Our lab on Saturday tested how rubber bands release and absorb energy. When talking about change, chemists refer to the system and its surroundings. The system is the reaction and everything that is under study. The surroundings are everything in the universe that are not under study. To understand the system, we must ask many questions. For example, does the rubber band produce or absorb heat? There is also more than one type of system. There is the open system, where energy can pass between the system and the surroundings. Second, there is the closed system, which is isolated from the surroundings and heat cannot enter or leave the system. When heat energy is released from a system, this is an exothermic change. When the system releases energy, its surroundings gain energy. A good example of this would be how the air surrounding a burning match is hot. This means the enthalpy of the reaction is negative. When the heat energy is absorbed from the system, this is referred to as an endothermic change. While the system absorbs energy, the surroundings give off the energy, meaning that the enthalpy of the reaction is positive. Energy can never be destroyed, which we learned from the law of the conservation of mass. When a reaction occurs the particles also change and they move around in the system. The change in disorder is called entropy. Order and entropy have an inverse relationship. As particles get more disorderly, the entropy increases, making entropy positive. As particles get more orderly, the entropy decreases, making entropy negative. Gas to Solid would be an example of a reaction in which order increased. Whether or not a reaction will occur depends on all of these factors. The relationship between all of these is delta H – temperature X delta S. If this equations yields a negative number, it is spontaneous. If delta H is negative and delta S is positive, the reaction will be spontaneous. If delta H is positive and delta S is negative then the reaction will be not be spontaneous. If delta H is negative and delta S is negative, the reaction will be depend on the temperature because this factor will allow you to figure out whether delta H or delta S is larger. If both factors are negative then the reaction will be spontaneous. If both factors are positive, the reaction will be spontaneous only if delta S is larger. Polymers are molecules made of strings of monomers attached to each other. Entropy and enthalpy are also important in understanding these. In this lab, we saw the rubber band contract and stretch. When the band is stretched, both enthalpy and entropy are lowered because when it gets stretched the molecules align themselves, which equals less disorder. When the band is contracted both the enthalpy and entropy are higher because the molecules are disordered. The entropy is favored in a contracted band and the enthalpy is favored in a stretched band. In nature the contacted state is preferred. Which, if you think about it makes sense because if left alone, a rubber band will be relaxed and it only stretches if someone purposely does that, it won’t happen spontaneously.
In this lab, we examined two important questions when it comes to reactions: Will it continue to occur? and How fast will it occur?
In this lab we learned about chemical reactions that continue to occur on their own after they start. If a reaction can continue to occur without someone or something affecting it in any way, it is spontaneous. We determined the factors that affect how quickly a reaction will occur. In a reaction that produces heat energy, controlling the spread of the reaction will allow you to control the speed at which heat is produced. To control chemical reactions, you would need to be able to predict when the reaction would happen spontantiouly and know what can be done to increase or decreases the reaction speed. This is where we discuss thermodynamics, which deals with the relations between heat and other forms of energy. Another theory is known as kinetics. There are only two factors that affect the spontaneity of a reaction: one is if the change gives off heat energy when it occurs or absorbs heat energy when it occurs. The second is if the change results in particles becoming more disordered or less disordered. The two rules that apply to these questions are that lowered energies are more stable than higher energies and that everything tends to become more disorganized over time. Heat energy changes endothermic and exothermic reactions. When chemical reactions occur, bonds in the reactants break and form new bonds which become the products. Breaking bonds requires energy input so breaking bonds is an endothermic change. Conversely, forming bonds releases energy so bond forming is an exothermic change. Activation energy is defined as the bump in the graph which measures the reactants energy and must always be positive. In this lab we tested different substances in water and observed the reactions. Mg and water reacted very slowly and only bubbled minimally. Mg and NaCl with water was much quicker and formed many more bubbles.
Electrolytes are substances that break into charged particles when dissolved in water and can conduct electricity. Electrolytes contain charged such as ions and they can move around freely. Non-electrolytes are molecules that do not form ions and do not conduct electricity. They cannot conduct electricity because they do not form charged particles. Every battery is made up of two half cells. One of them called anode which is responsible for oxidation, and the other half cell called cathode is where reduction takes place. This is why on a battery there is both a positive and a negative side. In today’s lab, we used zinc and copper to put in the battery. Zinc experienced oxidation, meaning it lost two electrons. The electrons of zinc then went to the reduction part of the reaction where copper is. Copper gained the two electrons and formed neutral copper metal with copper ions. The mass then increased because it was then attached to the copper.
