When chemists are speaking of a change, they refer to the system and the surroundings. The system and the surroundings make up the universe. When considering change, you think about what the system is doing. For example, you ask, “Is the enthalpy of the system increasing or decreasing?” etc. If the system is an open system, energy is able to pass between the system and the surroundings. However, if the system is a closed system is it isolated from the surroundings and heat can neither enter nor leave.
When heat energy is release from the system, the surroundings gain energy and the system loses energy. When a system loses energy there is a loss in the amount of enthalpy the system contains. The change in enthalpy is negative. On the other hand, when heat energy is absorbed by the system, the surroundings must give that heat energy to the system because energy must come from somewhere. The law of conservation of energy states that energy cannot be created or destroyed. It is merely transferred from one place to another. When a system gains energy the change in enthalpy is positive.
When a change takes place, the particles inside the system are rearranged. The rearrangement of these particles are either more or less disorganized. If the particles become more disorganized the entropy has increased but if the particles become more organized the entropy has decreased.
The changes in enthalpy and entropy are the two factors that affect the spontaneity of a change. Exothermic changes drive a process toward spontaneity because the products have lower energy than the reactants from which they are formed. Lower energy states are more favorable. Also an increase in entropy drives a process toward spontaneity because nature tends to become more disorganized over time. There is a third factor that affects spontaneity, that is the temperature at which the process occurs. There is something called Gibbs free energy which conveniently combines the three factors that affect spontaneity. Gibbs free energy can be used to tell whether a change will happen spontaneously. If the change in Gibbs free energy is negative the change will be spontaneous. If it is positive the change will not happen spontaneously. The equation for Gibbs free energy is ΔG=ΔH−TΔS.
Polymers are molecules made up of a long chain of repeating monomers joined together. Materials made of polymers exhibit unusual behaviors. When the rubber band is stretched the entropy is low because the molecules are lined up and the enthalpy is also low because the aligned molecules are able to experience attractions to each other. When the rubber band is contracted the entropy is high because the molecules are tangled and the enthalpy is high because the molecules are farther apart from other molecules. The rubber band contracted more when we heated it up and stretched when it cooled.
Polymers are probably the most versatile substance that technology has produced. Scientists have learned how to control the properties of polymers like varying the composition of the monomers, the chemical types of monomers and the degree of cross-linking between polymer strands. A field in which polymers are used is medicine. There is a polymer called a thermoplastic polymer which is used to stitch the working parts of the mechanical heart to the heart itself. Thermoplastics are usually polyesters or polymers where the repeating monomer forms an ester linkage between an organic acid and an alcohol. Polymers are used a lot in the biomedical field. For example, they are used to provide skin grafts for the treatment of serious burns and they are used in prosthetic limb technology. Polymers can be so sturdy they are used to construct cars and buildings. Also they can be so elastic that they are used to make trampolines.
The two questions chemists ask about any chemical reaction they plan to use is, “Will it continue to occur?” and, “How fast will it occur?” In our lab we explored a chemical reaction that continues to occur on its own. This reaction is called spontaneous. Spontaneous reactions are changes that, once begun, continue without an input of energy. Chemists need to be able to predict when reactions can occur spontaneously. There are two sets of theories, thermodynamics which answer the question, “Can a reaction occur spontaneously?” and kinetics which answer the question, “How fast can a reaction occur?” Thermodynamics is the study of how heat and other forms of energy are involved in chemical and physical reactions. Kinetics is the study of reaction rates and how they can be affected by variables such as temperature.
To determine if a change can occur spontaneously there are two factors. The first factor that affects spontaneity is if the change gives off heat when it occurs or absorbs heat energy. The second factor that affects spontaneity is if the change results in particles becoming disordered or less disordered. Two rules apply to these two questions. Lower energies are more stable than higher energies therefore energy changes in chemical reactions tend to occur in ways that allow the substances to end up with lower energy. Changes that release energy tend to be favored. Also everything tens to become more disorganized therefore the changes in which particles become more spread out are favored.
If a change both releases energy and becomes more disordered the change is definitely spontaneous. Vice versa, if the change absorbs energy and becomes more ordered the change is definitely not spontaneous.
The mains points on chemical reactions are:
- When chemical reactions occur, bonds in the reactants break and new bonds form to make products.
- Breaking bonds needs energy input therefore bond-breaking is an endothermic change.
- Forming bonds releases energy therefore bond-forming is an exothermic change.
- The overall change can be endothermic or exothermic depending on whether the total energy input or the total energy output is greater.
Chemists call en energy change that occurs at a constant pressure “change in enthalpy” and the symbol is delta H. A negative delta H means the reaction was exothermic and a positive delta H means the reactions was endothermic. To represent the enthalpy change you can draw an energy diagram. This diagram shows the progress of the reaction from reactants to products on the horizontal axis and shows the change in potential energy that occurs as substances progress from reactants to products on the vertical axis.
The relative locations of the reactant energy and product energy indicate whether the overall enthalpy change is endothermic or exothermic. For an exothermic reaction, if the starting point (reactants) is higher than the ending point (products), the system releases heat energy therefore the delta H is negative. For an endothermic reaction, if the energy of the reactants is lower than that of the products, the system absorbs heat energy therefore the delta H is positive.
