Chapter 5: Thermochemistry Part 3
Use the average bond enthalpies in Table 5.4 to estimate ΔH for the combustion of ethanol.
-1255 kJ/mol
Given the following standard enthalpy change, use the standard enthalpies of formation in table 5.3 to calculate the standard enthalpy of formation of CuO(s): CuO(s) + H2(g) = Cu(s) + H2O(l) ΔHo = -129.7kJ
-156.1 kJ/mol
Two steps to determine enthalpy of reaction from bond enthalpy
1. Energy added to break bonds between reactants not in the products. Enthalpy of the system is increased by the sum of the bond enthalpies of the bonds that are broken 2. Energy released as bonds are formed between products not present as reactants. Enthalpy of the system is lowered by the sum of the bond enthalpies of the bonds that are formed
For which of these reactions at 25 C does the enthalpy change represent a standard enthalpy of formation? For each that does not, what changes are needed to make it an equation whose ΔH is an enthalpy of formation? a. 2Na (s) + 1/2 O2(g) = Na2O(s) b. 2K (l) + Cl2(g) = 2KCl(s) c. C6H12O6 (s) = 6 C (diamond) + 6 H2(g) + 3O2 (g)
A B K should be solid and reduce moles by 2 C carbon should be graphite note diamond and elements must be reactants not products
coal
A fossil fuel that forms underground from partially decomposed plant material solid, hydrocarbons - high molecular weight
What is the enthalpy of formation for element in most stable form?
Element in most stable form has a standard enthalpy of formation of zero
Biofuels
Fuels, such as ethanol or methanol, that are created from the fermentation of plants or plant products. bioethanol - most common
nuclear energy
The potential energy stored in the nucleus of an atom energy released by fission or fusion of atomic nuclei
standard state
a reference state for a specific substance defined according to a set of conventional definitions atmospheric pressure (1 atm) temperature of interest usually 25C
When a bond breaks, energy is
absorbed
Bond enthalpy in polyatomic molecules
average together total bond enthalpy CH4 = C + 4H ΔH = 1660kJ Each C-H bond has enthalpy of 415 kJ/mol (divide by 4)
natural gas
contain hydrocarbons methane (mostly) , ethane, propane, butane
Is breaking a bond endothermic or exothermic?
endothermic
standard enthalpy of formation
enthalpy change if 1 mole of compound in standard state were formed directly from its elements elements (standard state) = compound (1 mole in standard state) most stable form of element used if if element exist in more than one form under standard conditions
What is the bond enthalpy equal to in a diatomic molecules?
enthalpy of the reaction
Is forming a bond endothermic or exothermic?
exothermic
Stronger bonds have
greater bond enthalpy molecule less likely to have chemical change
greater carbon and hydrogen in fuel lead to
greater fuel value
petroleum
liquid fossil fuel; oil hydrocarbons
When a bond forms, energy is
released
The energy released in a exothermic reactions is
released as heat
What is the standard enthalpy change of the reaction?
sum of the standard enthalpies of formation of the products minus the standard enthalpies of formation of the reactants
What does the magnitude of any enthalpy change depend on?
temperature, pressure, and state (gas, liquid, or solid)
bond enthalpy
the energy needed to break one mole of bonds in gaseous molecules under standard conditions positive number because energy is supplied from surroundings use letter D to represent bond enthalpies D(Cl-Cl) equal to enthalpy of the reaction
fuel value
the energy released when 1 g of a substance is combusted
Average bond enthalpy can be used to estimate
the enthalpies of reactions ΔHrxn which bonds are broken and new bonds are formed can be used to know if reaction is endothermic or exothermic
enthalpy of formation
the enthalpy change that accompanies the formation of a substance from the most stable forms of its component elements heat of formation
Standard enthalpy change
the enthalpy change when all reactants and products are in their standard states ΔHo o = standard state conditions
If the total energy of the newly formed bonds in larger (when using bond enthalpies to calculate enthalpy of the reaction) then
the reaction is endothermic
If the total enthalpy of the broken bonds in larger (when using bond enthalpies to calculate enthalpy of the reaction) then
the reaction is endothermic
enthalpy of reaction from bond enthalpies
ΔH reaction = sum(bond enthalpies of bonds broken) - sum(bond enthalpies of bonds formed)