1	Core Energetics I: Enthalpy
2	Learning Objectives Concepts: Kinetic energy, potential energy calorie, Calorie, Joule Kinetic theory, kelvin, Heat, source, sink, state, universe, system, surroundings, boundary isolated system, closed system, open system, Laws of Thermodynamics, First Law of Thermodynamics, conservation of energy, equilibrium, thermal equilibrium Skills: Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in kelvins. Express the first law of thermodynamics in terms of thermal sources and sinks.
3	Energy: Potential Energy Potential energy is the energy an object possesses by virtue of its position relative to another object. An object above the surface of the earth is said to possess potential energy because of it position relative to the earth. Because of its position, it experiences gravitational force of attraction from the earth and therefore it possesses gravitational potential energy. (On the surface however it is said to possess zero potential energy).
4	Energy: Potential Energy Opposite poles of two magnets held some distance apart possess potential energy due to the force of attraction between them. If they are near enough and let go, they will move toward each other and then will possess zero potential energy relative to each other when attached to each other. So, potential energy exists between two or more objects where a force is operating between or amongst them. Charged particles also exert force of attraction (and repulsion) to each other and therefore possess potential energy as a consequence. Electrons in an atom possess potential energy, for example, because of the electrostatic force of attraction between it and the nucleus. (At an infinite distance from the nucleus or any positively charged particle, the potential energy of the electron is said to be zero).
5	Energy: Electrostatic Potential Energy Atoms or molecules or ions within a substance also possess electrostatic potential energy again because of the electrostatic force of attraction (bonds) between them. (We will have more to say about this when covering bonding, see Core Bonding: Intermolecular Forces.) Electrostatic potential energy, E_d, between two oppositely charged particles, Q₁ and Q₂, a distance d apart is given by the following equation:
6	Energy: Electrostatic Potential Energy If ionic, it’s again the attractions between the electrons and the nuclei in the ions and the attraction between the oppositely charged ions in the crystalline structure. Gravitational potential energy relative to each other in the cases of electrons, protons and atoms or molecules or ions is negligible by virtue of them being so small! For a given substance such as iodine, liquid iodine molecules possesses more potential energy than molecules in solid iodine. In other words, liquid iodine contains more potential energy than solid iodine. Gaseous iodine molecules in general possesses more potential energy than molecules in liquid iodine. In other words, gaseous iodine contains more potential energy than liquid. The reason is that moving from solid to liquid to gas, the molecules get farther apart and therefore the potential energy increase.
7	Energy: Electrostatic Potential Energy That is also why the density of solid iodine is the highest followed by that of liquid and then gaseous iodine. In general then gaseous state of a substance contains the most potential energy, followed by the liquid state, and then the solid state. And also that for a substance, the gaseous state of a substance is the least dense, followed by the liquid state and then the solid state, with the one single exception: that of water and ice!
8	Nature of Energy & Heat Force is a push or pull on an object. Work is the product of force applied to an object over a distance:
9	Energy & Kinetic Theory Caloric theory however was not able to explain every effect of heat, such as that produced by friction. In 1798 The American Benjamin Thompson observed that if you rubbed to pieces of metal together you could produced heat for as long as the pieces were being rubbed. If heat was a fluid found between atoms in the substance, then the metal pieces should have produced less and less heat over time as the fluid got lost. Almost half-a-century later James Joule demonstrated that mechanical energy could be converted into an exactly equivalent amount of heat suggesting that heat is a form of energy rather than an actual fluid. In the nineteenth century, scientists suggested that heat results from the motion of particles of matter, called the kinetic theory (about which we will have more to say when covering the topic Kinetics).
10	Energy: Kinetic theory & Kinetic Energy Briefly, kinetic theory states that all particles possess motion. Particles in solids possess vibrational motion, in liquid and gas they possess vibrational, rotational and translational motion. Thus, all fundamental particles (atoms or molecules or ions) possess kinetic energy. Kinetic energy is the energy of motion:
11	Energy: Kinetic theory & Kinetic Energy The magnitude of the motion (speed) of particles depends on temperature and mass of the particles of the substance. For a given substance, the average kinetic energy of the fundamental particles the substance is made up of is directly proportional to the temperature of the substance (in Kelvin).
12	Energy Transfer: Heat Particles have no motion at 0 Kelvin. So therefore, since heat is the transfer of energy between two objects, we should be able to explain heat transfers in terms of kinetic theory. An object hotter then the ambient temperature such as a kettle of water just boiled for some tea transfers heat energy to the surrounding. The steam molecules and the water molecules in the kettle, and the atoms in the kettle have greater kinetic and potential energy than the air particles around it. The atoms in the kettle are vibrating at great speeds, much greater than the air molecules around it. When air molecules collide against the vibrating atoms in the kettle, these atoms transfer their motion and thus their energy to the air particles just as a fast moving snooker ball colliding into a slow moving one will transfer its energy to it and cause it to move faster. As the atoms in the kettle thus lose their energy, the water molecules and steam molecules inside the kettle transfer their energy to the kettle atoms and they themselves lose their kinetic and/or potential energies.
