Hey there! Sign in to join this conversationNew here? Join for free

Hess's Cycle Question Watch

Announcements
    • Thread Starter
    Offline

    3
    ReputationRep:
    Given the data:
    4NH3(g) + 3O2(g)  2N2(g) + 6H2O(l), H = -1530kJmol-1
    H2(g) + 1/2O2(g)  H2O(l), H = -288 kJmol-1

    Calculate the enthalpy of formation of ammonia (NH3)

    Answer: -49.5kJmol-1

    Did you get this answer, if so how? Thank you
    Offline

    2
    ReputationRep:
    (Original post by Frappé)
    Given the data:
    4NH3(g) + 3O2(g)  2N2(g) + 6H2O(l), H = -1530kJmol-1
    H2(g) + 1/2O2(g)  H2O(l), H = -288 kJmol-1

    Calculate the enthalpy of formation of ammonia (NH3)

    Answer: -49.5kJmol-1

    Did you get this answer, if so how? Thank you
    So you know H2 + 0.5Os = H2O has a delta H value of -288, in the first equation there are 6 H2O. so 6 x -288 = -1728

    therefore, 4NH3 = 2N2 where delta H now equals -1728 + 1530
    this gives 4NH3 = 2N2 a delta H value of -198
    since there are 4 NH3, -198 divided by 4 gives -49.5

    hope this helps!
    Offline

    2
    ReputationRep:
    The only thing you get closer to when trying to solve a problem concerning Hess's law is death.


    Posted from TSR Mobile
    Offline

    21
    ReputationRep:
    1) Write the equation for formation of NH3.
    So, N2+3H2>2NH3

    2) Look at the equations given; there is w 4NH3 in the first eqn so scale up your equation from 1:
    2N2+6H2>4NH3

    3) write both equations given then try work out how to make the above equation in 2)

    4) Reverse first eqn so 4NH3 is in the right. This will reverse H = 1530KJ

    5) Multiply 2nd eqn by 6 to get 6H2
    H= 6*-288= -1728Kj

    6) Now it should cancel to give your desired eqn of formation of Ammonia.
    H=1530-1728=-198Kj

    7) However, this is for 4 moles of Ammonia but you are looking for just 1 mol of ammonia because its formation.

    8) Scale it right down so you are only producing NH3 not 4NH3

    9) You should now get -49.5 Kjmol-1

    If you don't get that^

    Spoiler:
    Show
    Divide by the eqn by 4 to get 1/2N2+3/2H2>NH3. -198/4=-49.5kj
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by Amellia123)
    So you know H2 + 0.5Os = H2O has a delta H value of -288, in the first equation there are 6 H2O. so 6 x -288 = -1728

    therefore, 4NH3 = 2N2 where delta H now equals -1728 + 1530
    this gives 4NH3 = 2N2 a delta H value of -198
    since there are 4 NH3, -198 divided by 4 gives -49.5

    hope this helps!
    Thank you!!
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by Mystery.)
    1) Write the equation for formation of NH3.
    So, N2+3H2>2NH3

    2) Look at the equations given; there is w 4NH3 in the first eqn so scale up your equation from 1:
    2N2+6H2>4NH3

    3) write both equations given then try work out how to make the above equation in 2)

    4) Reverse first eqn so 4NH3 is in the right. This will reverse H = 1530KJ

    5) Multiply 2nd eqn by 6 to get 6H2
    H= 6*-288= -1728Kj

    6) Now it should cancel to give your desired eqn of formation of Ammonia.
    H=1530-1728=-198Kj

    7) However, this is for 4 moles of Ammonia but you are looking for just 1 mol of ammonia because its formation.

    8) Scale it right down so you are only producing NH3 not 4NH3

    9) You should now get -49.5 Kjmol-1

    If you don't get that^
    Spoiler:
    Show
    Divide by the eqn by 4 to get 1/2N2+3/2H2>NH3. -198/4=-49.5kj
    Thank you!!
    Offline

    21
    ReputationRep:
    (Original post by Frappé)
    Thank you!!
    You're welcome!
    Hope it made sense
    Offline

    2
    ReputationRep:
    (Original post by Frappé)
    Thank you!!
    You're welcome! I hope it made sense!
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Should Spain allow Catalonia to declare independence?
    Help with your A-levels

    All the essentials

    The adventure begins mug

    Student life: what to expect

    What it's really like going to uni

    Rosette

    Essay expert

    Learn to write like a pro with our ultimate essay guide.

    Uni match

    Uni match

    Our tool will help you find the perfect course for you

    Study planner

    Create a study plan

    Get your head around what you need to do and when with the study planner tool.

    Study planner

    Resources by subject

    Everything from mind maps to class notes.

    Hands typing

    Degrees without fees

    Discover more about degree-level apprenticeships.

    A student doing homework

    Study tips from A* students

    Students who got top grades in their A-levels share their secrets

    Study help links and info

    Can you help? Study help unanswered threadsRules and posting guidelines

    Sponsored content:

    HEAR

    HEAR

    Find out how a Higher Education Achievement Report can help you prove your achievements.

    Groups associated with this forum:

    View associated groups
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.