VCE Chemistry Question Thread

[thenerdygangster]

Thank you

Another question: why is the cathode usually identified from the anode by its higher position in the electrochemical series? From my knowledge, the cathode is the site where reduction occurs, and the higher up the series, the stronger the oxidation strength. From this logic, shouldn't the cathode be lower than the anode in the series?

Someone correct me if I'm wrong as my chem knowledge is a bit rusty. If i remember correctly, an element that has strong oxidation strength will be reduced. Hence, it will occupy a position higher up on the electrochemical series

[Syndicate]

Someone correct me if I'm wrong as my chem knowledge is a bit rusty. If i remember correctly, an element that has strong oxidation strength will be reduced. Hence, it will occupy a position higher up on the electrochemical series

yea, that's totally fine 

EDIT:
Just to add: All the standard electrode potential values of elements are calculated relative to H2 (which why H2 has an electrode potential of 0.00). Remember that oxidation strength refers to denote how a certain element oxidises another element, it doesn't mean that the element is being oxidised itself (it also doesn't mean it cannot be oxidised as well).

edit 2: grammar fix :p

[seth]

Thank you both thenerdygangster and Syndicate! Another question (promise, last one :p!): what is the purpose of the salt bridge? I understand that the cations provided move towards the anode to balance out the negatively charged ions left from the oxidation reaction (and the opposite for the cathode), but why is the balanced charge (i.e neutrality) needed for the reactions to continue?

[Syndicate]

Thank you both thenerdygangster and Syndicate! Another question (promise, last one :p!): what is the purpose of the salt bridge? I understand that the cations provided move towards the anode to balance out the negatively charged ions left from the oxidation reaction (and the opposite for the cathode), but why is the balanced charge (i.e neutrality) needed for the reactions to continue?

Anions move towards the anode, and the cations move towards the cathode (why? Because after oxidation, the element gain a positive charge(think about oxidation/ reduction pairs), so an anion moves towards the anode to cancel out the charges, and vice versa). The purpose of a salt bridge is to act as an internal circuit, to allow the electrons to move from the anode to the cathode. If the charge builds at one side, then the like charges will repel and the electrons will not flow. So the cations move towards the cathode, which contains the element that is reducing (meaning it gains electrons) in order to prevent polarisation of the cell (accumulation of charge at one side of the cell).

no flow of electrons = 0 voltage 

[deStudent]

http://m.imgur.com/a/MjWfs
I can't find the error in my working but I'm getting 556 for the value of the answer but it should be 5.5?

Thanks

[Shadowxo]

http://m.imgur.com/a/MjWfs
I can't find the error in my working but I'm getting 556 for the value of the answer but it should be 5.5?

Thanks

I can't see any error in your working, it's probably the book making a mistake

[peterpiper]

Hi I'm a bit stuck with this galvanic cells question:

Background:
There's an inert platinum for the anode and a copper cathode. Hydrogen gas is being pumped (or something) into the solution where the anode is, so that it oxidises and becomes H+ ions in solution. Meanwhile, at the cathode, Cu(II) ions are reducing and being deposited onto the copper cathode.

The question asks how the pH levels of each solution change when current is flowing...And this is where I need help...

Also, I'm a bit confused about the nature of pH levels as well. I know it's the concentrations of OH- and H+ but like when current is flowing through these solutions, do the concentrations change because of the presence of electrons flowing in the solutions? Or do there actually have to be OH and H+ ions involved in the redox reaction for us to see changes in the pH levels?

Thanks in advance!

[Gogo14]

1. In exothermic reactions, is the energy in the bonds of the reactants>energy of product bonds? In checkpoints, it says that the products have a stronger bond, does that mean that a stronger bond has less energy?
2. When asked to write thermochemical equations and are given a heat of combustion, do we write the equation with the coefficient of the fuel being 1 and the coefficient of oxygen being a fraction? Or do we write it as all having non fractional coefficients?
3. when ur rounding with sigfigs, do u take the molar mass into consideration if it has the smallest amount or do you round only according to the sigfigs in the question?

[Shadowxo]

Hi I'm a bit stuck with this galvanic cells question:

Background:
There's an inert platinum for the anode and a copper cathode. Hydrogen gas is being pumped (or something) into the solution where the anode is, so that it oxidises and becomes H+ ions in solution. Meanwhile, at the cathode, Cu(II) ions are reducing and being deposited onto the copper cathode.

