The thermo chemical equations for the formation of water are: How much energy is needed to convert 2 mol of liquid water to water vapour? - Wired Faculty

Question

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The thermochemical equations for the formation of water are:

$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis$
$increment subscript straight r straight H to the power of 0 space equals space minus 483. space 8 space kJ$
$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space increment subscript straight r straight H to the power of 0 space equals space minus 571.8 space kJ$
How much energy is needed to convert 2 mol of liquid water to water vapour?

Solution

We aim at equation
$2 straight H subscript 2 straight O left parenthesis straight l right parenthesis space space space rightwards arrow space space space 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis semicolon space space space increment subscript straight r straight H to the power of 0 space equals space ?$
Given that
$left parenthesis straight i right parenthesis space 2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space space space rightwards arrow space space space 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis semicolon space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space increment subscript straight r straight H to the power of 0 space equals space minus 483.8 space kJ left parenthesis ii right parenthesis space 2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space space space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space increment subscript straight r straight H to the power of 0 space equals space minus 571.8 space kJ$
Subtracting equation (ii) from equation (i), we have,
$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space minus space space 2 straight H subscript 2 left parenthesis straight g right parenthesis space minus space straight O subscript 2 left parenthesis straight g right parenthesis space space space space rightwards arrow space space 2 straight H subscript 2 straight O space left parenthesis straight g right parenthesis space minus space 2 straight H subscript 2 straight O left parenthesis l right parenthesis semicolon space space space space increment subscript straight r straight H to the power of 0 space equals space minus 483.8 space plus space 571.8 space space space space space o r space space space 2 straight H subscript 2 straight O left parenthesis l right parenthesis space space space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis semicolon space space space increment subscript straight r straight H to the power of 0 space equals space 88 space k J$

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Thermodynamics

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