forms methyl ethanoate, which is transesterification.

Remember that there is equilibrium associated with both reactions.

[missbrb]

100.0 ml of a 0.0100 M aqueous solution of calcium hydroxide will absorb carbon dioxide according to the equation:

 Ca(OH)2(aq) + 2CO2(g) = Ca(HCO3)2(aq)

 What is the maximum volume of Co2 at SLC that could be absorbed by the solution in mL?

Completely stuck with this question.

[Bri MT]

100.0 ml of a 0.0100 M aqueous solution of calcium hydroxide will absorb carbon dioxide according to the equation:

 Ca(OH)2(aq) + 2CO2(g) = Ca(HCO3)2(aq)

 What is the maximum volume of Co2 at SLC that could be absorbed by the solution in mL?

Completely stuck with this question.

1. Find the amount of CA(OH)2  in mols (using n=cv)
2. Use that to find the amount of CO2(g) in mols (using the equation)
3. Find the volume of CO2(g) (using n * Vm)

note: the equations assume that volume is measured in litres

[missbrb]

Sorry guys! I have one more question I'm stuck on!

[sophomania]

Sorry guys! I have one more question I'm stuck on!

1. convert 1000kg of iron to mole
2. using stoichiometry, find out how many moles of carbon dioxide you have
3. using n= V/Vm , calculate volume of carbon dioxide produced

[Yertle the Turtle]

Sorry guys! I have one more question I'm stuck on!

I shall assume that it is b)(ii) that you need help on since it is circled.
Find the number of moles of Fe in 1000kg by using n=m/M
Then use mole ratio to find the number of moles of CO2 produced and find the volume of CO2 by using n * Vm as in the answer to the previous question.

[missbrb]

I'm still getting the wrong answer and I don't know why.
Is the answer that my teachers put beside the question correct? Because I'm getting no where near that. I have no idea where I'm going wrong.

[sweetcheeks]

I'm still getting the wrong answer and I don't know why.
Is the answer that my teachers put beside the question correct? Because I'm getting no where near that. I have no idea where I'm going wrong.

I get an answer of around 6.6 x 10^5 L. If you could post your working it would make it much easier to see if you have made a mistake or not (I suspect that the given answer may not be correct.)