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)