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GECH 119 Energy. Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University. Chapter Objectives. Assignment. Read Chapters 9 in Investigating Chemistry: A Forensic Science Perspective
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GECH 119Energy Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University
Assignment • Read Chapters 9 in Investigating Chemistry: A Forensic Science Perspective • For future tests and quizzes you should be able to do problems 1 – 24 in Chapter 9:
What is energy? • Energy is the ability of a system to perform work. • What is work? • The cost of moving an object a certain distance against a force. • Units of energy • Joules (SI) • Erg • calorie (4.184 J) • Calorie (nutritional = 1000 cal) • BTU
Types of energy • Energy exists in two types. • Kinetic Energy (movement) • Potential Energy (position) • Forms of energy • Heat • Electrical • Chemical • Nuclear • Electromagnetic
Sources of Energy • Sun • Windmills • Waves • Hydroelectric plants • Food • Fuels • Nuclear • Alpha Decay • Beta Decay • Gamma Radiation
Where does energy we use go? • There are three laws of energy. • The First Law of Thermodynamics • Also known as the Law of Conservation of Energy • The energy of the universe cannot be created or destroyed. It can only be moved from one place to another.
Where does energy we use go? • Second Law of Thermodynamics • The entropy of the universe is always increasing. • Third Law of Thermodynamics • Only a perfect crystal at absolute zero has an entropy of ZERO
Energy Efficiency • It is impossible to transfer 100% of the energy from one place to another. • We always end up losing energy due to friction or collisions. • The energy is still transformed (moved) just not to where we want it.
Thermodynamics • Every process has a certain amount of energy change associated with it. • If a process has no change in energy then that process is the act of doing NOTHING. • Thermodynamics is the branch of science that studies the movement of energy as heat. • Thermochemistry is the branch of chemistry that studies the movement of energy in chemical reactions.
Chemical Energy • The energy stored in fuels which is released during the chemical reaction (combustion) is a form of potential energy (chemical potential). • Energy is not a thing but a property of a given substance or object. • The property of where the object is located, how fast it is traveling or how it is made up. • In a chemical compound the energy is “stored” in the bonds that hold the compound together.
Heat vs Temperature • We have previously discussed this to some extent. • Heat is energy and it is manifested in how fast molecules travel. • The faster the molecules travel the more likely they are to collide with one another or the walls of a container. • The more collisions there are with the wall, the more energy is transferred to the container’s walls.
Heat vs Temperature • Temperature is just a relative measurement of the object we are probing and a reference point. • There are several temperature scales: • Centigrade/Celsius • Fahrenheit • Kelvin/Absolute
Temperature Scales • Fahrenheit Scale • The scale was originally based on 4 values (now only 3). • 0F = temp of a water/ice/salt mixture • 30F = Water freezing • 90F = Body Temperature • 240F= Water boiling • Since the first is no longer used, the temperature depends on the composition of the mixture. The other 3 have been adjusted to 32, 98.6 and 212.
Temperature Scales • Celsius/Centigrade • Based on the boiling and freezing points of water and using the traditional decimal system. • Kelvin Scale • Based on experimental Pressure vs Temperature results involving gases at different volumes. It clearly demonstrated the existence of an absolute temperature (0 K). • Current low temperature record is 0.15K in 2000
Electromagnetic Radiation • Light • The human eye can detect EM radiation with wavelengths in the range of 400-800 nm. • The longer the wavelength the smaller the energy of the radiation • The human eye is most sensitive to green light (500nm). • The human eye can see yellow light (550nm) the farthest.
Light • Light is made up of particles that contain energy but have no mass, “photons” • Light contains two characteristics: • Wavelength: distance between the same point in adjacent waves. • Symbolized l in m. Visible light in nm. • Frequency: number of waves passing a given point in a second. • Symbolized n in (1/s of Hz).
Water wave (ripple). Amplitude is not a characteristic of a wave. Amplitude: determines how bright or dimmed the light is
Photoelectric effect • In the experiment electrons are ejected from a metal sheet when light shines on it. • However, the electrons are ejected only if the light exceeds a certain “threshold” frequency. • Violet light, for example, will cause potassium to eject electrons, but no amount of red light (which has a lower frequency) will lead to ejected electrons.
Excitation and Relaxation • If an electron is excited it absorbs energy. (heat, current, light) • Then it MUST relax, as it relaxes it emits energy (heat, light). • This is how fireworks work. • They absorb energy from the powder explosion and then as electrons relax they emit light. • Different metals emit different colors.
Energy • Some reactions need energy to take place and some release energy. • In order to boil water we must provide energy. • When we burn methane we obtain energy. • A reaction that needs energy it is said to be ENDERGONIC. • A reaction that releases energy as it takes place it is said to be EXERGONIC.
Thermicity • Thermicity • Themicity is a bit more specific it deals with just heat and not energy in general. • Explains the direction of heat flow in a process. • A reaction that requires heat to proceed is Endothermic. • A process that releases energy as it takes place is Exothermic. • Changes in heat are measured by changes in Enthalpy (DH).
Enthalpy • In the case of exothermic reactions DH is (-) since the final state has less heat than the initial state. • Endothermic reactions have DH that is (+) since energy went into the system. • DH° is the change in heat at standard conditions. • Standard conditions are • 25°C, 1atm pressure, 1M, pure substances.
