Chapter 3 Stoichiometry 2 PDF

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CHEMISTRY 120/121

Chapter 3 Stoichiometry

Overview: 

Combustion Analysis



Chemical Formula and Molecular Models



Chemical Equations



Theoretical, Actual, and Percent Yield

Stoichiometry Ch 3.7-3.10

Combustion Analysis: Combustion analysis is a common technique for analyzing compounds (mainly organic compounds which contain ____________________, ____________________, and ____________________). In combustion analysis, we ____________________ a known mass of compound and weigh the amounts of ____________________ made. By knowing the mass of the product and composition of constituent elements in the product, the original amount of constituent elements can be determined.

Note: All the original C forms ____________________, and the original H forms ____________________, the original mass of O is found by subtraction.

Once the mass of all the constituent elements in the original compound have been determined, the empirical formula can be found.

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Chapter 3 Stoichiometry

Example: Combustion of a 0.8233 g sample of a compound containing only carbon, hydrogen and oxygen produced the following: CO2 = 2.445 g and H2O = 0.6003 g. Determine the empirical formula of the compound.

Example: The smell of dirty gym socks is caused by the compound caproic acid. Combustion of 0.844 g of caproic acid produced 0.784 g of H2O and 1.92 g of CO2. If the molar mass of caproic acid is 116.2 g/mol, what is the molecular formula of caproic acid? (MM C = 12.01, H = 1.008, O = 16.00)

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Chemical Formulas and Molecular Models Compounds are generally represented with a ____________________ ____________________. The amount of information about the structure of the compound varies with the type of formula. All formulas and models convey a limited amount of information – none are perfect representations. All chemical formulas tell what elements are in the compound. 1) Empirical Formulas describe the kinds of elements found in the compound and the ____________________ of their atoms. 

They do not describe how many atoms, the order of attachment, or the shape.



The formulas for ionic compounds are empirical.

2) Molecular Formulas describe the kinds of elements found in the compound and the ____________________ of their atoms. 

They do not describe the order of attachment, or the shape.

3) Structural Formulas describe the kinds of elements found in the compound, the number of their atoms, the order of atom attachment, and the kind of attachment. 

They do not directly describe the 3D shape, but an experienced chemist can make a good guess at it.



Use lines to represent covalent bonds.



Each line describes the number of electrons shared by the bonded atoms.

4) Models show the 3D structure along with all the other information given in the structural formula. 

Ball-and-stick model s use balls to represent the ____________________ and the sticks to represent the ____________________ between them.



Space-Filling Models use interconnected spheres to show the ____________________ ____________________ of atoms connecting together.

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Molecular Formula

Chapter 3 Stoichiometry

Structural Formula

Ball-and-stick model

Space-filled model

Recall: The amount of every substance used and made in a chemical reaction is ____________________ to the amounts of all the other substances in the reaction (due to Law of Conservation of Mass)

Chemical Equations:



The total number of atoms is conserved



Change in physical state can occur o Solid o Liquid o Gas o Aqueous solution



Can indicate reaction conditions o Heat, temperature

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The study of the numerical relationship between chemical ____________________ in a chemical reaction is called ____________________. The coefficients in a balanced chemical equation specify the relative amounts in moles of each of the substances involved in the reaction.

** Review balancing equations on your own

Example: According to the following equation, how many moles of water are made in the combustion of 0.10 moles of glucose? C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

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…but in a laboratory, we weigh our solid reactants before a reaction. How can we convert the number of moles of a reactant to mass? How can we decide how many grams of products we could produce? We use ____________________ ____________________ along with molar mass of compounds.

Steps in calculating the masses of reactants and products in a chemical reaction (Stoichiometric calculation)” 1. Balance the equation for the reaction 2. Convert the masses of substances to moles 3. Use the balanced equation to setup the appropriate mole ratios 4. Use the mole ratios to calculate the number of moles of the desired substances 5. Convert from moles back to masses

Example: How many grams of O2 can be made from the decomposition of 100.0 g of PBO2? PbO2 (s) → PbO (s) + O2 (PbO2 = 239.2 g/mol, O2 = 32.00 g/mol)

For reactions with multiple reactants, it is likely that one of the reactants will be completely used before the others. When this reactant is used up, the reaction ____________________ and no more products are made. The reactant that limits the amount of product is called the ____________________ ____________________ (limiting reagent). 14 J. Zhou Summer 2018

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Chapter 3 Stoichiometry

Note: The limiting reactant gets completely consumed. Reactants not completely consumed are called ____________________ reactants.

The amount of product that can be made from the limiting reactant is called the ____________________ ____________________. Steps in solving stoichiometry problems involving limiting reagents. 1. Write and balance the equation for the reaction 2. Convert known masses of reactants to moles 3. Determine the limiting reagent using the number of moles of each reactant and the appropriate mole ratios 4. Use the amount of limiting reagent and appropriate mole ratios to compute the amount of desired product 5. Convert moles back to grams using molar mass (if necessary)

Example: How many moles of Si3N4 can be made from 1.20 moles of Si and 1.00 mole of N2 in the reaction Si

+

N2

→

Si3N4

Example: Suppose we combine 10.0 g of aluminum with 35.0 g of chlorine in the following reaction: Al (s)

+ Cl2 (g)

→

AlCl3 (s)

How much aluminum chloride (in grams) will be produced in this reaction?

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Chapter 3 Stoichiometry

The amount of product that can be made from the limiting reactant is called the ____________________ ____________________. The actual amount of product made in a chemical reaction is called the ____________________ ____________________. The percentage of the theoretical yield that was actually attained is called the ____________________ ____________________:

Example: If 4.61 g of N2 are made from reactaing 9.05 g of NH3 with 45.2 g of CuO, what is the percent yield? NH3 (g)

+ CuO → N2 (g)

+ Cu (s) + H2O

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