CHM 114 Lecture Notes - Lecture 3: Chemical Formula, Strong Electrolyte, Limiting Reagent

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22 Sep 2017

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Chm 114-ch3 (chemical reactions and reaction stoichiometry) and ch4 (reactions in. Stoichiometry: the relationships among the quantities of reactants and products involved in chemical reactions: uses an understanding of atomic masses, chemical formulas, and the law of the conservation of mass (see ppt for definition) Chemical equations: use the following format: reactants (starting material(s)) products (end material(s), equations must be balanced according to the law of the conservation of mass, never change the subscripts when balancing an equation. Types of chemical reactions (see definition in ppt: combination/synthesis reactions, decomposition reactions, combustion reactions, always has the same formula. Hydrocarbon (g) + o2(g) co2(g) + h2o (g) Formula weight (fw): the mass of the collection of atoms represented by a chemical formula. Molecular weight (mw): the mass of the collection of atoms represented by the chemical formula for a molecule [unit=amu: describes molecules only. *both fw and mw are found in the same method.

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I need help doing my lab homework, I missed lab that day because, I was sick and it's due.

Chemical Reactions and Equations Learning objectives To observe physical and chemical properties associated with chemical changes To be able to give evidence for the occurrence of a chemical reaction To write a balanced chemical equation To identify areaction as a combination, decomposition, precipitation, single or double replacement, or combustion reaction ntroduction When a substance undergoes a physical change, it changes its appearance but not its composition. For example, when ice melts to form water, it undergoes a physical change from solid to liquid. In a chemical change, the composition of a substance is changed to give a new substance with different physical and chemical properties. For example, when silver tarnishes, the shiny silver (Ag) changes to a dull gray silver sulfide (AgaS). Not only has the chemical formula changed from Ag to Agus, but also the physical properties (shininess) of the substance have changed. Some examples of physical and chemical changes are listed in Table 1 below. Table 1. Comparison of Physical and Chemical Changes Examples of Physical changes Examples and Evidence of Chemical Changes Change in state (melting, freezing, evaporating Formation of a gas (bubbles) subliming) Formation of a solid (precipitate) Change in size Tearing Change in Color Heat is given off or absorbed A solid dissolving in a solvent In a chemical reaction, atoms in the reactants are rearranged to produce new combinations of atoms in the products. However, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. This is true according to the Law of Conservation of Mass, which states that the total mass remains constant during a chemical reaction. In other words, matter cannot be created or destroyed in a chemical reaction. In a chemical equation, the reactants are shown on the left and the products are on the right. An arrow in between indicates that a chemical reaction occurs. Finally, the physical state of each compound is designated in parentheses after the chemical formula. Solid, liquid, gas, and aqueous (dissolved in water) are shown as(s,(n,(g), and (aq), respectively.

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Introduction

Decomposition of Sodium Chlorate Mass, Moles, and the Chemical Equation

Sodium chlorate is used as a source of oxygen in emergency oxygen generators. So- called oxygen canisters or oxygen candles are found on airplanes, submarines, even the space station â anywhere oxygen might be in short supply in case of an emergency. Sodium chlorate decomposes upon heating or in the presence of metals to give oxygen gas. What is the chemical equation for the decomposition of sodium chlorate?

Concepts

ï· Moles ï· Balanced chemical equation

ï· Molecular formula ï· Stoichiometry

Background

Sodium chlorate, NaClO₃, is a colorless, odorless, white solid that melts at 248 Â°C. When heated above 300 Â°C, it begins to lose oxygen. The ultimate products of the thermal decomposition of sodium chlorate are oxygen gas and a white solid. Based on the molecular formula of sodium chlorate, three possible reactions will account for the loss of oxygen gas upon heating (Equations 1-3). Note that equations 1-3 are not balanced.

Equation 1 2NaClO₃(s) â 2NaClO₂(s) + O₂(g)

Equation 2 NaClO₃(s) â NaClO(s) + O₂(g)

Equation 3 2NaClO₃(s) â 2NaCl(s) + 3O₂(g)

What is the actual chemical equation for the thermal decomposition of sodium chlorate? All of the possible sodium-containing products n Equations 1-2 are real compounds: sodium chlorite, NaClO₂; sodium hypochlorite, NaClO; and sodium chloride, NaCl. All are white solids at room temperature. It is possible to determine the chemical equation for the decomposition of sodium chlorate by applying the principles of stoichiometry to the masses of the reactants and products.

