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Confused on which formula to use where

  1. Apr 11, 2005 #1
    I keep getting confused on which formula to use where.
    q = mc(delta)t
    (delta)H = n(delta)Hx
    n(delta)H = mc(delta)t

    I don't know which ones to use, I keep using the wrong formula.. :cry:
    How do I know the difference?
     
  2. jcsd
  3. Apr 11, 2005 #2
    The first equation there looks like it is Energy Transferred. This should be used to find the amount of energy a substance needs to change a specific degree of temperature.

    E.g. 50g of water needs 5212.5J of energy to change 25°C or °K as
    E = 4.17 x 50 x 25 = 5212.5J

    The others I can't say I have seen before.

    The Bob (2004 ©)
     
  4. Apr 11, 2005 #3

    Gokul43201

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    Q : How do you decide, in general, what is the correct formula to use ?

    A : Usually, if all the quantities but one, in a given equation/formula, are known (or have been found using other formulae), then the remaining quantity can be determined from this formula. If this is the quantity you are required to determine/calculate, then this is the formula to use.

    Do you know what quanties each of the symbols in those formulae represent ?
     
  5. Apr 12, 2005 #4

    GCT

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    To my knowledge the third isn't even correct, enthalpy pertains to open systems and is certainly not equivalent to q, heat.

    with closed/isolated systems use the first equation, in gen chem. this is almost exclusively with problems relating to calorimeters.

    The second equation is usually used in finding the net standard enthalpy of a reaction.
     
  6. Apr 13, 2005 #5
    Thank you :biggrin:

    Gokul43201 - yes I do
     
    Last edited: Apr 13, 2005
  7. Apr 13, 2005 #6

    ShawnD

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    This one says the energy change in a system (heat) is proportional to the temperature change of a material, the mass of the material, and the heat capacity of the material. q is in terms of energy of the object you are looking at, so increasing in temperature means q is positive.

    example: A frying pan is cooled from 100 to 0 C and you want to find how much energy was released. Since this involves the amount of energy in the frying pan decreasing, q will be negative.

    This one says the energy change in the system is proportional to the molar enthalpy of a reaction times the moles in said reaction. Enthalpy Hx is in terms of stored energy in the compound, so exothermic reactions will have a negative Hx (energy is leaving the compounds in the form of heat). Likewise, H will be negative for exothermic reactions

    example: Ethanol has a molar enthalpy of combustion of -1366.8 KJ/mol, what is the total enthalpy change when 5 moles of ethanol is burned? Since combustion is exothermic, H will be negative.


    This is a HORRIBLE equation. Whoever told you this wants you to fail. It's technically right, but it becomes impossible to use when you start adding too many terms. Can you imagine what this equation would be if you had 10 things heating and cooling? What would the signs be?

    Break it down like this: the sum of energies is always constant. Suppose you have 2 things happening at once. You are burning ethanol in a frying pan, and just suppose that all of the energy is between the ethanol and the frying pan.

    ethanol combustion
    energy change = nHx

    frying pan heating
    energy change = mc(delta)T

    Remember that I said energy is always constant.... then wouldn't that mean the sum of all changes is 0? If I add the two above equations together I get one ultimate equation
    0 = nHx + mc(delta)T
    This equation will absolutely never fail. The ethanol is burning... so the ethanol itself is actually losing stored energy. The frying pan is exposed the released heat from the burning ethanol, so it's gaining energy.
    You can tack an infinite number of terms to that and it will always work. You could have 10 different materials heating and cooling at the same time and 10 chemical reactions happening at the same time, and as long as you add them all together in a closed system, the sum is always 0.

    Just remember these things with that above equations
    -all terms are always added
    -change is always final - initial
    -exothermic terms are always negative
    -endothermic terms are always positive
     
    Last edited: Apr 13, 2005
  8. Apr 14, 2005 #7
    wow thanks Shawn. It explains alot :biggrin:
     
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