Bazza's 3/4 chemistry questions

[oneoneoneone]

For 4) They're effectively the same technique, but colorimetry is simplified and less accurate in a way, since you're selecting a range of wavelengths (The complementary colour) rather than just a single wavelength.

[AllAboutTheLGs]

I think with the ethane w/ chlorine, it just undergoes substitution twice? Can't think of any other reason haha

[pi]

1) I don't think so

[charmanderp]

I think with the ethane w/ chlorine, it just undergoes substitution twice? Can't think of any other reason haha

If you leave it long enough and have a large excess of chlorine gas, I'm pretty sure it would keep chlorinating the ethane molecule.

[sin0001]

1) The lowest alkyl chain for R that you can have is a methyl chain

[WhoTookMyUsername]

Do we need to put states for organic pathway reactions? Do we need to balance the equations?


For gas chromatography why do you vaporise the sample? Is it solely to turn into gaseous form or is it also to remove water ( why does this help )

[AllAboutTheLGs]

In the past exams, states weren't required, so I doubt it would be. Only balance equations and put states if it asked for a balanced equation, or if they specify to give states

[WhoTookMyUsername]

UNIT 4

...



If the heat of reaction is given in kj / mol
And if A+b=C and heat of  reaction = 200 kj / mol

Why does
2A + 2B = 2C have a heat of 400 kj/ mol?

Shouldnt the "per mole" mean that the number stays the same no matter the number  of mol?

2) i thought that bonds breaking releases energy? My textbook says that " energy released as bonds form in products "

[synchronise3]

1) Think about it this way, you have x amount of substance A and x amount of substance B, the amount of heat (energy) required to convert them into C is 200kj but when you double the amount of A and B, you require a larger amount of heat (energy) and since there is double the amount of A and B you require double the amount of energy equating to 400kj.

2) I thought the same, but remember the activation energy is the energy required to break bonds, so from this we can deduce that energy is required to break the bonds. (Another example from unit 3, remember that UV light is required to break the bonds in Cl2, Uv light essentially provides the energy to split the Cl2 molecule.

hope that heled

[WhoTookMyUsername]

Think about it this way, you have x amount of substance A and x amount of substance B, the amount of heat (energy) required to convert them into C is 200kj but when you double the amount of A and B, you require a larger amount of heat (energy) and since there is double the amount of A and B you require double the amount of energy equating to 400kj.

hope that heled

yes but the unit isn't Kj. it's Kj/mol

UNIT 4

...



If the heat of reaction is given in kj / mol
And if A+b=C and heat of  reaction = 200 kj / mol

Why does
2A + 2B = 2C have a heat of 400 kj/ mol?

Shouldnt the "per mole" mean that the number stays the same no matter the number  of mol?

2) i thought that bonds breaking releases energy? My textbook says that " energy released as bonds form in products "

[synchronise3]

Think about it this way, you have x amount of substance A and x amount of substance B, the amount of heat (energy) required to convert them into C is 200kj but when you double the amount of A and B, you require a larger amount of heat (energy) and since there is double the amount of A and B you require double the amount of energy equating to 400kj.

hope that heled

yes but the unit isn't Kj. it's Kj/mol

UNIT 4

...



If the heat of reaction is given in kj / mol
And if A+b=C and heat of  reaction = 200 kj / mol

Why does
2A + 2B = 2C have a heat of 400 kj/ mol?

Shouldnt the "per mole" mean that the number stays the same no matter the number  of mol?

2) i thought that bonds breaking releases energy? My textbook says that " energy released as bonds form in products "

Yep, it's kj/mol meaning Kj per mole, so when the mole is increased so is the Kj.
Hard to explain - maybe i'll leave it to someone else but this is the way i rmbr it.

edit: Refer to previous post in response to Q2

[Aurelian]

If the heat of reaction is given in kj / mol
And if A+b=C and heat of  reaction = 200 kj / mol

Why does
2A + 2B = 2C have a heat of 400 kj/ mol?

Shouldnt the "per mole" mean that the number stays the same no matter the number  of mol?

2) i thought that bonds breaking releases energy? My textbook says that " energy released as bonds form in products "

Just view the "per mole" as somewhat of a convention, meaning "for the equation as written".

That is, if I say:

3A + 5B ---> A3B5  dH = -1000kj/mol

All this means is that "when 3 mol of A react with 5 mol of B to produce 1 mol of A3B5, the change in enthalpy for the reaction is -1000kj".

Hence, using this understanding, it's quite clear why 6A + 10B ---> 2A3B5 would give a dH = -2000kj/mol; there's twice as much stuff reacting, so there will be twice as much the change in enthalpy!

Formally, we call heats of reaction extensive properties, in that they depend on the amount involved in the process (compared to intensive properties, where the value is independent of the amount, e.g. temperature).

[WhoTookMyUsername]

thanks for the help!

2) I thought the same, but remember the activation energy is the energy required to break bonds, so from this we can deduce that energy is required to break the bonds. (Another example from unit 3, remember that UV light is required to break the bonds in Cl2, Uv light essentially provides the energy to split the Cl2 molecule.
hope that heled

yeah lol i understand this bit

2) i thought that bonds breaking releases energy? My textbook says that " energy released as bonds form in products "

Is this just phrased badly?

At the time new bonds are formed in the product molecules, the broken bonds release energy ?
or am i missing something?

[synchronise3]

2) i thought that bonds breaking releases energy? My textbook says that " energy released as bonds form in products "

Is this just phrased badly?

At the time new bonds are formed in the product molecules, the broken bonds release energy ?
or am i missing something?

The textbook doesnt have much detail into this but I suppose it's just convention to know that Energy is required to break bonds and Energy is released when bonds form.

[kenhung123]

When products form, reactant bonds break, and new bonds form so both processes take place