5.3 Enthalpy

We need to consider some of the concepts of thermodynamics as we focus on thermochemistry in this chapter.

Substances act as a source of energy, meaning that they can be added or removed.

The energy in a substance is stored in its atoms.
Increased translations, vibrations, or rotation of the atoms or molecules are some of the ways in which the greater kinetic energy can be found.

When thermal energy is lost, the motions become less intense.

When a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings.

If the metal wire is immersed in hot water, it will absorb heat from the water, or if you bend the wire back and forth, it will become warmer.
The internal energy of the wire is increased by both processes.
When the system doesn't work on the surroundings, or when heat is lost from the system, energy is transferred out.

During the operation of an internal combustion engine, an example of this occurs.

The reaction of gasoline and oxygen is cold.
Some of the energy is given off as heat and some is used to push the cylinder.
The system and surroundings are the substances involved in the reaction.
The internal energy of the system decreases when heating and doing work on the surroundings.
How to determine the amount of work involved in a chemical or physical change will be considered.

The exothermic combustion reaction of a fuel such as gasoline into energy of motion is shown in this view.

The value of a state function is dependent on the state that the system is in.

An example of a state function is elevation.
If you are on the summit of Mt.
It doesn't matter if you hiked there or parachuted there, you are at an altitude of 5895 m. The distance traveled to the top of Kilimanjaro is not a state function.
You could either take a direct route or take a circuitous path to the summit.
The distance traveled is not a state function, but the elevation reached is.

There are two different routes to the summit.

Both have the same change in elevation, but they have different distances traveled, and it depends on the path.

The heat given off when you operate a Bunsen burner is equal to the enthalpy change of the methane combustion reaction that takes place, since it occurs at the constant pressure of the atmosphere.

The changes in matter and energy are represented by a thermochemical equation.

When 1 mole of hydrogen gas and 12 mole of oxygen gas at some temperature and pressure change to 1 mole of liquid water at the same temperature and pressure, 286 kJ of heat are released to the surroundings.

12 x (-286kJ) is 2.

The enthalpy change of a reaction depends on the physical state of the reactants and products of the reaction, so these must be shown.
286 kJ of heat is released when 1 mole of hydrogen gas and 12 mole of oxygen gas change to 1 mole of liquid water at the same temperature and pressure.
Only 242 kJ of heat can be released if water forms.

When 1 mole of HCl reacts, the enthalpy change is -58 kJ.

A gummy bear has 2.67 grams of sugar.

43.7 kJ of heat is produced when it reacts with 7.19 g potassium chlorate.

We have 2.67 g x 1 mol 342.3 g and 7.19 g x 1 mol 122.5 g.

-5960 kJ is themol x -43.7 kJ 0.0587 mol KClO.

Enthalpy changes are calculated for reactions in which both reactants and products are at the same conditions.

The IUPAC standard state for chemists does not specify a temperature, but refers to materials under a pressure of 1 bar and solutions at 1 M. The enthalpy change symbol will have a superscripted "o" in it.

We will use a subscripted " " to designate the temperature since it is not technically standard.

enthalpy change is used to indicate a process occurring under these conditions.

The enthalpy changes for many chemical and physical processes can be found in the reference literature.

The amount of heat produced when one mole of ethanol undergoes complete combustion at 25 degC and 1 atmosphere pressure is called the enthalpy.

Some of the substances that have been measured are listed in Table 5.2.

The process of burning gasoline is a highly exothermic one.

Assuming isooctane is the same as that of gasoline, we can determine the amount of heat produced by burning 1.00 L of gasoline.
The isooctane has a density of 0.692 g/mL.

The burning of gasoline is very hot.

We can perform conversions between units until we arrive at the desired amount of heat or energy.

One of the necessary conversions is provided by the enthalpy of combustion.
This value is given in Table 5.2 as -5460 kJ per 1 mole of isooctane.

33,100 kJ of heat is produced by the combustion of 1.00 L of isooctane.

As reserves of fossil fuels diminish and become more costly to extract, the search is ongoing for replacement fuel sources for the future.