In this lab we learned about light. The naked human eyes can only see a small range of wavelengths, known as the visible light spectrum. However based on certain light’s energy’s, we are unable to see certain wavelengths such as x-ray and infrared light. All waves travel at speed of light, which is (3.00*10^8) km per second. We can also figure out the frequency of the light from the equation c(3.00*10^8)=λ(wavelength in meters)*f(frequency). The longer the wavelength, the lower the frequency. Same goes with the energy of the light. The longer the wavelength, the lower the energy. Therefore, infrared light has the longest wavelength and the lowest frequency. Energy is identified with the equation E=h(Planck’s constant+6.63*10^-34)*f(frequency in s^-1).
In order for an atom to emit light, its electron starts off in the ground state and needs energy for it to move up to the excited state. If enough energy is added to the atom, it reaches the exited state. However, the atoms want to be in the ground state. In order to achieve this, they must release energy which we see in the form of light. This is known as fluorescence. However, some light using a more complicated process called phosphorescence. The electron first needs to have enough energy to reach the excited state and later fall to the middle state where it can emit light like what we saw on the glow tape. The different colors appear depending on how much energy you add in the first place, because then different amounts of energy will be released to get back to the ground state. The only colors with the frequencies to make phosphorescence are blue and violet which have the highest frequencies.
As we learned today in Chem class, a mole of any single atom or molecule has a mass equal to its atomic mass which is measured in grams. To find the atomic mass of an element, look on the period table. It’s the bottom number in the square with all the information on that element. Molar mass is also listed on the periodic table. To use oxygen as an example, it has a molar mass of 16 amu (unit used for molar mass). However, O2 (2 oxygen atoms bonded together) means that one mole of those oxygen molecules be 32g, twice as much as the single oxygen atom. Imagine the molecule was not made of the same element. In that case, you would add the masses together.
A single mole of pretty much any gas at STP (standard temperature and pressure) has the same volume, which is 22.4 L. If temperature decreases, it will take up less space, and if it increases, the gas will take up more space. The temperature and the volume of the gas have a direct relationship.
Stoichiometry is referred to by chemists as the calculation which involves figuring out the number of moles of a chemical in a reaction based on the number of moles of the other chemicals in the chemical equation. The coefficients in the equations act as the proportions with the number of moles of the reactants and products. A ratio is thus created that will always stay fixed no matter what. So if you put in 4 times as much of the reactants, you will get 4 times as much of the products. Stoichiometry becomes important to chemists because with this method conversions from mass to moles or vice versa are much easier. Another part of this method is what chemists call dimensional analysis, which just makes sure everything is expressed in terms of the same quantifiable measurements. Often times in a reaction, chemists will find that not all of the reactants show up in the products, which is just a result of many different ways a small parts of the product can be lost in a reaction. The amount that is missing is called the percent yield of a reaction.
A basic concept that was demonstrated in this lab was that matter can undergo changes. As we all know, matter is usually found in 1 of 3 states: solid, liquid, or gas. However, there is also variation in how/where matter is stored. Matter was very much involved and the basic concept of the fact that matter can undergo a change. Despite the variables in the condition of matter, the amount of energy that can be found in the universe is a constant. This means that matter can never be destroyed. If energy is added or removed in any way, the matter does not disappear, but rather goes someplace else.
The most basic example of this would be water. If you have a pot of water boiling on the stove (adding heat energy) for long enough, you will see that the water will start to boil, meaning it’s heating up. If you continue to keep it on high heat, you will begin to see that water disappear. However, it has not vanished from existence, but it now simply exists in a different state, a gaseous one. This happens for a reason. In adding heat energy, you are making the molecules move faster creating more friction and movement, hence an increase in temperature. Conversely if you freeze something, the molecules are slowing down towards the point of barely moving at all, and very little heat is being produced. This movement will also result in the breaking/forming of bonds, which is the real reason behind the phase change. If you have a glass of water with nothing being done to it, the bonds holding together the molecules are what is making sure that the liquid water stays liquid water. When you add/remove heat energy the bonds are broken, resulting in a different phase. These bonds also hold the water together in its shape. Although water doesn’t really have a shape, it has a more defined shape than water vapor.
