Hess' Law and Enthalpy Cycles | A-level Chemistry | OCR, AQA, Edexcel
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- เผยแพร่เมื่อ 24 ส.ค. 2019
- Hess' Law and Enthalpy Cycles in a Snap!
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The key points covered in this video include:
1. Hess’ Law
2. Enthalpy Cycles
a. Enthalpy Change of Combustion
b. Enthalpy Change of Formation
Hess’ Law
Germain Hess, a chemist and doctor, proposed Hess’ Law. Hess’ Law states that: The enthalpy change of reaction is independent of the route taken.
Enthalpy Cycles
The enthalpy change of a reaction cannot always be directly measured. We can use enthalpy cycles to help us to indirectly calculate the enthalpy change. An enthalpy cycle is a pictorial representation showing the alternative routes of reaction between reactants and products. Enthalpy cycles are triangular. In each corner there should be the same number of each atom. As well as energy, matter is also conserved. The sides of the triangle are arrows.
Enthalpy Cycle Example: Combustion
We can use the enthalpy changes of combustion to find the enthalpy change of the reaction. Carbon and Hydrogen can be combusted to form carbon dioxide and water. Benzene can be combusted to form carbon dioxide and water.
Enthalpy Cycle Example: Formation
We can use the enthalpy changes of formation to find the enthalpy change of the reaction. Nitrogen monoxide and oxygen can be made from nitrogen and oxygen, Nitrodgen dioxide can be formed from nitrogen and oxygen.
Summary
Hess' Law states that the enthalpy change of a reaction is independent of the route taken
Enthalpy Cycles are a pictorial representation of the routes of reaction
a. Triangle shape
b. Chemicals are the corners of the triangle
c. Arrows are the sides
i. These show the direction of the chemical reaction
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The first 5 minutes of this video explained it better than my teacher did in one hour.
periodt
Literally 😂
Omg same
literally
this amazing video helped me actually solve an enthalpy question!! it was only one mark but proudest moment of my life. Thank you so much!!!
EXTREMELY HELPFUL THANK YOU VERY MUCH TO WHOEVER MADE THIS VIDEOS FREE FOR EVERYONE
i swear i dont work for them, but prom cuz y not
look at their free rev sources. Their flashcards are awesome
@@iamsabz364 my guy am done with school lmfao today was my last day sheeeeeeesh 😩😭
@@ayowhat6139 ay congrats man!
happy for you!
@@iamsabz364 Oh thanks Sabz thats really sweet
@@ayowhat6139
Is it just me or did one part of the vid repeat itself
Maybe she meant to edit it out but forgot...
YES
No,
i found it weird as well in the first part that the there was a 2N in the intermediate of the cycle. But then they fixed it in the repeated one but having the intermediate as N2 which is the natural form of nitrogen at r.t.p cause this is a enthalpy change of formation
@@shawarmababa i am so thankful for this comment. Was confused for a sec
16:56 there is a mistake as it should be 9N2 not 5N2 for the constituent elements
yh i was confused af
EXACTLY
on the same question, how did she get the 10O2 for the constituent elements?
i don't see where that comes from
@@ronjohnson831 (5x4)=20 oxygens (O) divide that by two - 20/2=10 (O2)
Thank God i was like tf
AMAZING VIDEO I FINALLY UNDERSTAND HESS CYCLES THANK YOU
The video was really helpful. You explained it really well and I loved the diagrams and example questions
Excellent teaching style, I comprehended really well .
Thank you so much , you literally saved my life from being miserable , literally thank you so much , I love you 😂❤️in a good way .
this was so helpful definitely a walk in the park now
THANK YOU SO MUCH! I tried understanding this on my own for a few weeks to no avail, BUT NOW I CAN DO HESS CYCLE QUESTIONS. THANK YOU, Truly.
Day 1 in quarantine.. *I’m going mad.. I’m watching A Level lessons 6 months early*
Same, nice idea. Are you still watching?
Same. Are you guys also gassed that ur 1/3 through the course ?
@@orangecamo_cinnamonrolls306 yep, as levels will probably be easy because of this
@Sirius Orange @Daily Football Its good to get a head start on stuff like this when you get the chance🤷🏼♂️
@@tomcurtis1827 ikr, I also study it because I find it interesting. Other than that I would get bored otherwise as we have too much time with this quarantine.
You are an amazing teacher ❣️
Okay I'm probs just being really stupid but I cant work out for the life of me why at 12:07 ish she says she putting in arrows of combustion but she puts in arrows of formation when it says to uses enthalpy change of combustion? can someone help please?!
Thank so much it really helped me.
TYSM THIS VIDEO IS SO HELPFUL!!!
your videos r really superb as it really makes us understand
cheeyaz lads i am very grateful
incredibly valuable!
Nice for your Understanding
Lovely video
This is brilliant
May I just ask, it may sound stupid but how did I get 9 O2?
great vid thanks
How to know in which direction the arrow must be put?
Thank you so much ♥
THANK YOU THANK YOU THANK YOU THANK YOU
Thanks, good work.
anyone else hears Emma Watson speaking?
ExAcTlY! lol
Other than similarities in accent, she sounds nothing like Emma Watson
It's veeeeeery true
I feel like Hermione is explaining me Potions
Jazak Allah. Thank you.
