OCR AS - Chemistry Unit F322 - Chains, energy and resource - REVISION!

Scroll to see replies

you know for hess's law....if you don't draw the cycle but just have it as like products - reactants (for formation) and you get the correct answer..full marks?

Reply 521

Bright

Someone please teach me hess cycles I've forgotten it!!! :frown:

I know how to draw it like formation pointing up and combustion down with products CO2 and H20 at the bottom, and I know the labels A B C, but how do you label when each one is?

Reply 522

sixthformer

Original post by touran22

you know for hess's law....if you don't draw the cycle but just have it as like products - reactants (for formation) and you get the correct answer..full marks?

unless it says to draw one, no need

it works either way

but it's easier to use one, although everyones different

double check though

Reply 523

sixthformer

Original post by lolo-x

if anyone wants to see some notes, i've attached mine here and pleeease let me know if there's anything wrong or missing that i need to know about! :smile:

these were awsome

Reply 524

Killjoy-

Original post by Bright

Someone please teach me hess cycles I've forgotten it!!! :frown:

I know how to draw it like formation pointing up and combustion down with products CO2 and H20 at the bottom, and I know the labels A B C, but how do you label when each one is?

Finding enthalpy of reaction using enthalpy of formation:

sum of enthalpies of formation for products - sum of enthalpy of formation for reactants. (You go against the arrows for reactants.)

Finding enthalpy change of reaction using enthalpy of combustion:

sum of enthalpies of combustion for reactants - sum of enthalpy of combustion for products. (Again, you go against the arrows but for products)

For enthalpy change of reaction (as opposed to enthalpy change of formation and combustion) remember it is associated with the molar quantities in the equation, not for one particular molecule. So if the enthalpy change you have calculated is for a substance that appears as two moles, double your answer etc.

If you think of the elements forming and then reacting to make the RHS of the equation for formation. (A way to think about it.)

And in the same way think of the combustion products forming and then reacting 'backwards' to create the products in the equation.

Its easier to explain with diagrams, if you are still not sure ask your teacher.

Reply 525

SmithytheDrummer

It's weird. In the exams they tend to give you the enthalpy change of combustion and ask you to calculate formation or vice versa. The only way I have found for that to work is if you reverse the signs on the enthalpy changes and proceed as normal :s-smilie:

Reply 526

medusa cascade

I really hope that the free radical and electrophilic mechanisms come up a lot...

Reply 527

godknows

hey can someone plz explain to me how does ch3ch2ch2ch2oh change to ch3ch3chch2 when reacted with an acid/heat :s-smilie:

question 3 from june 2010 paper

Reply 528

amberandmoss

Original post by godknowsprerna

hey can someone plz explain to me how does ch3ch2ch2ch2oh change to ch3ch3chch2 when reacted with an acid/heat :s-smilie:

question 3 from june 2010 paper

this reaction is an elimination reaction. for a primary alcohol, water is removed and a C=C double bond is formed. the reaction mechanism is not on the specificiation, ( so you wouldn't be expected to work it out the other way round)
but i think you're meant to know the process.

Reply 529

godknows

Original post by amberandmoss

thank you.. i get sooo confused with all these reactions :frown:

i keep forgetting them.. if u r not busy could u plz jus summarise all the reactions/conditions we need to know :frown:

they r sooooooo confusing.. like in the one above how would u know if they jus gave u the first formula n wrote acid n heat tht they r removing water :/

Reply 530

lolo-x

is it true to say that in an exothermic reaction, more energy is required to make bonds than to break bonds?

Reply 531

SmithytheDrummer

Original post by lolo-x

is it true to say that in an exothermic reaction, more energy is required to make bonds than to break bonds?

Energy isn't taken in when the bonds are made. Exothermic reactions are where the energy created when the bonds are formed is greater than the energy required to break the bonds in the products.

Reply 532

Moiiu

Original post by lolo-x

is it true to say that in an exothermic reaction, more energy is required to make bonds than to break bonds?

If bonds made are stronger than bonds broken, the reaction is exothermic

If bonds broken are stronger than bonds made, the reaction is endothermic

Reply 533

lolo-x

Original post by SmithytheDrummer

Energy isn't taken in when the bonds are made. Exothermic reactions are where the energy created when the bonds are formed is greater than the energy required to break the bonds in the products.

that makes more sense, i could tell mine didn't seem right :smile:

thank you!

Reply 534

lolo-x

Original post by Moiiu

If bonds made are stronger than bonds broken, the reaction is exothermic

If bonds broken are stronger than bonds made, the reaction is endothermic

i see

thank you

Reply 535

godknows

hey does anyone know how to do q 3) (iii) from jan 2010 .. plz help !

Reply 536

SmithytheDrummer

Original post by godknowsprerna

thank you.. i get sooo confused with all these reactions :frown:

i keep forgetting them.. if u r not busy could u plz jus summarise all the reactions/conditions we need to know :frown:

they r sooooooo confusing.. like in the one above how would u know if they jus gave u the first formula n wrote acid n heat tht they r removing water :/

Reaction Conditions

Alkene to Alkane- Hydrogen gas, 150 degrees, Ni catalyst
Alkene to Alcohol- Water (g), 300 degrees, Phosporic acid catalyst
Alcohol to Alkene- Acid catalyst, heat under reflux
Alkane to Halogenoalkane- UV light
Alcohol to Ester- Carboxylic acid, Acid catalyst, Heat
Halogenoalkanes to- NaOH, Hydrolysis, Reflux
Alcohol
Alkene to Halogenoalkane- RTP
Primary alcohol to Aldehyde- Distil with Acidified dichromate ions
Primary Alcohol to COOH acid- Reflux with Acidified dichromate ions
Secondary Alcohol to Ketone- Reflux with Acidified dichromate ions

(edited 12 years ago)

Reply 537

touran22

in the exam do you guys think its better to flip to the end and do the long mark questions first?

Reply 538

Killjoy-

Original post by godknowsprerna

hey does anyone know how to do q 3) (iii) from jan 2010 .. plz help !

Put a negative sign in front of your value for Q (since its an exothermic reaction so heat is evolved) and divide by the number of moles of glucose that you calculated earlier.

Reply 539

godknows

Original post by SmithytheDrummer

Reaction Conditions

Alkene to Alkane Hydrogen gas, 150 degrees, Ni catalyst
Alkene to Alcohol Water (g), 300 degrees, Phosporic acid catalyst
Alcohol to Alkene Acid catalyst, heat under reflux
Alkane to Halogenoalkane UV light
Alcohol to Ester Carboxylic acid, Acid catalyst, Heat
Halogenoalkanes to NaOH, Hydrolysis, Reflux
Alcohol
Alkene to Halogenoalkane RTP
Primary alcohol to Aldehyde Distil with Acidified dichromate ions
Primary Alcohol to COOH acid Refluz with Acidified dichromate ions
Secondary Alcohol to Ketone Reflux with Acidified dichromate ions

THANK YOU SOO MUCH DEAR :smile:

life saver!

hey if u r not busy could u plz explain to me how to find the enthalpy change of combustion when enthalpy change of formations are given :s
q -3 c) jan 2010 paper :frown: