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Hess's law chemistry watch

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    Hi, So in Hess's law I've spent ages trying to understand the method with arrows pointing in different directions, but then i came across this video: https://www.youtube.com/watch?v=y1odUs75JXc which gave a different method. could i just use this method in the exam and for the rest of chemistry, or would it matter that much that i don't understand the arrows method? thanks
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    yes although that method doesn't really help you understand the different enthalpies involved in the reactions, it just looks like you're remembering a method whereas Hess cycles are a very helpful tool in chemistry, everywhere.

    when you start a2, hess cycles will be a lot easier to use.
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    (Original post by BTAnonymous)
    yes although that method doesn't really help you understand the different enthalpies involved in the reactions, it just looks like you're remembering a method whereas Hess cycles are a very helpful tool in chemistry, everywhere.

    when you start a2, hess cycles will be a lot easier to use.
    im just completely lost on hess's law cycles. i haven't found a single resource explaining where the elements with the mole ratios come from and how you know which direction the arrows are pointing.
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    (Original post by Bertybassett)
    im just completely lost on hess's law cycles. i haven't found a single resource explaining where the elements with the mole ratios come from and how you know which direction the arrows are pointing.
    ok. so you have your reaction. write it out. if you're given enthalpy of formation then you write the elements in their standard states beneath the equation. the definition of enthalpy of formation is the enthalpy change when 1 mole of a substance is formed from it's constituent elements in their standard states.

    For example:

    From here, you can see that we have the reaction of C2H4(g) + HCl --> C2H5Cl

    Below the equation you can see the constituent elements which make up these compounds - these elements are carbon,hydrogen and chlorine which you can clearly see. It is important to remember that the elements are in their standard state so ensure you write O2/H2 and not just O/H for the constituent elements. you can also see that these elements are balanced accordingly.

    you will be given enthalpy of formation data for these compounds in the question.

    now the arrows are pointing up because remember our definition; the enthalpy change when 1 mole of a substance is FORMED FROM ITS CONSTITUENT ELEMENTS IN THEIR STANDARD STATES.
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    (Original post by BTAnonymous)
    ok. so you have your reaction. write it out. if you're given enthalpy of formation then you write the elements in their standard states beneath the equation. the definition of enthalpy of formation is the enthalpy change when 1 mole of a substance is formed from it's constituent elements in their standard states.

    For example:

    From here, you can see that we have the reaction of C2H4(g) + HCl --> C2H5Cl

    Below the equation you can see the constituent elements which make up these compounds - these elements are carbon,hydrogen and chlorine which you can clearly see. It is important to remember that the elements are in their standard state so ensure you write O2/H2 and not just O/H for the constituent elements. you can also see that these elements are balanced accordingly.

    you will be given enthalpy of formation data for these compounds in the question.

    now the arrows are pointing up because remember our definition; the enthalpy change when 1 mole of a substance is FORMED FROM ITS CONSTITUENT ELEMENTS IN THEIR STANDARD STATES.
    thanks for the reply, but how did you know that 2c reacts with 0.5 h etc?
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    (Original post by Bertybassett)
    thanks for the reply, but how did you know that 2c reacts with 0.5 h etc?
    the 2C isn't reacting with 2.5H. (not 0.5H, it's 2.5H). these are the constituent elements. Our actual reaction is C4H4 + HCl --> C4H4Cl

    the 2C, 2.5H2 and 0.5Cl2 is just showing us what elements make up the reactants and products. you say it's confusing to put these elements here. true, but remember the hess's law; the enthalpy change of a reaction is independent of the route taken. in other words, it doesn't matter what reaction/route you use, the overall enthalpy change will be the same if you start and end with the same products and reactants.

    the 2 is in front of the C to balance it (because we have two carbons on both sides of the reaction).

    same with the hydrogen as we have 5 hydrogens on both sides of our reaction (2.5 times 2 = 5 hydrogens)

    and same with chlorine. we have 1 chlorine on both sides of the reaction. it's 0.5Cl because remember, chlorine in it's standard state is Cl2, so to get 1 chlorine we multiply by 0.5.
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    (Original post by BTAnonymous)
    the 2C isn't reacting with 2.5H. (not 0.5H, it's 2.5H). these are the constituent elements. Our actual reaction is C4H4 + HCl --> C4H4Cl

    the 2C, 2.5H and 0.5Cl is just showing us what elements make up the reactants and products. you say it's confusing to put these elements here. true, but remember the hess's law; the enthalpy change of a reaction is independent of the route taken. in other words, it doesn't matter what reaction/route you use, the overall enthalpy change will be the same if you start and end with the same products and reactants.

    the 2 is in front of the C to balance it (because we have two carbons on both sides of the reaction).

    same with the hydrogen as we have 5 hydrogens on both sides of our reaction (2.5 times 2 = 5 hydrogens)

    and same with chlorine. we have 1 chlorine on both sides of the reaction. it's 0.5Cl because remember, chlorine in it's standard state is Cl2, so to get 1 chlorine we multiply by 0.5.
    this has been really helpful, i think i see know. why do you have two arrows going up to one side but 1 going up to the other, and does this matter and what is this for?
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    (Original post by Bertybassett)
    this has been really helpful, i think i see know. why do you have two arrows going up to one side but 1 going up to the other, and does this matter and what is this for?
    yes so remember ourdefinition for enthalpy of formation: the enthalpy change when 1 mol of a substance is formed from it's constituent elements.

    in this case, our substances are: C2H4, HCl and C2H5Cl.

    we have two substances on the reactants side which means we are forming two substances from our constituent elements. Hence, we have two arrows coming up (pointing up) from the elements to the reactant side because we have two substances being formed to the reactant side.