You are most welcome! 😊
How do i find the intermediate? I understand the rest its just that part that I'm confused on
why is the intermediate at 6:22 "2N(g)" where as at 8:36 its "N2(g)" could u explain why?
i think the fact that at 6:22 its 2 nitrogen atoms and at 8:36 its a diatomic nitrogen molecule may mean that the have different levels of enthalpy. Maybe 2N has higher enthalpy than N2? I not sure
I can’t draw the arrows, i don’t understand the direction. Can anyone help me with it?
6:30
(a) Under standard conditions
(b) Understand those conditions
It’s obvious but I wrote it anyway
17:33 why is it 5N2 I thought it’s supposed to be 9N2. Do we not count the nitrogen’s on the 4CH3NHNH2
mfana your a snake
Seems like an error in the video to me
Its just a mistake but it wont effect the final answer. I also got 9N2
i am struggling with getting intermedate products
how do we create that constituent table so confused.......
They give it to you in the exam
on the last question 22:03 does anyone know how she got 6CO2?? im probs being slow but i thought there was only 6 oxygens not 12
It's a combustion, so they're reacting the molecules already there with oxygen (aside from the oxygen already there). That's why you get more oxgen in the molecules after the combustion has taken place
in combustion isnt the answer always supposed to be negative
they use the wrong formula.. they do.. reactant - product.. instead of product - reactant.. and they use the wrong standard enthalpy of h2(g) which is zero..
Thank youu
7:08 why are we not adding enthalpy of O2(g) for calculating A?
I’m not sure if the correct definition but , an easy way during exam is just calculate the values that they give in the table since those are the ones to be calculated . Someone correct me if I’m wrong
I think it's because the element is forming the element, in other words the element is forming itself so there is no enthalpy change, so it's just zero.
it's already present as O2(g), so no bonds have to be made or broken in order to move to the next stage of the reaction where it is also present as O2(g)
Subscribing!!!
at 12:59 arent the arrows for combustion meant to be facing towards the elements
Also the enthalpy change of combustion is always negative. I think they made a mistake
The cycle you draw depends on the type of data you're given, rather than what you're asked to calculate 🙂
At 2:16 the products should be replaced with intermediate and the arrows need to shift too
22:00 there's 18 O atoms in the intermediate stage?
yes I do agree with you, I have to say, although there are lots of errors, this video still provides me with enormous and immeasurable amount of help, I suppose that the author is deliberately making these errors and hoping that we can find them and correct them, which is an ultimate way of teaching
How did you get 4C+5N2+12H2+10O2 as the intermediate?
Its should be 9N2
@@shellykrypton5611 thought it should be 18 cuz 5N2O4
@@UziiWeb3 yeahh you are right
Anyone else notice the oxygens don't balance in the enthalpy change of formation for glucose?
Meeeeeee it had me going crazy...lol that’s what brought me here wanted to be sure I’m not the only one ☺️
Meeee
meeeee
it is correct tho cus glucose is combusted in the presence of 6mols of oxygen so that makes 12 oxygen
I'm having a really big prob with hess law but born haber cycle is simple however this one confuses me too much.
Cheers hermione
is it just me or is it the other way round? like for 21:58 it says to work out enthalpy change of formation but she’s working out combustion… or am i wrong?
Hi, combustion data is used to work out enthalpy of formation and formation data is used to work out enthalpy of combustion. Confusing, I know!
I still don't get how the arrows work
I thought combustion means arrow pointing downwards?
i have the same question
The cycle you draw depends on the type of data you're given, rather than what you're asked to calculate 🙂
16:54 how is it 10 oxygen and not 5?
16:09
So I keep getting 2830 not -2830 am a lil confused coz I got all the other calculations correct
Do you know where you went wrong yet?? Its probs a silly mistake
It should always be delta H negative for combustion reaction because we always lose energy.
Reegan Dalli it clear was a fat silly mistake, crazy looking back at this now 4 months later and realising this is the least of my worries now haha
Elsie Mazarura I pretty sure i did products - reactants rather than reactants - products so I got 2830 instead of -2830 thanks!
@@anagomes6587 Yeah, if you use the formula like me, then that's how it is, I had the same problem and wasted an hour trying to figure out where I went wrong lol.
Uko juu 😏👍
amazed
16:56 10 N? shouldn't it be 18?
philadelphia national school is the beast
Ily
12:13 why is it 9O2 instead of 6O2
Check the 6Oxygen already found in the glucose molecule
@@elsiemazarura3711 but on the left hand side altoger there is 18 oxygen molecules. So why isn't it 902
after watching this video, I doubt all the knowledge I have
16:50 should be 9N2 not 5N2
you put the wrong standard enthalpy.. it is H2O not H2. 4:30 H2(g) is zero in standard enthalpy..
It do be making sense tho
17:09 I’m sure N2 should be a 9 not a 5
Is it just me or there is a problem with 3:25 ???
Yes. Wonder where in the reaction oxygen joined them lol
Combustion gives CO2 and H2O
What the fuck is an intermediate
R u Indian?
Didnt understand shite
I hate ur British accent but like your video...lol
i hate you american
@@gamingmaniac5611 I'm not american LMAO
@@gamingmaniac5611 I am not America wither
Her voice is what makes it easier to understand so stop being a hater u prick
Thought is was only England that take A-levels, why would you bee watching the video if you weren't doing the course?
And if you are doing A-levels, I hardly doubt you would have heard there English speakers during lessons e.c.t?