Some of the most promising fuels are derived from algae.

Among the world's fastest growing organisms are the species of algae used.

50% of the algal weight is oil, which can be converted into fuel.

It can yield more energy than other crops.
Some strains of algae can grow in water that is not suitable for growing other crops.
Biodiesel, butanol, methane, and even jet fuel can be produced by algae.

The US Air Force is currently producing jet fuel from algae at a cost of under $5 per gallon.

A variety of renewable fuels are created by the conversion of sunlight and carbon dioxide into oil by algae.

For more on the problem ofpentagon fuel, see http://www.theguardian.com/environment/2010/feb/12/algae-solve-pentagon-fuel-problem.

Click on the process to create the biofuel.

These values are useful for computing or predicting enthalpy changes for chemical reactions that are impractical or dangerous to carry out, or for processes that are difficult to make measurements.

If we have values for the appropriate standard enthalpies of formation, we can determine the enthalpy change for any reaction, which we will practice in the next section.

The carbon present in the reactants at a pressure of 1 atm and 25 degC is the most stable form of carbon under these conditions.

You can find a table of standard enthalpies of formation of many common substances in these values.

The standard enthalpy of formation of an element in its most stable form is zero under standard conditions, which is 1 atm for gases and 1 M for solutions.

The energy that drives this reaction in the upper atmosphere comes from ultraviolet radiation.

There are two ways to determine the amount of heat involved in a chemical change.

It is difficult to investigate and make accurate measurements in some reactions.
It is convenient to be able to determine the heat involved in a reaction without having to do an experiment.

The enthalpy is a state function and it is valid because it depends on where a chemical process starts and ends.

The reaction of carbon with oxygen to form carbon dioxide can be thought of as a two-step process.

The products are at a lower enthalpy than the reactants.

If we divide an equation by a number, the enthalpy change should also be divided by the same number.

This is a less straightforward example of the thought process involved in solving a law problem.

If the corresponding enthalpies of formation of the reactants and products are available, we can use Hess's law to determine the enthalpy change of any reaction.

The stepwise reactions we consider are: decomposition of the reactants into their component elements for which the enthalpy changes are proportional to the negative of the enthalpies of formation of the reactants, followed by re-combinations of the elements The standard enthalpy change of the overall reaction is the same as the sum of the standard enthalpies of formation of the reactants.

This procedure is used in the general equation.

The capacity to do work is called energy.

The potential energy is based on relative position, composition, or condition.
Energy is not created or destroyed when it is converted from one form to another.

The thermal energy of Matter is due to the average ke of its molecules.

The energy that is transferred between objects at different temperatures is called heat.
Chemical and physical processes can release heat.
The joule is the SI unit of energy, heat, and work.

Specific heat and heat capacity are measures of the energy needed to change the temperature.

The amount of heat released by a substance depends on a number of factors.

Calorimetry measures the amount of thermal energy transferred in a process.

This requires careful measurement of the temperature change that occurs during the process.
The amount of heat produced or consumed in the process is calculated using known mathematical relations.

Calorimeters are used to minimize energy exchange between the system being studied and its surroundings.

Coffee cup calorimeters are used by introductory chemistry students, while bomb calorimeters are used to determine the energy content of food.

Under standard state conditions, reactions occur at 298 deg.

enthalpy of formation, fusion, and vaporization are examples of enthalpy changes.

If the reactants and products of a reaction are available, the enthalpy change can be calculated.

You wouldn't sit around a burning match on a fall evening to stay warm if the temperature was the same.

There are several energy transitions that take place during the typical operation of an automobile.

A piece of unknown substance weighs 44.7 g and requires 2110 J to increase its temperature.

A piece of unknown solid substance weighs 437.2 g and requires 8460 J to increase its temperature.

An aluminum kettle weighs more than one kilo.

Most people find waterbeds uncomfortable if the water temperature is not maintained.