The question asks how the pH levels of each solution change when current is flowing...And this is where I need help...

Also, I'm a bit confused about the nature of pH levels as well. I know it's the concentrations of OH- and H+ but like when current is flowing through these solutions, do the concentrations change because of the presence of electrons flowing in the solutions? Or do there actually have to be OH and H+ ions involved in the redox reaction for us to see changes in the pH levels?

Thanks in advance!

pH is a measure of acidity. pH = -log₁₀[H₃O+], So (at 25º) a pH of 1 is very acidic- high concentration of H+ / H3O+ ions (H+ and H3O+ are interchangeable), pH of 7 is neutral, and pH of 14 is basic.
So pH doesn't depend on electrons at all, only the concentration of H+ (you can also find it using the concentration of OH- at 25ºC). So the pH only changes when the concentration of either OH- or H+ ions are produced / used up. So yes, there have to be OH- and H+ ions involved in the redox reaction to see changes in the pH levels

1. In exothermic reactions, is the energy in the bonds of the reactants>energy of product bonds? In checkpoints, it says that the products have a stronger bond, does that mean that a stronger bond has less energy?
2. When asked to write thermochemical equations and are given a heat of combustion, do we write the equation with the coefficient of the fuel being 1 and the coefficient of oxygen being a fraction? Or do we write it as all having non fractional coefficients?
3. when ur rounding with sigfigs, do u take the molar mass into consideration if it has the smallest amount or do you round only according to the sigfigs in the question?

1. Yes, the energy in bonds of reactants > energy of products. The products have a stronger bond but it's not that important to know, but when they have a stronger bond they have less energy and are more stable, and it takes more energy to release the bonds.
2. Usually you use whole numbers, but sometimes equations use the coefficient of the fuel being 1 and others being fractions. I'd always use whole numbers and change the ∆H accordingly.
3. Yes you take molar mass into consideration, usually it's to 1 decimal place, so usually it's to 3 sometimes 4 sig figs. I usually round according to sig figs in each step (I use the numbers in the calculator for the rest of the problem - only round what I write down)

[usernameincorrect]

Anyone aware if we need to know
1. Galvanic cell examples (primary cels like button cells, zinc air cells etc)
2. The different types of fuel cells and their different half equations?? Cheers!

[sweetiepi]

Anyone aware if we need to know
1. Galvanic cell examples (primary cels like button cells, zinc air cells etc)
2. The different types of fuel cells and their different half equations?? Cheers!

In the previous study design, there was a need to know these. However I do believe there is more of a focus towards the fuel cells in the new study design. Therefore, I'd say yes.
(I haven't seen the new SD in ages, so I could be completely misleading here, haha)

[usernameincorrect]

In the previous study design, there was a need to know these. However I do believe there is more of a focus towards the fuel cells in the new study design. Therefore, I'd say yes.
(I haven't seen the new SD in ages, so I could be completely misleading here, haha)

Ok, thank you!! Just to be safe I'll learn the fuel cells 

[Butterflygirl]

Can someone help me with this question please?

There are three metals X, Y and Z. Z is the strongest reductant then X then Y.
These three metals form ores. Which metal is likely to be most difficult to extract and easiest to extract? Explain your answer.

Answer is that Z is hardest to extract and Y is easiest.


Thanks!

[peterpiper]

Can someone help me with this question please?

There are three metals X, Y and Z. Z is the strongest reductant then X then Y.
These three metals form ores. Which metal is likely to be most difficult to extract and easiest to extract? Explain your answer.

Answer is that Z is hardest to extract and Y is easiest.


Thanks!

The stronger a reductant is, the less likely it will want to accept electrons and reduce; the more violently it will oxidise with water when in solution. Therefore if we're extracting it from an ore or something, the strength of the reductant will make it harder to extract; if it's a weaker oxidant than water, then other measures will have to be taken like extracting the metal from the molten form of the ore (using inert electrodes).

Disclaimer: I'm doing vce chem like you and I'm no expert; so you might want to verify this with one of the moderators of this board or your teacher.

[vasuk]

hello doing a gas stoichiometry question
it says there are 45kg of octane & my step 1 is to find the mole of octane
which  I am doing by n = 45000/114.0
what is the correct no. of sig figs for my answer? does the 114.0 count? or do i go by the 45 kg?