Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    5
    ReputationRep:
    Just from a past paper this question I don't seem to understand the answer.

    The standard enthalpy change for the reaction of calcium hydroxide with
    hydrochloric acid was found by reacting 0.0100 mol of solid calcium hydroxide
    with 50.0 cm3 of a 1.00 mol dm–3 solution of hydrochloric acid (an excess), in a polystyrene cup. The temperature rose from 21.2 °C to 26.7 °C.


    Q) Calculate the standard enthalpy change, ΔH ○, for the reaction. Include a sign and units in your answer.


    I have established that the change in enthalpy is 1149.5J, but the mark scheme says this must be divided by 0.01 moles in order to get the standard enthalpy change. I infact divided it by 0.02 seeing as there is a 1:2 ratio between the calcium oxide and the water produced,

    Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq) + 2H2O(l)

    , which for neutralisations is what I think the energy change has to be divided by. But could someone help me out on this? Thanks!!
    Offline

    13
    ReputationRep:
    Look up the definition for standard enthalpy change- the enthalpy change to form 1 mole of a water.

    Edit: Just realised it's even simpler.

    You divide the energy change by the number of moles right? Well you should use the compound which is limiting (not in excess). Therefore if you have 0.01mol of calcium hydroxide, you use that.
    Offline

    3
    ReputationRep:
    The balanced equation involves two moles of HCl for every 1 mole of Ca(OH)2
    • Thread Starter
    Offline

    5
    ReputationRep:
    Ah I see, silly mistake. Thanks to both of you!
 
 
 
  • 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
    Brussels sprouts
    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.