Question

When a student mixed `50mL` of `1M HCI` and `150mL` of `1M NaOH` in a coffee cup calorimeter, the temperature of the resultant solution increases from `21^(@)C` to `27.5^(@)C`. Assuming that the calorimeter absorbs only a negligible quantity of heat, that the total volume of solution is `100mL`, its density `1gm-mL^(-1)` and that its specific heat is `4.18 J g^(-1)`. calculate:
a. The heat change during mixing.
b. The enthalpy change for the reaction
`HCI(aq) +NaOH(aq) rarr NaCI(aq) +H_(2)O(aq)`

Answer 1

(a). Number of moles of HCl and NaOH added
`=(MV)/(1000)=(1xx50)/(1000)=0.05`
Mass of mixture `=Vxxd=100xx1=100g`
Heat evolved, `q=msDeltaT=100xx4.18xx(27.5-21.0)`
`=100xx4.18xx6.5J=2717J=2.717kJ`
(b). The involved reaction is:
`HCl(aq.)+NaOH(aq.)toNaCl(aq.)+H_(2)O`
`DeltaH=`Heat evolved per mol
`=(-2.717)/(0.05)=-54.34kJ`