Hess’s Law
• In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same
whether the reaction takes place in one step or in a series of steps.
• Ex: the oxidation of nitrogen to produce nitrogen dioxide
– can be carried out in two distinct steps:
– the sum of these two steps gives the net, or overall, reaction:
The principle of Hess’s law
Characteristics of Enthalpy Changes
• Two characteristics of ΔH for a reaction:
– If a reaction is reversed, the sign of Δ H is also reversed.
– The magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced
reaction are multiplied by an integer, the value
of Δ H is multiplied by the same integer (i.e.,
an extensive property)
Example 9.6
• Diborane (B2H6) is a highly reactive boron hydride, which was once considered as a possible rocket fuel for the
U.S. space program. Calculate ΔH for the synthesis of diborane from its elements, according to the equation
• Using the following data:
Solution:
• This procedure can best be done by focusing on the reactants and products of the required reaction:
Hydrogen?
Water ?
Standard Enthalpies of Formation
• Not all the values ofΔH for the chemical
processes can be measured in a calorimeter.
– Calculate ΔH for chemical reactions and physical changes by using standard enthalpies of formation – The standard enthalpy of formation ( ) of a
compound is defined as the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states.
– The superscript zero on a thermodynamic function indicates that the corresponding process has been carried out under standard conditions.
o
H
fΔ
o
H f
Δ
Standard State for a substance
• A precisely defined reference state:
– For a gas the standard state is a pressure of exactly 1 atm.
– For a substance present in a solution, the standard state is a concentration of exactly 1 M at an applied pressure of 1 atm.
– For a pure substance in a condensed state (liquid or solid), the standard state is the pure liquid or solid.
– For an element the standard state is the form in which the element exists (is most stable) under conditions of 1 atm and the temperature of interest (usually 25ºC).
* The International Union of Pure and Applied Chemistry (IUPAC) has adopted 1 bar (100,000 Pa) as the standard pressure instead of 1 atm (101,325 Pa). Both standards are now widely used.
Standard Enthalpies of Formation
• Ex:
per mole of product with the product in its standard state !!
The change in enthalpy for the overall reaction
• The enthalpy change for a given reaction can be calculated by subtracting the enthalpies of
formation of the reactants from the enthalpies of formation of the products:
– Elements are not included in the calculation since elements require no change in form.
– We have in effect defined the enthalpy of formation of an element in its standard state as zero.
Key Concepts for Doing Enthalpy Calculations
• When a reaction is reversed, the magnitude of ΔH remains the same, but the sign changes.
• When the balanced equation for a reaction is multiplied by an integer, the value of Δ H for that reaction must be multiplied by the same integer.
• The change in enthalpy for a given reaction can be calculated from the enthalpies of formation of the reactants and products:
• Elements in their standard states are not included in the Δ Hreaction calculations. That is, △Hf° for an element in its standard state is zero.
Example 9.7
• Using the standard enthalpies of formation listed in Table 9.4, calculate the standard enthalpy change for the overall reaction
that occurs when ammonia is burned in air to form nitrogen dioxide and water. This is the first step in the manufacture of nitric
acid.
) ( 6
) ( 4
) ( 7
) (
4 NH
3g + O
2g → NO
2g + H
2O l
Solution:
– The pathways:
Solution:
– Step (a): Decomposition of NH3(g) into elements
• Since
– Step (b): O2(g) is an element in its standard state:
– Step (c): Synthesis of 4 mol of NO2 (g) from the elements:
• Thus,
– Step (d): Synthesis of 6 mol of H2O(l) from the elements:
• Thus,
) 0
( =
ΔHob
• To summarize:
Example 9.8
• Methanol (CH
3OH) is sometimes used as a fuel in high-performance engines. Using the data in Table 9.4, compare the
standard enthalpy of combustion per gram of methanol with that of gasoline. Gasoline is actually a mixture of compounds, but
assume for this problem that gasoline is
pure liquid octane (C
8H
18).
• The combustion of methanol
• The combustion of octane
For 2 mol (or 64 g) of methanol
For 2 mol (or 228.4 g) of octane The enthalpy of combustion per gram of octane is
about twice that per gram of methanol !!
