Chemistry

posted by .

When given a chemical reaction, how would I know whether a reaction is endo or exothermic and whether work is done on the system or by the system. Without using any calculations.

So I have a couple reactions which I have to use chemical reasoning to to determine.

2 Cl(g) --> Cl2(g)
H2O(l) --> H2O(g)
2 H2(g) + O2(g) --> 2 H2O(l)
2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)
3 NaN3(s) --> Na3N(s) + 4 N2(g)

  • Chemistry -

    Lets use H2O(l) ==> H2O(g) as an example.
    We know it takes energy to turn water into steam. We know that because we must heat it someway (electrically, gas burner, etc). So since we must ADD energy, it is an endothermic equation. We could write it this way.
    H2O(l) + energy (or heat) ==> H2O(g) and we would write delta E = + some number.
    When looking at the work part of it, compare delta n, the number of mols gas products vs mols gas reactants. In the case of the H2O conversion, we have 1 mol products and zero mol reactants. The equation for work is -p(V2-V1).
    Larger n on products side (delta n is positive)(it expands) means work is negative that means work is done ON the surroundings BY the system (the reaction).

  • Chemistry -

    In general, going from s or l to g is endothermic. That answers 2-5. Now on 1, the Cl2 is a more stable, less energetic than Cl, so it must be a exothermic reaction.

    These are general ideas, with lots of exceptions.

  • Chemistry -

    And what I wrote holds for constant P and T.

  • Chemistry -

    Oh, alrite so then heres what I get.

    2 Cl(g) --> Cl2(g) exo-on
    H2O(l) --> H2O(g) endo-by
    2 H2(g) + O2(g) --> 2 H2O(l) exo-on
    2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g) endo-by
    3 NaN3(s) --> Na3N(s) + 4 N2(g)endo-by

  • Chemistry -

    I'm inclined to think Li and NaN3 reactions are exothermic but I don't have any data to prove that.

  • Chemistry -

    2 Li(s) + 2 H2O -> 2 LiOH (aq) + H2(g)

    why is this exothermic?

  • Chemistry -

    single replacement

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. chemistry

    Label each of the following reactions as exothermic or endothermic ("exo" or "endo"), and according to whether work is done on or by the system ("on" or "by")?
  2. chemistry URGENT (2)

    Label each of the following reactions as exothermic or endothermic ("exo" or "endo"), and according to whether work is done on or by the system ("on" or "by")?
  3. Chemistry

    Label each of the following reactions as exothermic or endothermic ("exo" or "endo"), and according to whether work is done on or by the system, or no work is done at all ("on", "by" or "none")?
  4. Chemistry I

    For the methanol combustion reaction 2CH3OH(â„“) + 3O2(g) ----> 2CO2(g) + 4H2O(g) estimate the amount of P [change]V work done and tell whether the work was done on or by the system. Assume a temperature of 27â—¦C. 1. 2.5 …
  5. Chemistry

    A chemical system within a sealed 1 L reaction vessel is described by the following reversible reaction equation: 2H2S(g) <---> 2H(g) + S2(g) (a) the reaction quotient intially (b) the order of concentration of all three components …
  6. science, chemistry

    2Al(s)+ 3Cl2(g)=2AlCl3(s) For the above reaction calculate: A) delta H, then state whether the reaction is endothermic or exothermic B) delta s, then whether the reaction becomes more or less disorder. C) delta G, the state whether …
  7. Chemistry

    H2 and F2 react according to the following equation, forming HF. H2(g) + F2(g)  2HF(g); H° = –271 kJ If H2(g) and F2(g) were mixed in a thermally insulated vessel, the reaction that occurred would be a. endothermic, …
  8. Chemistry

    In a three reactions system the first two reactions are endothermic with DeltaH values of 138kj and 241kj. if the final reaction is exothermic what is the minimum value of DeltaH3 that would make the overall reaction exothermic?
  9. Chemistry

    In a three reactions system the first two reactions are endothermic with DeltaH values of 138kj and 241kj. if the final reaction is exothermic what is the minimum value of DeltaH3 that would make the overall reaction exothermic?
  10. Chemistry

    In a three reactions system the first two reactions are endothermic with DeltaH values of 138kj and 241kj. if the final reaction is exothermic what is the minimum value of DeltaH3 that would make the overall reaction exothermic?

More Similar Questions