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122-10

# 122-10 - Standard Enthalpies of Formation The standard...

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Standard Enthalpies of Formation The standard enthalpy of formation of a substance, denoted H f o , is the enthalpy change for the formation of one mole of a substance in its standard state from its component elements in their standard state. Note that the standard enthalpy of formation for a pure element in its standard state is zero.

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Standard Enthalpies of Formation The l aw of summation of heats of formation states that the enthalpy of a reaction is equal to the total formation energy of the products minus that of the reactants. Σ is the mathematical symbol meaning “the sum of”, and m and n are the coefficients of the substances in the chemical equation . . ) reactants ( H m ) products ( H n H o f o f o - =

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A Problem to Consider You record the values of H f o under the formulas in the equation, multiplying them by the coefficients in the equation. - You can calculate H o by subtracting the values for the reactants from the values for the products. ) 9 . 45 ( 4 - ) 0 ( 5 ) 3 . 90 ( 4 ) 8 . 241 ( 6 - ) g ( O H 6 ) g ( NO 4 ) g ( O 5 ) g ( NH 4 2 2 3 + +
How is theheat of sublimation, H sub , theenthalpy changefor the reaction: H 2 O (s) H 2 O (g) related to H fis and H vap ?

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Fuels Food fills three needs of the body: - It supplies substances for the growth and repair of tissue. - It supplies substances for the synthesis of compounds used in the regulation of body processes. - It supplies energy. About 80% of the energy we need is for heat. The rest is used for muscular action and other body processes
Fuels A typical carbohydrate food, glucose (C 6 H 12 O 6 ) undergoes combustion according to the following equation. + ) g ( O 6 ) s ( O H C 2 6 12 6 kJ -2803 H ); l ( O H 6 ) g ( CO 6 o 2 2 = + One gram of glucose yields 15.6 kJ (3.73 kcal) when burned.

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Fuels A representative fat is glyceryl trimyristate, C 45 H 86 O 6 . The equation for its combustion is: + ) g ( O ) s ( O H C 2 2 127 6 86 45 kJ -27,820 H ); l ( O H 43 ) g ( CO 45 o 2 2 = + One gram of fat yields 38.5 kJ (9.20 kcal) when burned. Note that fat contains more than twice the fuel per gram than carbohydrates contain.
Figure 6.15: Sources of energy consumed in the United States (1996).

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Fuels Fossil fuels account for nearly 90% of the energy usage in the United States. Anthracite, or hard coal, the oldest variety of coal, contains about 80% carbon. Bituminous coal, a younger variety of coal, contains 45% to 65% carbon. Fuel values of coal are measured in BTUs (British Thermal Units).
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