Present Sources of Energy
• By the process of photosynthesis, plants store energy that can be claimed by burning the plants themselves or the decay products that have been converted to fossil fuels
Energy sources used in the United States
Petroleum and Natural Gas
• Petroleum:
– a thick, dark liquid composed mostly of compounds called hydrocarbons that contain carbon and
hydrogen. Most likely formed from the remains of
marine organisms that lived c.a. 500 million years ago.
– It consists mostly of hydrocarbons having chains that contain from 5 to more than 25 carbons.
• Natural gas:
– usually associated with petroleum deposits, consists mostly of methane but also contains significant
amounts of ethane, propane, and butane.
Hydrocarbons
Coal
• From the remains of plants that were buried and subjected to pressure and heat over long periods of time
• Coal “matures” through four stages: lignite, subbituminous, bituminous, and anthracite. Each stage has a higher carbon-to-oxygen and carbon-to-hydrogen ratio.
most valuable least valuable
Effects of Carbon Dioxide on Climate
• Radiant energy from the sun:
– ~ 30% is reflected into space by the atmosphere
– Some is absorbed by plants to drive photosynthesis and by the ocean to evaporate water
– MOST is absorbed by soil, rock and water, resulting an increase in the temperature of the earth’s surface.
(In turn radiated from the heated surface mainly as infrared radiation (often called heat radiation))
• Molecules in the atmosphere, principally H
2O
and CO
2, strongly absorb infrared radiation,
trapping it in the earth’s atmosphere
Greenhouse Effect
• H
2O
– The atmosphere’s water content is controlled by the water cycle (evaporation and precipitation) and the average content remains constant over the years.
• CO
2– The increase of the CO2 concentration was 16% from 1880 to 1980.
– Some projections indicate that the CO2 concentration may be double in the 21 century what it was in 1880.
The earth’s average temperature could increase by 3ºC.
FIGURE 9.14
The earth’s atmosphere is
transparent to visible light from the sun. This visible light strikes the earth, and part of it is changed to infrared radiation. The infrared
radiation from the earth’s surface is strongly absorbed by CO2, H2O, and other molecules present in
smaller amounts (for example, CH4 and N2O) in the atmosphere. In effect, the atmosphere traps some of the energy, acting like the glass in a greenhouse and keeping the earth warmer than it would
otherwise be.
Diagram of Storage of CO 2
New Energy Sources
• Should consider economic, climatic and supply factors.
• Several potential energy sources:
– The sun (solar)
– Nuclear processes (fission and fusion) – Biomass (plants)
– Synthetic fuels
– Others
Coal conversion
• Transportation costs for solid coal are high,
more energy-efficient fuels are being developed
→ gaseous fuel → convert coal from a solid to a gas requires reducing the size of the molecules
• Coal gasification:
– A process which breaks down the coal structure.
– Many of the carbon-carbon bonds are replaced by carbon-hydrogen and carbon-oxygen bonds as the coal fragments react with the water and oxygen.
– The product consists of synthetic gas, or syngas (a mixture of carbon monoixde and hydrogen) and
methane gas.
Coal Gasification
Coal…
• In the gasification process:
• Methanol formation
– can be used in the production of synthetic fibers, plastics and gasoline
• Coal slurry
– Pulverized coal mixed with water
– Might replace solid coal and residual oil (a heavy oil from petroleum accounting for 13% of U.S. petroleum imports.)
endothermic exothermic
Hydrogen as a Fuel
• The heat of combustion of H
2(g) per gram is about 2.5 times that of natural gas:
• Only water as the product
• Three main problems:
– The costs of production – Storage and transport
• On metal surface, H2 decomposes to atoms, migrates into the metal, causing structural changes that make its brittle
• Relative small amount of energy that is available per unit volume of hydrogen (1/3 of methane)
Hydrogen
• Current production:
– Highly endothermic; treating methane with steam is not an efficient way
• Water splitting:
– requires 286 kJ per mole of liquid water; not economically feasible. Other methods?
Hydrogen from water
• Electrolysis of water
– Still too expensive
• Thermal decomposition of water
• Biological decomposition of water
– Only small-scale currently
Lower temp?