What Is Energy Quizlet Chemistry?
Energy is a fundamental concept in chemistry that describes the capacity to do work and drive physical or chemical changes. Some key energy concepts covered in chemistry include:
Forms of Energy – Energy can exist in many different forms, such as chemical energy, thermal energy, electromagnetic radiation, nuclear energy, and more. Chemists study how energy is transferred between these different forms.
Energy Transformations – Reactions and chemical changes involve energy transformations, where some forms of energy are converted to others. Exothermic reactions release energy, while endothermic reactions absorb energy.
Thermochemistry – The branch of chemistry concerned with measuring heat changes during chemical reactions and transformations. Enthalpy, entropy, and free energy are important thermodynamic properties.
Chemical Energy – The potential energy stored in the bonds between atoms within molecules. Reactions release this energy when bonds are broken and new bonds form.
Nuclear Energy – The energy stored in the nucleus of an atom and released in nuclear reactions. Radioactive decay releases nuclear energy.
Electromagnetic Energy – Energy associated with electromagnetic radiation, including radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Absorption or emission of this energy leads to electronic transitions.
This overview covers the key energy concepts relevant to chemistry that will be explored further throughout this content.
Forms of Energy
Energy comes in many different forms that can be categorized as either potential or kinetic energy. Potential energy is stored energy based on an object’s position or arrangement. For example, a ball held at the top of a ramp has potential energy due to gravity. Kinetic energy is energy of motion that changes depending on an object’s speed and direction. For instance, the same ball rolling down the ramp now has kinetic energy.
Other forms of potential energy include:
- Chemical energy stored in the bonds between atoms and molecules
- Nuclear energy from forces holding nuclei together
- Strained energy stored in elastic materials
Additional forms of kinetic energy include:
- Radiant energy like light and heat that travels in waves
- Sound energy from vibrating molecules
- Motion energy of ocean waves
Energy transforms between potential and kinetic states. For example, chemical energy in gasoline is converted to kinetic energy to move a car. The law of conservation of energy states that the total energy in a closed system remains constant though it may change forms.
Energy Transformations
Energy can be converted from one form to another during chemical reactions and physical changes. The two main types of energy transformations are endothermic and exothermic.
Endothermic reactions absorb heat energy from the surroundings. They require an input of energy to proceed and have a positive change in enthalpy (ΔH). Some examples of endothermic processes are melting, vaporization, thermal decomposition, and chemical reactions like photosynthesis. As heat is absorbed during an endothermic reaction, the temperature of the surroundings decreases.
Exothermic reactions release heat energy to the surroundings. They give off energy as they proceed and have a negative change in enthalpy (ΔH). Some examples of exothermic processes are freezing, condensation, combustion reactions, and explosive chemical reactions. As heat is released in an exothermic reaction, the temperature of the surroundings increases.
The direction of heat flow determines whether a process is endothermic or exothermic. Heat flows from surroundings to system in an endothermic process, while heat flows from system to surroundings in an exothermic process. By analyzing enthalpy changes, we can classify chemical reactions and phase changes as either endo or exothermic.
Thermochemistry
Thermochemistry focuses on the heat transfer that occurs during chemical reactions and physical changes. Exothermic reactions release heat, while endothermic reactions absorb heat. The transfer of heat is an indication that a chemical reaction is taking place.
When heat is released or absorbed during a reaction, the system gains or loses energy. This energy change is called the enthalpy change (ΔH). The enthalpy change of a reaction depends directly on the bonds broken and formed during the reaction. Breaking bonds requires an input of energy, while forming bonds releases energy. Stronger bonds tend to release more energy when formed.
The enthalpy change of a reaction can be calculated based on the energies required to break and form bonds. The enthalpy change also determines whether a reaction is endothermic or exothermic. Exothermic reactions have a negative enthalpy change, indicating that energy is released. Endothermic reactions have a positive enthalpy change, showing that energy is absorbed.
By measuring the heat transfer of a reaction, the enthalpy change can be determined experimentally. This allows unknown enthalpy values to be determined for specific chemical changes. The measured heat can also be used to find other important quantities like internal energy and bond energies.
Chemical Energy
Chemical energy is the energy stored in the bonds between atoms that make up molecules. Atoms form chemical bonds with other atoms by sharing or transferring electrons. The attraction between the positively charged nuclei and the negatively charged electrons is what holds the atoms together in molecules or crystal lattices.
When atoms bond together, energy is either released or absorbed. The amount of energy stored in the bonds depends on the types of atoms involved and how they are bonded. Strong chemical bonds store more energy than weaker bonds. This potential energy stored in chemical bonds is called chemical potential energy.
When chemical bonds break, the stored chemical potential energy is released. This often occurs in chemical reactions when old bonds are broken and new bonds are formed. The energy released can be in the form of kinetic energy, light, or heat. Exothermic reactions release energy, while endothermic reactions absorb energy.
For example, the hydrocarbon molecules in fuels like gasoline, natural gas, and propane contain high levels of chemical potential energy in their hydrocarbon bonds. When these bonds are broken during combustion reactions with oxygen, large amounts of heat are released which can be used to power cars, stoves, power plants, etc. Foods also contain chemical potential energy that is released when metabolic processes break down nutrient molecules.
The study of transforming chemical potential energy during reactions and transferring it into other forms of energy is called thermochemistry. Quantifying the changes in energy that occur during chemical reactions is key to understanding how chemical energy systems work.