Bonds must always be broken in the reactants in order to form new bonds in the products. Some initial amount of energy must be applied to the system for the reaction to begin. This is represented by an initial “bump” in the curve to get from reactants to products. The height of this bump is called the activation energy. The intermediate state that is a combination of reactant and product atoms is called the activation barrier.
To speed up a reaction you can use a catalyst. A catalyst is a substance that speeds up a chemical reaction without being used up itself. It works by providing a lower energy pathway which gives the starting materials enough energy to get over the energy barrier.
In this lab we worked with conductive and non conductive solutions. We used a conductivity tester to help us determine if a solution conducted electricity. A substance that dissolves in water to make solutions that conduct electricity are called electrolytes. The solutions we tested were made of substances that were dissolved in distilled water. Only the substances that broke into charged particles when dissolved were electrolytes.
In order for a substance to conduct electricity there must be charged particles present and they must be able to move around. Some compounds dissolve in water to form charged particles called ions. These compounds are normally made of a positively charged metal ion and a negatively charged ion to balance the charge. For example, table salt (NaCl); consists of sodium ions (Na^+) with charges of +1 and chloride ions (Cl^-) with charges of -1. When NaCl crystals dissolve in water the sodium and chloride ions separate from the crystal and become surrounded by water. Surrounded by water, the charged ions are able to move around therefore NaCl is able to conduct electricity.
Molecular compounds do no break up into ions when they dissolve. This is because molecules do not from charged particles in a solution therefore they do not conduct electricity. Molecules that do not form ions in a solution are non-electrolytes.
A battery is made up of two half cells. The half cell where oxidation takes place is called the anode (the negative battery electrode). The other half cell where reduction takes place is called the cathode (the positive battery electrode).
While the battery is operating the zinc metal electrode is undergoing a reaction. The zinc ions are entering the zinc nitrate solution while the electrons are traveling through the wire. The electrons reach the copper metal electrode where reduction occurs. When electrons reach the copper cathode they enter into reduction. Copper ions from the solution combine with electrons to make neutral copper metal atoms. The copper metal electrode then increases in mass because copper metal is attaching to it. The solution was originally prepared by dissolving Cu(NO3)2 crystals in water. When the compound dissolves they are both surrounded by water molecules. As the positively charged copper ions are used up the negatively charged nitrate ions are being pushed out. They are forced to go through the salt bridge. They enter the zinc half cell where zinc ions are being created at the anode. Then there is a need for more negative ions to balance the increasing positive charge on zinc ions.
Electricity only runs spontaneously in one direction in a battery due to the relative activities of the two metal electrodes used in the half reactions. In our lab the Zn metal atoms were converting to Zn2+ ions and from Cu2+ ions to Cu metal atoms. In the metal activity series zinc metal is more reactive than copper metal therefor zinc will react spontaneously with Cu2+ and Cu will not react spontaneously with Zn2+.
In this lab we looked at the colours that make up visible light using LED’s. Light is a form of energy known as electromagnetic radiation. Electromagnetic radiation is the energy that travels through space as waves. For example, X-rays, ultraviolet rays (UV), microwaves, radio waves and visible light. The electromagnetic spectrum is a list of electromagnetic radiation in order of wavelength.
Light is characterized by its wavelength and its energy. The wavelength of light is the distance between two corresponding points of a wave. It could be from crest to crest, or from trough to trough. The units for wavelength is nanometers. (a nanometer = 1*10^-9 meters)
All waves of light travel at the same speed in a vacuum. This speed is called the speed of light. In a vacuum the speed of light is 3.00*10^8 m/s. There is also something called frequency. Frequency is the number of cycles of a wave that occur in a second. The frequency and wavelength of light are related:
- c = λ * f
- c = speed of light = 3.00*10^8 m/s
- f = frequency in waves per second
- λ = wavelength in meters
Waves with longer wavelengths have relatively low frequencies. Waves with shorter wavelengths have relatively high frequencies. (as wavelength increases, frequency decreases) The energy of light is related to the frequency and wavelength of the light.
- E = h*f
- E = energy in Joules
- h = Planck’s constant (6.63*10^-34J)
- f = frequency in s^-1
Light that has a long wavelength has a less energy than light that has a short wavelength. The wavelengths in visible light range from 700nm to 400nm. When the wavelength becomes shorter, the energy increases. The wavelength of light determines if the light has enough energy to interact with the electrons in an atom.
The valence electrons of the atom take part in the chemical reactions. The electrons absorb energy going from ground state to an excited state. The excited state means the electrons have been raised to a higher energy sub level. Eventually the electrons will fall from the excited state back down, given up energy. When light is emitted from atoms or molecules it is called fluorescence. This process is when the electron absorbs energy, becomes excited then quickly releases energy as visible light when coming back to the ground state. When the glow-in-the-dark tape gave off light, this process was called phosphorescence. With phosphorescence the electron is at the excited state but temporarily gets stuck there in this intermediate state depending on the conditions like the temperature. This results in a delayed emission of light.