    we have one substance on the product side which means we are forming one substance from our constituent elements. Hence, we have one arrow coming up (pointing up) from the elements to the product side because we are forming one substance to the product side.

    these arrows are used to calculate the enthalpy change of the reaction we are trying to find. What we do is follow the arrows from the reactant side down to the elements then up towards the products.

    when you move in the opposite direction to the direction an arrow is pointing then you invert the sign. For example, lets take ours:



    we start from the reactant side and move down the two arrows. because the two arrows are pointing up and we are moving down against the arrows, we need to invert the signs. this means the +52.2 becomes a -52.2 and the -92.3 becomes a +92.3. Why does this happen? because the arrows show us the enthalpy of formation but we are going the opposite way to which the arrows point so we use opposite signs; logic.

    then we move from the elements towards the products. because this arrow is pointing up and we are also moving up with the arrow, we do not change the sign; it stays as -109.

    all that is left to do is add everything up:

    -52.2 + 92.3 - 109 = enthalpy change for our reaction.
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    (Original post by BTAnonymous)
    yes so remember ourdefinition for enthalpy of formation: the enthalpy change when 1 mol of a substance is formed from it's constituent elements.

    in this case, our substances are: C2H4, HCl and C2H5Cl.

    we have two substances on the reactants side which means we are forming two substances from our constituent elements. Hence, we have two arrows coming up (pointing up) from the elements to the reactant side because we have two substances being formed to the reactant side.

    we have one substance on the product side which means we are forming one substance from our constituent elements. Hence, we have one arrow coming up (pointing up) from the elements to the product side because we are forming one substance to the product side.

    these arrows are used to calculate the enthalpy change of the reaction we are trying to find. What we do is follow the arrows from the reactant side down to the elements then up towards the products.

    when you move in the opposite direction to the direction an arrow is pointing then you invert the sign. For example, lets take ours:



    we start from the reactant side and move down the two arrows. because the two arrows are pointing up and we are moving down against the arrows, we need to invert the signs. this means the +52.2 becomes a -52.2 and the -92.3 becomes a +92.3. Why does this happen? because the arrows show us the enthalpy of formation but we are going the opposite way to which the arrows point so we use opposite signs; logic.

    then we move from the elements towards the products. because this arrow is pointing up and we are also moving up with the arrow, we do not change the sign; it stays as -109.

    all that is left to do is add everything up:

    -52.2 + 92.3 - 109 = enthalpy change for our reaction.
    ah ok thanks so much for this. sorry to be annoying but how would you do it if you were given the enthalpy of combustions?
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    (Original post by Bertybassett)
    ah ok thanks so much for this. sorry to be annoying but how would you do it if you were given the enthalpy of combustions?
    it's alright.

    enthalpy of combustion: the enthalpy change when 1 mol of a substance is completely burned in oxygen. this means only CO2 (carbon dioxide) and H20 are formed.

    Example:



    so write out the products of combustion below the reaction which are always CO2 and H2O for complete combustion as done like above.

    now using our definition, it is the enthalpy change when we burn a substance to form CO2 and H20; this means the arrows go from the substance to CO2 and H20.

    In this case, C2H4 is burned. the draw an arrow to represent the enthalpy of combustion for C2H4 towards CO2 and H20. We are given the values for enthalpy of combustion of C2H4 so write that next to the arrow.

    there should actually be two arrows on this diagram going from the reactant side down towards H20 and C02 but the diagram hasn't put the second one in; make sure you do.

    again, product side, we burn C2H6 and we knw it's enthalpy of combustion so we draw an arrow from the product towards CO2 + H20.

    now follow the arrows from reactants to products just like before. See if you can invert the signs for yourself and show me
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    (Original post by BTAnonymous)
    it's alright.

    enthalpy of combustion: the enthalpy change when 1 mol of a substance is completely burned in oxygen. this means only CO2 (carbon dioxide) and H20 are formed.

    Example:



    so write out the products of combustion below the reaction which are always CO2 and H2O for complete combustion as done like above.

    now using our definition, it is the enthalpy change when we burn a substance to form CO2 and H20; this means the arrows go from the substance to CO2 and H20.

    In this case, C2H4 is burned. the draw an arrow to represent the enthalpy of combustion for C2H4 towards CO2 and H20. We are given the values for enthalpy of combustion of C2H4 so write that next to the arrow.

    there should actually be two arrows on this diagram going from the reactant side down towards H20 and C02 but the diagram hasn't put the second one in; make sure you do.

    again, product side, we burn C2H6 and we knw it's enthalpy of combustion so we draw an arrow from the product towards CO2 + H20.

    now follow the arrows from reactants to products just like before. See if you can invert the signs for yourself and show me
    thanks, so follow the direction of the cycle depending on the direction of the arrows you would change the sign of the enthalp change? also how do you work out the mole ratio of co2 and h2o?
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    (Original post by Bertybassett)
    thanks, so follow the direction of the cycle depending on the direction of the arrows you would change the sign of the enthalp change? also how do you work out the mole ratio of co2 and h2o?
    the balancing actually doesn't matter for the CO2 and H20 because it doesn't change the enthalpy of combustion. the thing which does affect it however is the mole ratios of the substances (in our case, C2H4 and C2H6 - however these are only moles of 1 so it doesn't matter(.

    however, if it was 2(C2H4) then you would do 2 x -1393 KJ/mol because if we remember of definition of enthalpy of combustion, it is the enthalpy change when 1 mol of a substance is completely burned in oxygen and we have 2 moles here so multiply by 2.

    so ensure the reaction equation is always balanced correctly.
 
 
 
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