Nuclear Energy
Nuclear energy comes from the splitting (fission) or joining (fusion) of atomic nuclei. Nuclear fission occurs when a heavy nucleus splits into two or more lighter nuclei, releasing energy in the process. This process is used in nuclear power plants to generate electricity. Nuclear fusion occurs when two light nuclei fuse together to form a heavier nucleus, releasing a tremendous amount of energy. Fusion is the process that powers the sun and stars.
In both fission and fusion reactions, mass is converted into energy according to Einstein’s famous equation E=mc2. Even a small amount of mass converted to energy produces a tremendous amount of energy because the speed of light (c) is an extremely large number. The energy produced during nuclear reactions comes from the binding energy that holds the nucleus together. When the nucleus splits (fission) or merges (fusion), some of this binding energy is released.
During nuclear fission, a neutron collides with a heavy nucleus like uranium-235, causing it to split into two lighter nuclei, plus more neutrons that can continue the fission chain reaction. The fission of uranium-235 produces a tremendous amount of energy that can be harnessed to generate electricity. During nuclear fusion, nuclei need to overcome electrostatic repulsion between positively charged protons. Fusion occurs naturally in stars where tremendous pressure and temperature ignite the reaction. Scientists are working to develop fusion power by using extremely high temperatures and magnetic confinement to bring nuclei close enough to fuse.
Electromagnetic Energy
Electromagnetic energy refers to energy that comes from electromagnetic radiation, which includes visible light. Light is a form of electromagnetic radiation that behaves like both a wave and a particle. The particle aspect of light is known as a photon. Photons carry packets of energy called quanta. The energy of a photon is directly proportional to its frequency according to Planck’s equation:
E = hf
Where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the photon. Higher frequency light like ultraviolet has higher energy photons than lower frequency light like infrared.
When a photon is absorbed by matter, its energy can be transferred to atoms and molecules. This electromagnetic energy from light can excite electrons to higher energy levels. It can also power chemical reactions through photochemical pathways. Photosynthesis in plants is one example, where the energy from sunlight is used to convert carbon dioxide and water into glucose and oxygen. Solar cells also convert electromagnetic radiation into electrical energy through the photovoltaic effect. Overall, electromagnetic energy from photons and light can be transformed into other useful forms of energy in matter.
Energy Diagrams
Energy diagrams, also known as reaction profiles or reaction coordinate diagrams, are graphical representations of the energy changes that occur during a chemical reaction. They provide insight into the energetics of a reaction by illustrating the positions of the reactants, products, intermediates, and transition states on the potential energy surface.
These diagrams plot the potential energy of the reactants and products on the y-axis versus the reaction progress on the x-axis. The reaction progresses from left to right, beginning with the reactants and ending with the products. The peak of the diagram represents the transition state, which is the highest energy state along the reaction pathway.
Some key features of potential energy diagrams include:
- The difference in energy between the reactants and products gives the enthalpy change (ΔH) for the reaction.
- The activation energy (Ea) is the energy required to reach the transition state from the reactants.
- The reactions can be categorized as exothermic (energy releasing) if the products are lower in energy than the reactants, or endothermic (energy absorbing) if the products are higher in energy.
- The relative energies of intermediates and transition states also provide information about the reaction mechanism.
Overall, energy diagrams are an important tool for visualizing the energetic landscape of chemical reactions. They provide critical insights that aid in the analysis of reaction thermodynamics and kinetics.
Laws of Thermodynamics
The laws of thermodynamics describe the relationships between thermal energy, heat, and work. The laws provide fundamental constraints on what types of thermodynamic processes are possible.
The first law of thermodynamics is essentially a statement of the conservation of energy. It states that the change in internal energy of a thermodynamic system is equal to the amount of heat supplied to the system minus the amount of work done by the system on its surroundings. This can be summarized mathematically as:
ΔU = Q – W
Where ΔU is the change in internal energy, Q is the heat, and W is the work. This law implies that energy cannot be created or destroyed, only transformed from one form to another.
The second law of thermodynamics introduces the concept of entropy. It states that the entropy of an isolated system always increases over time. Entropy is a measure of disorder, so the second law describes how isolated systems naturally evolve towards states of higher disorder over time.
The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero is exactly equal to zero. This law provides a fixed reference point that allows entropy values to be determined experimentally.
Together, these laws place fundamental limits on the efficiency of energy conversion processes. They show that perpetual motion machines are impossible, since thermodynamic systems will always increase in entropy over time rather than sustaining endless useful energy conversion.
Energy Calculations
Energy calculations are an important part of chemistry. Here are some examples of common energy calculations:
Heat Transfer: Calculating the amount of heat (q) transferred using the equation: q = mcΔT. Where m is mass, c is specific heat capacity, and ΔT is change in temperature.
Endothermic/Exothermic Reactions: Calculating the energy change of a chemical reaction using q = nΔH. Where n is moles of substance and ΔH is the enthalpy change of the reaction.
Bond Energy: Calculating bond energy using the equation: Bond energy = ΣD(bonds broken) – ΣD(bonds formed). Where D is the bond dissociation energy.
Reaction Activation Energy: Calculating activation energy (Ea) from an Arrhenius plot using the equation: ln(k) = -Ea/RT + ln(A). Where k is the rate constant, R is the gas constant, and T is temperature.
Calorimetry: Determining the heat of a reaction by measuring the temperature change it causes in a known amount of water. Using the equation: qreaction = -qwater.