13	Energy Transfer: Heat This process continues on until the excess kinetic energy possessed by the atoms in the kettle and water molecules have been transferred to the air particles Until the kinetic energies are equal, in other words, until the temperature of the kettle and the water has dropped to room temperature. The sum total of kinetic and chemical potential energy in a substance is called the total energy of the substance. And from here onwards, when mention is made of the energy of a substance, total energy (sum of kinetic and chemical potential energy) is to be understood. Additionally, from here onwards, heat, energy and enthalpy will often be used interchangeably. For the purposes of IB Diploma Chemistry, the three terms are equivalent.
14	Enthalpy change terms for physical changes For a given substance, total energy (kinetic and potential) of liquid is greater than that of solid. In other words, liquid bromine at 25°C has more kinetic and potential energy than solide bromine at -7°C (freezing point of bromine). Not only are the molecules in iquid bromine moving faster on average, they are also farther apart. At -7°C, however, liquid and solid bromine have the same kinetic energy, but liquid bromine has more potential energy. At -7°C the average motion of the particles is equal but the average distance between molecules in liquid is greater than in the solid. Similarly, total energy of gas is greater than that of liquid. Steam at 110° has more kinetic and potential energy than water at 25°C. At 100°C however, steam and water possess equal kinetic energy, but steam has more potential energy. And again, that’s why steam at 100°C scalds you more badly than boiling water at the same temperature.
15	Heating curve
16	Heating curve When a solid at below it’s melting point is heated, heat energy is converted to kinetic energy as the particles in the solid vibrate more more strongly. But, when sufficient heat energy has been added to arrive at (t₁) the melting point of the solid, further heat energy added converts into potential energy as the particles go from vibrating to flowing, the solid structure breaks down and it converts into liquid. Thus, heat energy at the melting point, converts into potential energy in the liquid until all the solid has melted (by t₂). Continual heating increases the kinetic energy of the particles in the liquid, until the boiling point of the liquid is reached (at t₃). From t₃ to t₄ the liquid starts evaporating. That is, the heat energy added at constant temperature here changes into potential energy of the particles allowing them to leave the surface of the liquid and bringing them farther apart. Continual heating of course increases the kinetic energy of the gaseous particles.
17	Phase Change & Energy Change That is why melting a solid requires energy, as does vaporizing a liquid. Again that’s why condensation of a gas requires cooling as does freezing of liquid. In other words, the following sequence is endothermic: solid ® liquid ® gas And the following sequence is exothermic: gas ® liquid ® solid All phase changes are possible under the right conditions (e.g., water sublimes when snow disappears without forming puddles).
18	Energy transfer Petrol for example obviously contains energy as you all know as it is used to drive vehicles for instance. Depending on the temperature of the petrol, its molecules will contain more or less kinetic energy, but the potential energy is what is converted into heat energy in the engine of a car to drive the piston and thus the motor vehicle. The component of the energy of petrol that drives the motor vehicle, the energy stored in the arrangements of the electrons and nuclei within the molecules that makes up petrol is often referred to as chemical potential energy or simply chemical energy. Potential energy can be converted into other forms of energy, such as kinetic energy. Petrol burnt to drive a motor vehicle undergoes conversion from chemical to heat to mechanical to kinetic energy. A bicyclist riding down a hill coverts gravitational potential energy into kinetic energy.
19	Enthalpy Total energy of a substance in chemistry is referred to as the enthalpy of the substance. (You will encounter a more precise and correct definition of enthalpy later on.) SI Unit for energy is the joule, J (kg m/s). We sometimes use the calorie instead of the joule: 1 cal = 4.184 J (exactly) However, a nutritional Calorie (Cal) is different from the unit cal: 1 Cal = 1000 cal = 1 kcal In this topic you will study the role of energy in chemical reactions.
20	Thermochemistry The examination of the relationships between chemical reactions and energy changes is an aspect of chemistry called thermochemistry. (or energetics) Thermochemistry looks at performance of work (which has a slightly different meaning in science), expenditure or consumption of energy by chemicals, and flow of heat between different chemical species. For example, work must be performed to compress a gas or to separate particles of opposite electrical charge (such as an electron and a nucleus). Chemical processes such as the burning of gasoline release energy in the form of heat. Chemical systems perform work and transfer heat, which essentially boils down to inter-conversion and/or changes in kinetic and potential energy of the substance(s) involved. The broader topic of the study of energy and its transformation is known as thermodynamics, of which thermochemistry is only a small sub-topic.
21	First Law of Thermodynamics, & Thermal Source & Sink In order to study work and energy transfers, you must have an understanding of the first Law of Thermodynamics which states that energy is conserved. That is, the amount of energy the universe contains is constant. Energy changes on earth: Biggest input: Sun. Biggest output: infra-red radiation to space. If we consider the sun to be the thermal source, then the earth is the sink. The two are almost equal, so the net energy content of earth is constant. There is some amount of photosynthetic capture of solar energy by the earth. And an equal outward radiation from radioactive energy from within the earth amounting to less than 1 per cent. What in everyday language we complain that we are running out of energy, we are referring to the spreading out of energy into less and less useful form. Mention is made of the first law here because study of all changes shows that every change is characterized by the spreading out of energy (exothermic reaction) and/or matter (endothermic reaction).
