5. Chemical energetics

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4 thoughts on “5. Chemical energetics

  1. This question refers to 2013 may/june paper 3 variant 34 question 1(d). The total volume used in measuring the temperature is increased but answers state that the accuracy is unaffected because the temperature rise will still be the same. How does the temperature rise remain the same?

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    • You are using the same concentrations to find the max temperature (that’s what they are referring to when they lazily say ” temp rise the same”). Using more volume of one substance will simply require a proportionate increased volume of the other liquid. Of course more moles of acid and base are reacting but it’s in a greater volume. Tamount of heat per cm3 of liquid hasn’t changed, hence the max temp is the same.
      Looking at it with ‘easier numbers’, 10cm3 of 1 molar HCL reacting with 10cm3 of 1 molar NaOH will give the same temperature rise as 20cm3 of 1 molar HCl reacting with 20 cm3 of NaOH. You have double the heat but in double the volume, so the temperature per cm3 (if you like to think of it that way) will be same as before.

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  2. Hi there. This question refers to Paper 1 May/June 2014 variant 13 question 24. Both methane and methanol produces the same number of moles of CO2, thus why is the official answer B? Besides, how to calculate the enthalpy in that question as the mass or even the number of moles of fuels isnt given?

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  3. Hi Mitrrah.
    Methanol releases less energy PER MOLE, compared to methane.
    So you would need to burn more moles of methanol (than methane) to get enough energy to heat from 20oC to 100oC. Hence more CO2 would be produced.

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