Safety Precautions:

Sodium chlorate is a strong oxidizing agent and a dangerous fire risk; it is slightly toxic by ingestion. Contact with metal powders or combustible organic compounds may cause fires. Keep away from contact with organic materials, including rubber stoppers, rubber tubing, etc. Avoid contact with eyes and skin. Do NOT dispose of exess sodium chlorate in the trash.

When heating sodium chlorate, use only a small amount of solid (0.25-0.40 g) in a Pyrex test tube. Inspect the test tube for chips and cracks before use and handle the test tube using a test tube clamp. Distribute the solid evenly along the bottom of the test tube and heat the test tube clamp. If any smoke is produced, remove the test tube from the flame until the reaction subsides. Do not inhale the smoke. Allow the test tube to cool completely on a wire gauze with a ceramic, heat-resistant center. Wear chemical splash goggles and chemical-resistant gloves and apron.

Procedure:

Measure all masses to the nearest 0.01 g and record all data in the data table.

Obtain two medium test tubes. Label them 1 and 2.

Measure and record the mass of each test tube.

Using a spatula, add approximately 0.2-0.4 g of sodium chlorate to the bottom of each test tube.

Measure and record the combined mass of each test tube and sodium chlorate.

Place test tube #1 in a test tube clamp. Holding the test tube in an almost horizontal position, gently tap the test tube to distribute the solid sodium chlorate evenly along the bottom one-third of the test tube.

Light a Bunsen burner.

Holding test tube #1 with the test tube clamp, slowly move the test tube back and forth through the burner flame to gently heat the solid. The solid should begin to melt and bubble slightly. Caution: Do not aim the opening of the test tube at anymore.

Quickly remove the test tube from the flame if a white smoke is given off from the open end of the test tube. The smoke represents a loss of starting material due to evaporation of molten sodium chlorate and will reduce the yield of product. A small amount of smoke may be inevitable and should not interfere with the accuracy of the results.

Continue to heat the test tube in the burner flame until no more bubbling is observed and the material in the test tube has solidified.

Allow the test tube to cool completely on a wire gauze with a ceramic center.

While test tube #1 is cooling, repeat steps 5-11 sing test tube #2.

When test tube #1 has cooled to room temperature, measure the combined mass of the test tube and its solid contents.

Reheat test tube #1 gently for an additional three minutes.

Cool the test tube on the wire gauze until it is room temperature. Measure the mass a second time. If the second mass differs from the first mass by more than 0.02 g, repeat the heating cycle a third time.

Repeat steps 13-15 using test tube #2.

Data Table:

Trial 1

Weight of beaker --- 68.1887g

Weight of test tube -- 19.3538g

Weight of NACLO3 -- 0.3341g

Everything together -- 87.9155g

1st time burning:-- 87.8851g

2nd time burning:-- 87.7630g

3rd time burning:-- 87.7626g

Trial 2

Weight of beaker --- 68.1886g

Weight of test tube -- 22.8518g

Weight of NACLO3 -- 0.3154g

Everything together -- 91.3508g

1st time burning:-- 91.2078g

2nd time burning:-- 91.2103g

Post-Lab Calculations and Analysis:

1) For each trial, find the initial mass of sodium chlorate and calculate the number of moles of sodium chlorate.

2) Based on the molar masses of the three possible solid products in Equations 1-3, and the number of moles of reactant in each case, calculate the expected masses of the three possible products for Trials 1 and 2.

3) Compare the actual mass of product obtained in Trials 1 and 2 with the calculated or expected masses. What is the balanced chemical equation for the decomposition of sodium chlorate? Explain.

4) Use the following equation to determine the percent error in the mass of solid obtained.

â (actual mass â expected mass) / expected mass

5) Consider the following potential sources of error in this experiment. Explain whether they would have caused the actual mass of solid product to be lower or higher than the expected value.

a)The sodium chlorate did not decompose completely.

b)The sodium chlorate was heated too fast, allowing considerable white smoke to escape from the test tube.

c)The sodium chlorate absorbed some moisture from the air before the experiment began and was not completely dry.

Provide a peer review

(Provide feedback relating to each rubric component (Article pros and cons, course application, defense of
position, and quality of writing). 5 points each making up 20 points.

