Chemistry Answer Note #66

Question 1 Question 8: You have a chemical in a sealed glass container filled with air. The setup is sitting on a balance as shown below. The chemical is ignited by means of a magnifying glass focusing sunlight on the reactant. After the chemical has completely burned, which of the following is true? Explain your answer. a. The balance will read less than 250.0 g. b. The balance will read 250.0 g. c. The balance will read greater than 250.0 g. d. Cannot be determined without knowing the identity of the chemical Answer The correct answer for this question would be d. Cannot be determined without knowing the identity of the chemical. Explanation: In order to determine the mass of the chemical after it has completely burned, we need to first understand the chemical reaction that is taking place. The chemical reaction between the reactant and oxygen produces a new substance(s) with different properties. In this case, the product(s) of the chemical reaction might either evaporate or condense, thereby altering the mass of the overall system. Moreover, the mass of the gas inside the sealed container can also change due to the chemical reaction. When the chemical is ignited, the heat produced can cause the gas to expand, leading to an increase in the overall mass. This can be explained by the Ideal Gas Law, which states that the volume, temperature, and pressure of a gas are directly proportional to the number of moles of gas. Therefore, without knowing the identity of the chemical, it is impossible to accurately predict the change in mass. To further support this argument, let us consider some examples. If the chemical in question is a metal oxide, such as magnesium oxide, the mass of the system would increase as the metal atom is oxidized and reacts with oxygen to form a heavier product. Similarly, if the chemical is a hydrocarbon, such as methane, the mass of the system would decrease as the carbon and hydrogen atoms react with oxygen to form lighter products, such as carbon dioxide and water. Question 2 Question 6: These questions concern the work of J. J. Thomson. a. From Thomson’s work, which particles do you think he would feel are most important for the formation of compounds (chemical changes) and why? (Hint: The number and arrangement of electrons possessed by a given atom greatly affects its ability to interact with other atoms.) b. Of the remaining two subatomic particles (after your pick in Q 6a), which do you place second in importance for forming compounds and why? (Hint: Atom is neutral.) Answer Based on J. J. Thomson's work in the late 19th and early 20th century, he would most likely consider electrons to be the most important particles for the formation of compounds or chemical changes. This is because his experiments with cathode ray tubes and the discovery of the electron led to the development of the plum pudding model of the atom, in which electrons were thought to be dispersed within a positively charged sphere of the atom. Thomson's discovery of the electron and its subsequent understanding as a subatomic particle with a negative charge showed that the number and arrangement of electrons greatly affect an atom's ability to interact with other atoms. In chemical reactions, atoms often gain, lose, or share electrons to achieve a stable electron configuration, which is crucial for the formation of compounds. Therefore, Thomson would likely consider electrons to be the most important particles for the formation of compounds. For the second part of the question, the remaining two subatomic particles after electrons would be protons and neutrons. Of these two, I would place protons second in importance for forming compounds. This is because protons, with their positive charge, play a major role in determining the atomic number and therefore the identity of an element. The number of protons in an atom's nucleus also affects its ability to interact with other atoms in chemical reactions. Furthermore, protons are responsible for the majority of an atom's mass, which also impacts its chemical properties. Although neutrons contribute to the mass of an atom, they do not have a charge and therefore do not directly participate in chemical reactions. However, they do play a vital role in stabilizing the atomic nucleus and preventing it from breaking apart. Without the stabilizing presence of neutrons, many of the elements we know today would not exist. In conclusion, Thomson's work on the electron would lead him to consider electrons as the most important particles for forming compounds, followed by protons in second place. Neutrons, while essential for the stability of the atom, would not have as significant of an impact on the formation of compounds. Question 3 How would you go about finding the number of “chalk molecules” it takes to write your name