22	System and Surrounding In the study of thermochemistry or energetics (and also chemical equilibrium), the region or particular quantity of matter of interest—the portion we single out for study—must first be defined. If transfer of energy is to be considered, one must be able to state where it is flowing from (what the source is) and what it is flowing into (what the sink is), for example. Since energy is conserved, any observed change is just the flow of energy from one to another. With chemical systems, the energy flow is between a system and the surrounding. The region that is singled out for study is defined as the system and everything else as the surrounding.
23	Source or Sink: Exothermic vs. Endothermic Depending the type of reaction, one of the two could be classified as a thermal source and the other a sink. In other words, whether the reaction is exothermic or endothermic. In general, in the case of an exothermic reaction, one that is characterized by the spreading out of energy mostly, the system is the source and the surrounding the sink. In other words, exothermic reactions give off energy which dissipates into the surrounding. In the case of an endothermic reaction, one that is characterized by the spreading out of matter mostly, the surrounding is the source and the surrounding the sink. In other words, endothermic reactions in general are accompanied by an increase in the number of substances. It must however be noted that there are three kinds of systems.
24	Three kinds of systems: 1. Open System 1. The set below of 20 g KBr in a crucible under three different physical conditions in which no reaction is taking place are all examples of open systems.
25	2. Closed system 2. The set of stoichiometric amount of Zn and dil. H₂SO₄ to the right in which a reaction could take place (a), is taking place (b), has taken place (c) (in the next slide) are examples of closed systems.
26	2. Closed system A closed system is capable of exchanging only energy with its surrounding. The containers are closed and therefore no matter can enter or leave them. However they are not insulated and therefore they are able to exchange energy. If the reaction produces energy (exothermic), then the system would lose energy to the surrounding (which is the case here). If the reaction absorbs or requires energy (endothermic), then the system would take energy from the surrounding.
27	3. Isolated System: Bomb Calorimeter Since a bomb calorimeter is completely covered and insulated, it can exchanges neither energy nor matter with the surrounding, and that is the characteristic of an isolated system. In some instances, it is important to specify whether the container (e.g. the flask and the syringe in the preceding example) is to be considered part of the system or part of the surrounding.
28	Summary Enthalpy of a substance is the sum total of all the kinetic and potential energy possessed by the different components of the substance. Every single chemical change is characterized by a spreading out of energy and/or matter. When energy spreads out it spreads from the system, the source, to the surrounding, the sink. When matter is spread out, it is brought about by the flow of energy from the surrounding into the system. Since, according to the first law of thermodynamics, energy is conserved there is no net change in the energy of the universe (which we will have more to say about later). A system that doesn’t exchange energy nor matter with its surrounding is referred to as an isolated system. One that exchanges both: open system, while one that exchanges only energy: closed system.
29	Practice Questions: Multiple Choice 1. For a given substance, such as water, A) the liquid state has more energy than either the gaseous or solid state B) the solid state has more energy than either the gaseous or liquid state C) the gaseous state has more energy than either the solid or liquid state D) the liquid state has the least energy of the three states E) the solid state does not have any energy 2. An object moving at a high speed always has A) more potential energy than one at rest B) less potential energy than one at rest C) more kinetic energy than one at rest D) less kinetic energy than one at rest E) equal potential and kinetic energy as one at rest
30	Practice Questions: Multiple Choice 3. Water at 70°C has ________________ one at 90°C. A) less potential energy than B) more kinetic energy than C) more potential energy than D) less total energy than E) the same total energy as 4. Ice at 0°C and ice cold water also at 0°C have A) particles that have different type of motion B) particles that on average have same amount of potential energy C) particles that are absolutely still D) particles that are of different sizes E) particles that are different
31	Practice Questions: Multiple Choice 5. Steam at 100°C A) will have the same potential energy as water at water at 100°C B) will scald more badly than water at 100°C C) will need heat to condense to water D) will consist of regularly arranged water molecules E) will consist of molecules moving very very slowly 6. Particles in alcohol at 50°C on average ___________ water at 50°C A) be moving at the same speed as those in B) will have the same kinetic energy as those in C) will have the same potential energy as those in D) will have fewer freedom of movement than those in E) will have greater freedom of movement than those in
32	Practice Questions 1. State in your own words the first law of Thermodynamics. 2. In your own words, define kinetic energy and give an example. 3. In your own words, define potential energy and give an example.
33	Practice Questions 4. Give an example of a process, chemical or physical, where there is conversion between kinetic and potential energy. Describe the process indicating how the conversion takes place. 5. Define the following: Open system: Closed system: Isolated system:
34	Practice Questions 6. Identify as many different energy transfer/transformation taking place in the following set up.