# **6.1 - The Nature of Energy**

* **Energy**: The capacity to do work or to produce heat
* The law of conservation of energy states that energy can be converted from one form to another but can be neither created nor destroyed
* The total energy content of the universe is constant
* **Potential energy**: Energy due to position or composition
  * Attractive and repulsive forces also lead to potential energy
* **Kinetic energy**: An object is energy due to the motion of the object and depends on the mass of the object **m** and its velocity **v**
* Heat involves a transfer of energy
* A state function refers to a property of the system that depends only on its present state
  * This leads to a very important characteristic of a state function: A change in the function in going from one state to another state is independent of the particular pathway between the two states
  * Work and heat are not state functions
* The system is the part of the universe on which we wish to focus attention; the surroundings include everything else in the universe
* **Exothermic:** A reaction results in the evolution of heat
* **Endothermic**: A reaction that absorbs energy from the surroundings
* The internal energy E of a system can be defined mso t\[precisely as the sum of the kinetic and potential energies of all the particles in the system
* **Thermodynamic**s: The study of energy and its interconversions
  * Thermodynamic quantities always consist of two parts: a number, giving the magnitude of the change, and the sign, indicating the direction of flow
  * The sign reflects the system’s point of view
  * To take the system’s point of view; q= -x denotes an exothermic process, and q=+x denotes an exothermic one
* A common type of work associated with chemical processes is work done by gas or work done to a gas
* For a gas expanding against an external pressure P, w is a negative quantity as required, since work follows out of the system
* For an ideal gas, work can occur only when its volume changes. Thus, if a gas is heated at a constant volume, the pressure increases but no work occurs

# **6.2 - Enthalpy and Calorimetry**

* Enthalpy is a state function. A change in enthalpy does not depend on the pathway between two states
* ▵H = q only at constant pressure
* The change in enthalpy of a system has no easily interpreted meaning except at constant pressure, where ▵H= heat
* At constant pressure, exothermic means ▵H is negative; endothermic means ▵H is positive
* **Calorimetry**: the science of measuring heat based on observing the temperature change when a body absorbs or discharges energy as heat
* Constant-pressure calorimetry is used in determining the changes in enthalpy for reactions occurring in solutions
* **Specific heat capacity**: The energy required to raise the temperature of one gram of a substance by one degree Celsius
* **Molar heat capacity**: The energy required to raise the temperature of one mole of a substance by one degree Celsius

# **6.3 - 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
* ▵H is not dependent on the reaction side
* Reversing the direction of a reaction changes the sign of ▵H
* Hints for using Hess’s Law: Work backward from the required reaction, using the reactants and products to decide how to manipulate the other given reactions at your disposal
  * Reverse any reactions as needed to give the required reactants and products
  * Multiply reactions to give the correct numbers of reactants and products

# **6.4 - Standard Enthalpies of Formation**

* The standard enthalpy of formation of a compound is defined as the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states.
* For most thermodynamic properties, we can measure only changes in the property
* The standard state is not the same as the standard temperature and pressure for a gas
* Enthalpies of formation are always given per mole of product with the product in its standard state.
* Enthalpy is a state function, so we can invoke Hess’s law and choose any convenient pathway from reactants to products and then sum the enthalpy changes along the chosen pathway.
* 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
  * Remember to multiply the enthalpies of formation by integers as required by the balanced equation.

# **6.5 - 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 over millions of years to fossil fuels
* **Petroleum**: A thick, dark liquid composed mostly of compounds called *hydrocarbons* that contain carbon and hydrogen.
* **Natural gas**: Associated with petroleum deposits, consists mostly of methane, but it also contains significant amounts of ethane, propane, and butane.
* Coal has variable composition depending on both its age and location.
  * Coal was formed from the remains of plants that were buried and subjected to high pressure and heat over long periods of time.
  * It matures through four stages: lignite, sub-bituminous, bituminous, and anthracite.
  * The energy available from the combustion of a given mass of coal increases as the carbon content increases
  * If coal were pure carbon, the carbon dioxide produced when it was burned would still have significant effects on the earth’s climate.
* The earth receives a tremendous quantity of radiant energy from the sun, about 30% of which is reflected back into space by the earth’s atmosphere
  * The temperature of the earth’s surface is controlled to a significant extent by the carbon dioxide and water content of the atmosphere
  * The average temperature of the earth’s surface is 298 K. It would be 255 K without the “greenhouse gases”

# **6.6 - New Energy Sources**

* There are several potential energy sources: the sun (solar), nuclear processes (fission and fusion), biomass (plants), and synthetic fuels
* To convert coal from a solid to a gas, therefore, requires reducing the size of the molecules; the coal structure must be broken down in a process called *coal gasification*
  * To convert coal from a solid to a gas, therefore, requires reducing the size of the molecules; the coal structure must be broken down in a process called *coal gasification*
* An industrial process must be energy efficient.
* Syngas can be used directly as a fuel and as a raw material to produce other fuels.
* coal slurries might replace solid coal and residual oil as fuels for electricity-generating power plants
* However, the water needed for slurries might place an unacceptable burden on water resources, especially in the western states.
  * The western states, especially Colorado, contain huge deposits of *oil shale,* which consists of a complex carbon-based material called *kerogen* contained in porous rock formations.
* These deposits have the potential of being a larger energy source than the vast petroleum deposits of the Middle East.
* Another potential source of liquid fuels is oil squeezed from seeds
  * It is hoped that oil-seed plants can be developed that will thrive under the soil and climatic conditions unsuitable for corn and wheat.
* The main advantage of seed oil as a fuel is that it is renewable

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Chemistry

# **6.1 - The Nature of Energy**

* **Energy**: The capacity to do work or to produce heat
* The law of conservation of energy states that energy can be converted from one form to another but can be neither created nor destroyed
* The total energy content of the universe is constant
* **Potential en

Chapter 6 - Thermochemistry

Georges Jackson

View the note Chapter 6 - Thermochemistry: 6.1 - The Nature of EnergyEnergy: The capacity to do work or to produce heatThe law of conservation of energy states that energy can be converted from one form to another but can be neither created nor destroyedThe total energy content of the universe is co...