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# Hess's law watch

1. Hi anyone know an easy and none confusing way of constructing a Hess cycle for standard enthalpy of formation and standard enthalpy of combustion reactions? I am studying it at school but am really confused on the whole use of the arrows and how to get a final answer for the enthalpy of the reaction.
2. (Original post by Logic4Life)
Hi anyone know an easy and none confusing way of constructing a Hess cycle for standard enthalpy of formation and standard enthalpy of combustion reactions? I am studying it at school but am really confused on the whole use of the arrows and how to get a final answer for the enthalpy of the reaction.
I go with clockwise = anti-clockwise when I teach it. Usually goes down well with the learners.
3. Aww cycles are confusing! Just remember these equations:

Standard Enthalpy of Formation= Sum of products-Sum of reactants.

Standard Enthalpy of Combustion= Sum of reactants-Sum of products.
CRP (almost like crap; combustion, reactants, products)

Mean Bond Enthalpy= Sum of bonds broken- Sum of bonds made.
BBBM (almost like BBM as in Blackberry Messenger)

I know it's silly but some people find this helpful.

Oh and please assume that all reactants and products in my equations are in their standard states and under standard conditions.

Posted from TSR Mobile
4. (Original post by Mimi97)
Aww cycles are confusing! Just remember these equations:

Standard Enthalpy of Formation= Sum of products-Sum of reactants.

Standard Enthalpy of Combustion= Sum of reactants-Sum of products.
CRP (almost like crap; combustion, reactants, products)

Mean Bond Enthalpy= Sum of bonds broken- Sum of bonds made.
BBBM (almost like BBM as in Blackberry Messenger)

I know it's silly but some people find this helpful.

Oh and please assume that all reactants and products in my equations are in their standard states and under standard conditions.

Posted from TSR Mobile
Is it not necessary to know the triangle/diagram way of figuring it out- even for A2? Can I just memorise those things? ^
5. (Original post by ChronicBoredom)
Is it not necessary to know the triangle/diagram way of figuring it out- even for A2? Can I just memorise those things? ^
Don't recommend trying to memorise equations without some logic behind it. Check out this interactive on Hess's law
6. (Original post by charco)
Don't recommend trying to memorise equations without some logic behind it. Check out this interactive on Hess's law
It's not that, I just don't like having to draw out the whole cycle. Will I lose marks if I don't draw the whole cycle?
7. (Original post by ChronicBoredom)
It's not that, I just don't like having to draw out the whole cycle. Will I lose marks if I don't draw the whole cycle?
If the question asks you to draw the cycle, then yes.
8. (Original post by charco)
If the question asks you to draw the cycle, then yes.
lol ok, thanks

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