Why do we care about energy? • Every process involves energy. • Energy costs money. • Production of energy depletes natural resources. • I have a molecule… where is the energy found? • It can be said that energy is stored in the bonds holding the molecule together. • This implies that when bonds break they release energy… that is not the case. • It costs energy to break a bond.
Energy in Chemicals • Energy is released when bonds are formed. • While it may cost energy to break a bond, when a bond is formed after that we hope we obtain more energy in return. • The energy transferred in a chemical reaction depends on three factors: • Starting materials. • Products made. • Amounts of chemicals.
Spontaneity • Reactions that occur without the help of humans are said to be spontaneous. • Some reactions need help to get going but no help after that. • Combustion of gas requires us to provide the initial spark but after that continues without our help. • Reactions that must continuously need human intervention are non-spontaneous. • Fire will not continue if we do not add fuel.
Chemical Combustion • Most well-known chemical reaction • Fire! • Fuel source + oxygen + heat • Remove any one and fire goes out • Provides gaseous products • Carbon dioxide and water • Light • Heat
Kinetics • Why do we have to provide energy for spontaneous reactions at the beginning? • Thermodynamic is only one important part of the process. • Kinetics explains the reason for the initial energy. • Imagine you want to slide a heavy desk… • It takes more energy to start the moving than to continue the process. • The initial energy is the “activation energy” • Activation energy is the minimum energy required to start a reaction.
Kinetics • Kinetics studies how fast reactions take place, rate of reaction. • There are some factors that affect this rate: • Temperature • Pressure • Concentration • Catalyst
Temperature & Kinetics • As we said before Temperature is a direct representation of molecular energy. • High temperature means a high rate of energy transfer to the thermometer or vessel’s walls. • This in turns means that many collisions are taking place. • Which means that particles are traveling fast. • When molecules travel fast and collide they will have enough energy to overcome the “activation energy”
Pressure & Kinetics • Pressure is a measurement of force in a given area. • If the pressure increases that means there is a higher force being applied to the same area. • This in turn means the molecules are pushing harder. • Only way for molecules to push harder is to collide with the wall harder. • This means molecules are traveling faster… see previous slide to see how this affect reactions.
Concentration & Kinetics • As the number of molecules in a reaction vessel increases the chance of collisions increases. • If there are no cars on the road other than your own the chance you will run into a car is rather low… if there are 300 cars on the road the chance of running into one increases… • More collisions means a higher probability of having a collision with the correct amount of energy to over come Ea.
Catalysts and Kinetics • Catalysts are chemical substances that help speed up a reaction but are not used during the chemical process. • They are used in one of the steps but are salvaged in a later step. • Amount of catalyst at start and end must be the same. • Amount of catalyst during the reaction may be lower than at start. • Catalysts work by lowering the activation energy. • Lower activation energy means less energy required to start the actual reaction.
Observation • Space dirt enters atmosphere • Most burn up - meteoroids • Some hit the surface – meteorites • Primarily made of iron (Fe) • As they enter our atmosphere • Iron reacts with oxygen to form Fe2O3
The Chemistry • Iron is oxidized • Oxygen is reduced, • Fe is the reducing agent • Oxygen is the oxidizing agent • Described by two half reactions: • Fe Fe+3 + 3e- • ½ O2 + 2 e- O-2
Movement of Electrons • Redox Reactions • Reduction • Oxidation • Described by Half Reactions • Important in electrochemical cells
Electrochemical Cells • The Battery • Electrons are moved around • Half reactions permit batteries to operate • Required part • Good electrical conductors • Metals held by metallic bonds • Electrons move in these bonds
Electrochemical Cell • Solid metal pieces • Electrodes • Immersed in solution of the ion • Salt Bridge • Electricity flows in wire • Electrons move through wire in one direction • Direction of current based upon half reaction
Half Reactions • Cu (s) ------- Cu+2 + 2 electrons • Zn (s) ------- Zn+2 + 2 electrons • Reactions are reversible • Electrons formed in one half reaction • Are used in second half reaction • One reaction goes left to right • Other reaction goes right to left
Half Reactions • Which goes which way? • Determined by reduction potential • Cu has larger reduction potential • Cu+2 + 2 electrons Cu (reduction) • Zn Zn+2 + 2 electrons (oxidation) • Electrons flow until Cu ions are used up
Battery • Dry Cell • No aqueous (water) solution • Electrolyte paste • Mn+4 + 1 electron Mn+3 (reduction) • Zn Zn+2 + 2 electrons (oxidation) • Mn on graphite is the cathode • Zn is the anode
Car Battery • Pb – acid battery • Anode – porous Pb plates • In sulfuric acid • Cathode – PbO2 in sulfuric acid • PbSO4 (lead (II) sulfate) is product • Coats the Pb plates battery is discharged • Charging reverses flow of electrons • Charge – discharge can occur many times
Fuel Cells • Continuously renewed reactants • Reactants flow in, products flow out • Electricity is produced • Hydrogen Oxygen fuel cell • Anode: Hydrogen is oxidized to water • Cathode: Oxygen is reduced to hydroxide • Used in space program • System of choice for automobile
Rusting • Redox reaction • Fe Fe+2 (oxidation) • O2 HO- (reduction) • Product is then converted into Fe2O3 • Orange powdery material flakes off surface • Fe is exposed • Rusting continues • 1/5 of new Fe produced replaces rusted Fe