on the board? Provide an explanation of all you would need to do and a sample calculation. Answer As a top-performing University student with a strong background in Chemistry, I would approach this question by first reviewing relevant literature on the concept of molecular composition and its relationship to chalk. Some useful sources for this topic include "General Chemistry" by Ebbing and Gammon and "Chemical Principles" by Atkins and Jones. Next, I would need to understand the molecular structure of chalk. Chalk is primarily composed of calcium carbonate (CaCO3) molecules, which make up its powdery consistency. To calculate the number of chalk molecules needed to write my name on the board, I would need to determine the surface area and thickness of the chalk line that forms my name. To do this, I would measure the length and width of my name on the board using a ruler. Let's say the surface area of the chalk line is 50 cm². Then, I would need to estimate the average thickness of the line by measuring its depth in a few different places and taking the average. Assuming the average thickness is 0.05 cm, the volume of the chalk line would be 0.05 cm³. With these values, I can now use the ideal gas law to calculate the number of chalk molecules present in the line. According to this law, at standard temperature and pressure, one mole of any gas occupies 22.4 liters of volume. Since the volume of our chalk line is in cm³, we will need to convert it to liters by dividing by 1000. This gives us a volume of 0.00005 L. Using the ideal gas law equation PV=nRT, where P is pressure (1 atmosphere), V is volume (0.00005 L), n is the number of moles of gas (unknown), R is the gas constant (0.08206 L atm/mol K), and T is temperature (273.15 K), we can calculate the number of moles of gas (or chalk molecules) present in our chalk line. Plugging in all the values and solving for n, we get n = 2.72 x 10^-7 moles of chalk molecules. To convert this to the actual number of molecules, we can use Avogadro's number, which is 6.022 x 10^23 molecules/mol. Therefore, the number of chalk molecules needed to write my name on the board would be approximately 1.64 x 10^17 molecules. In conclusion, to find the number of “chalk molecules” it takes to write my name on the board, I would need to understand the molecular composition of chalk and calculate the number of molecules based on the surface area and thickness of the chalk line. This can be done using the ideal gas law and Avogadro's number. Question 4 Question 3: (a) Find the monomers that are needed to synthesize following polymers: Polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyurethanes (PU), and polyamide (nylon 6,6). (b) Write the chemical equations to depict that synthesis these polymers. (Hint: Addition polymerization of ethene and substituted ethenes. Condensation polymerization of heterobifunctional monomers.) Answer Find the monomers that are needed to synthesize following polymers: Polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyurethanes (PU), and polyamide (nylon 6,6). Polyethylene (PE): The monomer needed to synthesize polyethylene is ethylene (C2H4). PE is formed by the addition polymerization of ethene molecules. Polypropylene (PP): The monomer needed to synthesize polypropylene is propylene (C3H6). PP is also formed by the addition polymerization of propylene molecules. Polystyrene (PS): The monomer needed to synthesize polystyrene is styrene (C8H8). PS is formed by the addition polymerization of styrene molecules. Polyvinyl chloride (PVC): The monomer needed to synthesize polyvinyl chloride is vinyl chloride (C2H3Cl). PVC is formed by the addition polymerization of vinyl chloride molecules. Polyethylene terephthalate (PET): The monomers needed to synthesize polyethylene terephthalate are ethylene glycol (C2H6O2) and terephthalic acid (C8H6O4). PET is formed by the condensation polymerization of these two monomers. Polyurethanes (PU): The monomers needed to synthesize polyurethanes are diisocyanates (RNCO) and diols (ROH). PU is formed by the reaction between these two monomers through condensation polymerization. Polyamide (nylon 6,6): The monomers needed to synthesize nylon 6,6 are adipic acid (C6H10O4) and hexamethylenediamine (C6H16N2). Nylon 6,6 is formed by the condensation polymerization of these two monomers. (b) Write the chemical equations to depict the synthesis of these polymers. Polyethylene: (n)CH2=CH2 → (-CH2-CH2-)n Polypropylene: (n)H2C=CH-CH3 → (-CH(CH3)-CH2-)n Polystyrene: (n)C6H5-CH=CH2 → (-C6H5-CH2-CH2-)n Polyvinyl chloride: (n)(C2H3Cl) → (-C2H3Cl)n Polyethylene terephthalate: (-HOOC-C6H4-COOH-) + (-HOCH2CH2-) → (-OOC-C6H4-COO-CH2CH2-)n + 2H2O Polyurethanes: (-RNCO-) + (-ROH-) → (-RNH-COO-R-)n + H2O Polyamide (nylon 6,6): (-COOH-(CH2)4-COOH) + (-NH-(CH2)6-NH-) → (-CO-(CH2)4-CO-NH-(CH2)6-NH)n + 2H2O