asadsadchild
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#1
Report Thread starter 10 months ago
#1
hi so basically i have tried several times to get into my account and i couldnt so had to create a new email and new student room email.
i am by now utterly frustrated. i dont understand hess' law at all. i have made so many goddamn notes and attempted so many questions and done everything that a perfect student should do yet IM STILL GETTING IT WRONG. i dont understand it and i now cannot stay calm because i need to understand it. like i dont even know what it is im getting wrong. it's all the questions and the bond enthalpy and everything and oh my god i feel like screaming. i dont get it. can someone please help me before i just errupt.
(i am in year 12 doing a level chemistry)
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charco
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#2
Report 10 months ago
#2
(Original post by asadsadchild)
hi so basically i have tried several times to get into my account and i couldnt so had to create a new email and new student room email.
i am by now utterly frustrated. i dont understand hess' law at all. i have made so many goddamn notes and attempted so many questions and done everything that a perfect student should do yet IM STILL GETTING IT WRONG. i dont understand it and i now cannot stay calm because i need to understand it. like i dont even know what it is im getting wrong. it's all the questions and the bond enthalpy and everything and oh my god i feel like screaming. i dont get it. can someone please help me before i just errupt.
(i am in year 12 doing a level chemistry)
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DJ Trump fan
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#3
Report 10 months ago
#3
(Original post by asadsadchild)
hi so basically i have tried several times to get into my account and i couldnt so had to create a new email and new student room email.
i am by now utterly frustrated. i dont understand hess' law at all. i have made so many goddamn notes and attempted so many questions and done everything that a perfect student should do yet IM STILL GETTING IT WRONG. i dont understand it and i now cannot stay calm because i need to understand it. like i dont even know what it is im getting wrong. it's all the questions and the bond enthalpy and everything and oh my god i feel like screaming. i dont get it. can someone please help me before i just errupt.
(i am in year 12 doing a level chemistry)
When you are asked a question on Hess' law they will give you a table of values. If they give you a table of delta Hf values (enthalpy of formation) them draw the arrows going up in the cycle. An easy way to remember this is don't **** it up. If they give you delta Hc balues in the table (enthalpy of combustion) draw the arrows going down. Then just use the values they give you in the table. Multiply each value by the number of moles in the balanced equation and follow the direction of the arrows. If you're given delta Hf in the table (arrows pointing upwards) then it's the total Hf on RHS - total Hf on LHS. If delta Hc is given in the table (arrows pointing down) then it's the total Hc on LHS - total Hc on RHS. Also learn your definitions of the standard enthalpy of formation, combustion, neutralisation and the standard enthalpy change of reaction. If they ask you to calculate the standard enthalpy of combustion of ethane for example you will need to consider the stoichiometry. 2C2H6 + 7O2 -> 4CO2 + 6H2O
So they would give you a table of delta Hf values. Draw arrows from the elements pointing upwards to the products and reactants. Then calculate the total delta Hf on the LHS and do the same for the RHS. Remember to multiply the delta Hf values by the no of moles. Then do RHS - LHS. Here is where the definitions come in to play. The enthalpy change of combustion is the enthalpy change when ONE mole of a substance completely reacts with oxygen. In the balanced equation 2 moles of ethane react so we will need to divide the final value by 2 to calculate it for 1 mole. If they asked for the standard enthalpy change then you would just leave it as it is and calculate for 2 moles and not divide by 2, cus standard enthalpy change is for the stoichiometry in the balanced equation they give to you, not for one mole. Hope this helps. It will click, you just have to be patient.
Last edited by DJ Trump fan; 10 months ago
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