Chemists refer to the energy it takes to overcome weaker intermolecular bonds as the input of energy. Which makes sense, seeing as that is the amount of energy you would need to input for the water (or any substance) to undergo any type of change. Another way you could create change in a molecule is by adding a different molecule to it. Perhaps there would be no reaction, like adding salt to water, but some molecules have explosive results when combined. In an equation, one side would be labeled the reactant, and the result would be the product. If heat energy is absorbed by the chemical reaction, it is endothermic. If it is released, it is exothermic. Endothermic is when the energy change from reactant to product is positive, whereas exothermic is negative.
Another aspect to molecules is how organized the atoms inside them are. Gases have their molecules spread out widely, whereas solids have theres packed in tightly, which gives solids there definite shape. Reactions that create gases from solid and liquids have an increase in disorder. The concept of disorder is referred to as entropy, written as “S”. Put simply, entropy is the measure of the organization of the atoms in a molecule. This too can be affected when energy is added/removed from a molecule in a chemical reaction.
Glass is a widely used material. From windows to plates to vases, glass has many uses in our daily lives. While glass in windows is clear, many glass is colored or stained. Glass is made by heating a mixture of silica, sodium carbonate, and calcium carbonate until it liquifies and eventually cools. Color is often created when impurities in the glass. To intentionally change the color of glass, metal oxides are added. For example, to make blue glass, we added cobalt oxide and copper oxide. The compounds absorb different colored light from the white light passing through and reflect the colors we see. In ceramics, colored glass is a result of is glazes made from metallic oxides such as iron, copper, cobalt, manganese, chromium and nickel. The colors are dependent on firing temperature, concentrations, and firing conditions. Glazes are suspensions of clay and minerals in water, and colors can be altered by the “recipes” used by artists to get the desired color.
In metals, atoms are held together by metallic bonds. Metallic bonds are formed by the atoms in the metal sharing valence electrons. To represent this bond, chemists use the electron-sea model. The large circles in this model are the cations, which are positively charged metal ions, and the smaller circles are electrons. Many metals have the property malleability, which means that that they can be shaped and are softer, but are not very strong. This is because in metallic bonds, electrons can freely move around the atom, so when the metal is hammered, for example, the electrons can move past each other. An alloy is a substance that has the properties of metal, but contains 2 or more elements. Even if one of these elements is a metal, that metal’s properties change when other elements are introduced. In this lab we heated and cooled steel. The annealed steel contains less carbon atoms between the iron atoms. This results in the steel becoming more pliable. The steel was then cooled quickly, the carbon atoms were locked into the crystalline structure. The electrons then had a difficult time sliding past each other. This resulted in hardened steel. The hardened steel was extremely hard and brittle, and was very easily broken. From there the steel was gently heated, allowing some of the electrons to move more easily. This resulted tempered steel. The tempered steel was a combination of the properties of annealed and hardened. It was hard, but also quite malleable.
Dyes are known as organic molecules that bond directly to a textile. This bond results in a color being soaked up by the material and giving our clothes and other materials color. In this lab we used onion skins, tea, and carrot greens to produce dyes. The chromophore is the part of the dye molecule that produces the colors we see. The mordant helps the dye to stick and not fade from the textile. We placed strings in the mordants before we dipped them in the dyes. When the dye is first introduced to the material, it forms an insoluble complex salt (also known as a lake) with the metal ions in the complex salt. Years ago most dyes were produced using natural ingredients, but now nearly all the dyes used to color our clothes and other materials are produced synthetically.
In this lab, the solid precipitates, which were the murky liquids, are insoluble compounds. When specific cations and anions are mixed, water-insoluble ionic compounds are formed, which we witnessed in the lab. When ions in different aqueous solutions are combined, an insoluble solid or precipitate is formed. The precipitate is an ionic compound that is created because specific ions are so strongly attracted to one another that they are removed from the liquid solutions during the chemical reaction. A double-replacement reaction is a type of precipitation reaction where a precipitate forms when one of the products is insoluble. Certain ions do not precipitate in reactions. These ions are known as spectator ions. Once they are taken out of the atomic equation, they form what is known as a net ionic equation.