Stoichiometry is traditionally a topic that is covered in each of the different levels at Argo Community High School (Argo). While the Honors Chemistry and Advanced Placement (AP) Chemistry courses stress a mathematical approach to mass and mole relationships, the Regular Chemistry course typically receives a lighter, scaled back approach due to a lower level of math skill and achievement. In Birdâs âStoichiometric Calculations Using Equivalent Molar Expressionsâ, the argument is made for a different mathematical approach using equivalent molar expressions instead of the standard unit or dimensional analysis method (2006). While this method does have some unique benefits for all students it would be difficult to implement completely at all levels of chemistry at Argo. One of the best aspects of using âequivalent molar expressionsâ is it shows the importance of the relationship of a balanced equation to the reacting moles (Bird, 2016). Students at Argo currently have two paths into Chemistry; a traditional biology first sequence and a nontraditional physics first sequence. The teachers in the freshman physics first sequence stress dimensional analysis throughout the course. For Sophomore Honors and AP Chemistry students stoichiometry occurs early in the semester and is continued throughout the school year. Although students have had at least one previous year using dimensional analysis, their proficiency in using dimensional analysis determines the studentsâ success or failure with stoichiometry. Students often rely on rote methods to arrive at a final answer but cannot always explain the value they calculated. Using the molar expressions method, each equivalent mole of reactant or product is determined by using the relationship of the coefficient to the starting or ending moles in the reaction. Through this method the students can see that the moles are equivalent and interchangeable, thus making a stronger connection to a particle approach to stoichiometry. Instead of a rote method of a series of dimensional analysis problems the student can quickly and easily determine the moles or other quantity desired in a balanced equation. Another positive aspect of the molar expression method is its flexibility in use with various topics. This method, combined with an ICE/BCA table (Initial, Change, End or Before, Change, After) approach, can be used to quickly inform the student of how much excess remains, how much product is formed, and what is present in the reaction vessel at the end of a reaction (May, 2017). Students in Honors and AP Chemistry at Argo struggle with determining the amount of excess reactant that remains at the end of a reaction. Various wrong methods to determine excess remaining are applied as it is a different calculation than their rote processes previously developed. Using molar expressions, combined with an ICE/BCA table, students can quickly see the amount remaining and will more accurately understand the process of the limiting reactants and the reaction itself. The students can also see that at the end there are some reactants and products remaining. Combining molar expressions with an ICE/BCA table has an added benefit of introducing ICE tables earlier in the curriculum. Although using equivalent moles can simplify stoichiometry calculations while also clarifying many mathematical misconceptions, the worst argument is the inclusion of the actual molar expression to the calculations as it is an unnecessary addition and one that would be detrimental to understanding the stoichiometry of the reaction. When students see a myriad of equations for the numerator they either quickly look for a method to memorize all of these diverse equations or simply assume it is too complicated to understand. If instead the mole was used and equivalent moles was determined instead of equivalent molar expressions, the work would become less clouded with variables. High School students may lose vigor when confronted with such a problem. Instead the students should rely on their ability to convert into moles by using molarity, the ideal gas law, or simply grams to moles, and then use equivalent moles, the process of stoichiometry can be kept separate from the method of determining moles in various ways. Using equivalent molar expressions, or just equivalent moles as I have argued, a connection can be made to both Extreme Whoosh Bottle Trio and Big Time Ethyl Alcohol Explosion videos provided by Flinn (Bracken, Dombrink, & Long, 2017; Marek, 2017). To make the demonstrations quantitative you could simply measure the amount of water generated in each demonstration. The Extreme Whoosh Bottle Trio lends itself particularly to a quantitative treatment as three different concentrations of alcohol were used. By determining the amount of water that should be generated by the 70% isopropyl solution and then comparing it to the amount of water that is actually generated, students can quickly see the application of stoichiometry as well as the use of equivalent molar expressions. The same treatement can be applied to the pure ethanol as well as the 95% isopropyl, allowing several examples as either an introduction or a summation set of demonstrations. The general idea of the equivalent molar expressions is applicable and should be used in all classes at Argo, with some modification (Bird, 2016). While the AP and Honors Chemistry students can definitely use the molar expression within the ICE/BCA table described within this paper, I have argued that the students will find it simpler without the molar expression, instead just using moles in the expression. But it would be advantageous to use molar expressions at the end of the year as a mathematical summation of several topics including percent composition, molarity, and ideal gases. However, the Regular Chemistry students would see no benefit of including molar expressions into the ICE/BCA table. The complexity of several different possible numerators would prove to be too cumbersome. Instead, it would be more appropriate to implement equivalent moles in those classes.

CHM 114 Lecture Notes - Lecture 3: Chemical Formula, Strong Electrolyte, Limiting Reagent

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