Unless it is heated, a waterbed with 892 L of water will cool from 85 to 72 degrees in 24 hours.
Estimate the amount of electrical energy required to keep the bed cool.
Assume that the density of water is 1.0 g/mL, and that 1 kilowatt-hour is 3.6 x 106 J.

A bottle of water at room temperature and a bottle of water at the same temperature were placed in a refrigerator.

The bottle of water cooled to the temperature of the refrigerator after 30 minutes.
The water cooled to the same temperature an hour later.
One student said that both bottles lost the same amount of heat because they started at the same temperature and finished at the same temperature.
A student thought that the bottle of water lost more heat because there was more water.
A third student thought that the bottle of water lost more heat because it cooled more quickly.
A fourth student thought that it was not possible to tell because we don't know the initial and final temperatures of the water.
Explain the error in each of the other answers if the answers are correct.

Coffee and water have the same density and heat.

Coffee has the same density and heat as water.

The temperature of the coffee and the spoon become equal when the spoon is placed in 180 mL of coffee.

Coffee has the same heat as water.

A 0.500g sample of KCl is added to 50.0 g of water in a calorimeter.

The approximate amount of heat produced by the solution and products is assumed to be 4.20 J/g degC.

The temperature increases when a 0.740-g sample of trinitrotoluene is burned in a bomb calorimeter.

The calorimeter has a heat capacity of 534 J/degC and has water in it.

One way of generating electricity is by burning coal to heat water, which produces steam that drives an electric generator.

To determine the rate at which coal is to be fed into the burner in this type of plant, the heat of combustion per ton of coal must be determined using a bomb calorimeter.
When 1.00 g of coal is burned in a bomb calorimeter, the temperature increases.
Determine the heat produced by burning a ton of coal if the calorimeter has a heat capacity of 21.6 kJ/degC.

16 calories is the number of calories in a cup of common sugar.

1100 calories can be found in a quart of premium ice cream.

A serving of breakfast cereals contains a lot of food.

The heat produced by the isooctane under standard conditions can be compared to the nutrition of the cereals.

130 calories can be found in a 1.0 ounce serving of the cereals.

Consider the conditions for which the data is reported.

125 kJ of heat is produced when 2.50 g of methane burns.

A sample of carbon is burned in a bomb calorimeter to make carbon dioxide.

The heat released from the reactants and products is proportional to the enthalpy of the fire.
The temperature of the calorimeter goes up from 26.74 to 27.93.

Sulfur dioxide was used in household refrigerators before the introduction of chlorofluorocarbons.

The hot water may be pumped through the radiators.

When 100 g of steam is cooled to 100 degC, what mass of water will provide the same amount of heat.

In 1774, Joseph Priestly prepared oxygen by heating red mercury(II) oxide with sunlight.

Determine the total energy change for the production of one mole of nitric acid.

H2O2 has been used to provide thrust in the control jets of various space vehicles.

The data was used under standard conditions.

The formation of propane has a enthalpy of 104 kJ/mol.

The enthalpy of formation of butane is 126 kJ/mol.

Gaseous fuels include propane and butane.

For this reaction and for the condensation of liquid methanol.

In the early days of automobiles, acetylene, C2H2, was burned to provide illumination at night.

Some cave explorers still use acetylene as a source of light despite no longer being used as an auto headlamps.

The enthalpy of gasoline is 1.28 x 105 kJ/gal, while the enthalpy of hard coal is -35 kJ/g.

The density of gasoline is the same as isooctane.

In Brazil, C2H5OH is used as a fuel for motor vehicles.

Among the 50 chemical compounds produced commercially in the largest quantities, ethylene, C2H2, is fourth.

Synthetic Ethanol is made from the reaction of ethylene with water in the presence of a suitable catalyst.

The same products are given by the metabolism of glucose, even though it reacts with oxygen in a series of steps in the body.

The air has 23% oxygen by mass.

During the month, the average density of air was 1.22 g/L.

In a house, electricity is efficient in producing heat.

In a coal-fired power plant, the efficiency of production and distribution is 40%.

2.26 kWh per pound is provided by a certain type of coal.