VCE Chemistry Question Thread

[cosine]

Yep, that's correct.

Cheers xD

Also if we have a gaseous equilibria: by increasing the volume, we would have a rapid change in colour as (dark brown) the amount of nitrogen dioxide is increased. But, would this colour change be changed if everything is kept constant?

Also To increase in volume initially causes the concentration of NO2 to increase, so that the concentration fraction is_____? And is K increased as the forward reaction proceeded? Why/why not?

[Acid]

Cheers xD

Also if we have a gaseous equilibria: by increasing the volume, we would have a rapid change in colour as (dark brown) the amount of nitrogen dioxide is increased. But, would this colour change be changed if everything is kept constant?

Also To increase in volume initially causes the concentration of NO2 to increase, so that the concentration fraction is_____? And is K increased as the forward reaction proceeded? Why/why not?

If everything is kept constant, we would expect moderate colour (somewhere between the colour of N₂O₄ and 2NO₂ because this system will be at equilibrium.

When the volume is increased, pressure is decreased, hence we get a net forward reaction and more product. Consequently the concentration fraction increases, but K remains constant (K is only affected by temperature). At this point concentration fraction will increase. Due to this new concentration fraction, the equilibrium will be disturbed, it will try to PARTIALLY oppose the change to re-establish equilibrium so that concentration fraction=K.
Remember, only temperature affects K; pressure, concentration and other factors only affect Q. Q will always try to become equal to K. When Q=K, the system is at equilibrium.

[cosine]

If everything is kept constant, we would expect moderate colour (somewhere between the colour of N₂O₄ and 2NO₂ because this system will be at equilibrium.

When the volume is increased, pressure is decreased, hence we get a net forward reaction and more product. Consequently the concentration fraction increases, but K remains constant (K is only affected by temperature). At this point concentration fraction will increase. Due to this new concentration fraction, the equilibrium will be disturbed, it will try to PARTIALLY oppose the change to re-establish equilibrium so that concentration fraction=K.
Remember, only temperature affects K; pressure, concentration and other factors only affect Q. Q will always try to become equal to K. When Q=K, the system is at equilibrium.

What does concentration fraction even mean?

[Acid]

What does concentration fraction even mean?

It means Q, reaction quotient.
Q=[products]/[reactants], however this is NOT at equilibrium. If it is at equilibrium, them we refer to it as K.

[cosine]

It means Q, reaction quotient.
Q=[products]/[reactants], however this is NOT at equilibrium. If it is at equilibrium, them we refer to it as K.

and what does reaction quotient mean? ...

[jessica666]

In the reaction I₂ (aq) + H₂O (l) <--> HOI (aq) + H⁺ (aq) + I^- (aq)

What effect would the addition of NaOH have on the equilibrium position? Would it react with the I- or H+ ??

thanks

[cosine]

How does a catalyst speed up a chemical reaction? It lowers the activation energy yes, but how so?

[mahler004]

How does a catalyst speed up a chemical reaction? It lowers the activation energy yes, but how so?

It provides an alternative pathway/mechanism by which the reaction can take place.

[cosine]

It provides an alternative pathway/mechanism by which the reaction can take place.

Can you give me a specific example? Also what does adsorb mean? "The iron solid catalyst adsorbs the reactants" ?

[cosine]

Few questions:

1. What does equilibrium yield mean? Is it just the amount of products that has been produced before equilibrium is established?
2. If we have an exothermic reaction, we need to decrease the temperature to increase the forward reaction. But would this not decrease the rate at which the reaction occurs? Or in other words, would the time to reach equilibrium take longer?

[Ancora_Imparo]

I think you're basically right with both, here's how I would explain them.

1. In any reaction and at any point in time, yield refers to the amount of product you have. Equilibrium yield simply refers to the yield after equilibrium has been reached, that is, the amount of product you have when the system has reached equilibrium.

2. So there are two separate concepts here: yield and rate of reaction
Increasing temperature will always increase the rate of both the forward and backward reactions and decreasing the temperature will always decrease the rate of both the forward and backward reactions.

The effect of temperature on yield, however, is dependent on the heat of reaction, as you have pointed out. So if we have an exothermic reaction, decreasing the temperature would shift the equilibrium to the right (as per Le Chatelier's Principle), meaning the forward reaction would predominate over the backward reaction until a new equilibrium is reached. Thus, the yield of the reaction is increased. However, the time it takes for this yield to be obtained will be longer, as decreasing the temperature will decrease the rate of reaction.

Hope that clarifies things.

[lzxnl]

Can you give me a specific example? Also what does adsorb mean? "The iron solid catalyst adsorbs the reactants" ?

Here's an example. VCE Chemistry tbh is pretty terrible at giving examples to certain in-depth questions.

Let's look at an esterification reaction. What normally happens in the presence of strong acid is that the carboxylic acid is protonated at the hydroxyl group. This makes the oxygen atom positively charged and greatly increases its electron-attracting ability, so the carboxyl carbon loses more electron density to the oxygen atom. This makes the more positive carbon now more juicy for anything negative, like say...an alcohol oxygen. That oxygen, being negatively charged because of its electronegativity and the OH bond, is now more attracted to the carbon than it was before without the strong acid. Without the acid catalyst, the alcohol oxygen would have to try to get attracted to the not-so-positive carbonyl carbon.
Also, the acid catalyst makes the OH group in the COOH a OH2 group now, so if the C-O bond breaks (which it does to allow a carbon-alcohol group bond to form), the leaving group is a water molecule. Without the acid catalyst, the leaving group would be a hydroxide ion and water is more stable than hydroxide. So in two ways, the acid catalyst makes the reaction easier to bring about.

Few questions:

1. What does equilibrium yield mean? Is it just the amount of products that has been produced before equilibrium is established?
2. If we have an exothermic reaction, we need to decrease the temperature to increase the forward reaction. But would this not decrease the rate at which the reaction occurs? Or in other words, would the time to reach equilibrium take longer?

Technically, decreasing the temperature doesn't 'increase the forward reaction'. You can't increase a reaction. It doesn't increase the reaction rate; what it does is it decreases the forward reaction rate less than that of the backwards reaction rate. It can be shown (using Arrhenius's negative-reciprocal exponential temperature dependence on rate) that higher activation energy reactions are more temperature-sensitive when it comes to the reaction rate. Exothermic reactions generally have less activation energies than their endothermic counterparts (not true if the reaction is multi-step but in VCE all reactions are single-step...LOL)

Can you give me a specific example? Also what does adsorb mean? "The iron solid catalyst adsorbs the reactants" ?

What happens there is that the iron catalyst literally forms bonds with the hydrogen molecules and breaks them up. You don't need to know how these bonds are made (oxidative addition to the iron atom if anyone is interested)

[knightrider]

What is the difference between covalent networks and covalent molecular ?

[lzxnl]

Covalent network = something like diamond in which the substance consists of regular crystals formed by covalent bonds between atoms; there are no discrete molecules as such and it's just one group of atoms all connected to each other.

Covalent molecular = substance exists as individual molecules like CO2 or H2O.

[knightrider]

Covalent network = something like diamond in which the substance consists of regular crystals formed by covalent bonds between atoms; there are no discrete molecules as such and it's just one group of atoms all connected to each other.

Covalent molecular = substance exists as individual molecules like CO2 or H2O.

Thanks lzxnl 

if you were given the formula for something how would you tell if its a Covalent molecular or Covalent network?

Say for example ethanol C2H6O