Stoichiometry
CHY113 OpenStax
Chapter 4: Stoichiometry of Chemical Reactions
Learning Objectives
4.1 Writing and Balancing Chemical Equations
• Derive chemical equations from narrative descriptions of chemical reactions.
• Write and balance chemical equations in molecular, total ionic, and net ionic formats.
4.2 Classifying Chemical Reactions
• Define three common types of chemical reactions (precipitation, acid-base, and oxidation-reduction)
• Classify chemical reactions as one of these three types given appropriate descriptions or chemical equations
• Identify common acids and bases
• Predict the solubility of common inorganic compounds by using solubility rules
• Compute the oxidation states for elements in compounds
4.3 Reaction Stoichiometry
• Explain the concept of stoichiometry as it pertains to chemical reactions
• Use balanced chemical equations to derive stoichiometric factors relating amounts of reactants and products
• Perform stoichiometric calculations involving mass, moles, and solution molarity
4.4 Reaction Yields
• Explain the concepts of theoretical yield and limiting reactants/reagents.
• Derive the theoretical yield for a reaction under specified conditions.
• Calculate the percent yield for a reaction.
4.5 Quantitative Chemical Analysis
• Describe the fundamental aspects of titrations and gravimetric analysis.
• Perform stoichiometric calculations using typical titration and gravimetric data.
Resources
4.1 Writing and Balancing Chemical Equations
• Derive chemical equations from narrative descriptions of chemical reactions.
• Write and balance chemical equations in molecular, total ionic, and net ionic formats.
Balanced chemical equations are mass balanced and charge balanced.
Molecular equation: CaCl2(aq) + 2AgNO3 (aq) ⟶ Ca(NO3)2(aq) + 2AgCl(s)
(Total) Ionic equation: Ca2+(aq) + 2Cl−(aq) + 2Ag+(aq) + 2NO3−(aq) ⟶ Ca2+(aq) + 2NO3−(aq) + 2AgCl(s)
Net Ionic equation: Ag+(aq) + Cl−(aq) ⟶ AgCl(s)
4.2 Classifying Chemical Reactions
• Define three common types of chemical reactions (precipitation, acid-base, and oxidation-reduction)
• Classify chemical reactions as one of these three types given appropriate descriptions or chemical equations
• Identify common acids and bases
• Predict the solubility of common inorganic compounds by using solubility rules
• Compute the oxidation states for elements in compounds
A precipitation reaction is one in which dissolved substances react to form one (or more) solid products.
Strategy: Memorize the solubility rules.
Table 4.1 Solubility Rules
An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another.
Strategy: Memorize which acids and bases are strong and which are weak.
Acid and Base Definitions
Arrhenius: An acid is a substance that dissociates in water to form hydronium ions (H3O+, also written as H+). A base is a substance that dissociates in water to form hydroxide ions OH-.
Bronsted-Lowry: An acid is a proton donor and a base is a proton acceptor.
Lewis: An acid is an electron pair acceptor and a base is an electron pair donor.
Table 4.2 Common Strong Acids
Strong acids completely dissociate. Use a one way arrow to indicate the dissociation. Hydrochloric acid is an example of a strong acid.
HCl(aq) + H2O(aq) ⟶ Cl−(aq) + H3O+(aq)
Weak acids only partially dissociate. Use a double-arrow to indicate partial dissociation. Acetic acid (CH3CO2H) is an example of a weak acid. Only about 1% of the acetic acid dissociates into hydronium and acetate ions.
CH3CO2H(aq) + H2O(l) ⇌ CH3CO2−(aq) + H3O+(aq)
Strong bases completely dissociate in water to form hydroxide ions. The most common strong bases are NaOH, KOH, LiOH, Mg(OH)2,Ca(OH)2, Ba(OH)2 - the alkali (group 1) and alkaline earth (group 2) metal hydroxides. (Note the one-way arrow in the reaction with water below.)
NaOH(s) ⟶ Na+(aq) + OH−(aq)
Weak bases react partially with water to produce hydroxide ions. Ammonia, NH3, is the prototypical week base. (Note the double-arrow in the reaction below.)
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq)
A neutralization reaction is a specific type of acid-base reaction in which the reactants are an acid and a base, the products are often a salt and water, and neither reactant is the water itself:
acid + base ⟶ salt + water
For example: Mg(OH)2(s) + 2HCl(aq) ⟶ MgCl2(aq) + 2H2O(l)
Oxidation-reduction reactions (commonly called redox reactions) are reactions in which electrons are transferred from reactant(s) to product(s) - which can be assessed by assigning oxidation numbers to each element in each compound.
Strategy: Learn how to assign oxidation numbers to each element in compounds.
Rules for assigning oxidation numbers.
It is helpful to split the overall reaction into individual equations called half-reactions.
2Na(s) ⟶ 2Na+(s) + 2e–
Cl2(g) + 2e– ⟶ 2Cl–(s)
Sodium (Na) loses electrons. Loss of electrons is oxidization. Sodium is oxidized. Sodium is called the reducing agent (the source of the electrons that are transferred).
Chlorine (Cl2) gains electrons. Gain of electrons is reduction. Chlorine is reduced. Chlorine is called the oxidizing agent (the recipient of the transferred electrons.
Rules for balancing redox reactions by the half reaction method.
For example, the reaction of sodium metal with gases chlorine exothermically yields the salt sodium chloride. See a YouTube of the reaction. 2Na(s) + Cl2(g) ⟶ 2NaCl(s)
4.3 Reaction Stoichiometry
• Explain the concept of stoichiometry as it pertains to chemical reactions
• Use balanced chemical equations to derive stoichiometric factors relating amounts of reactants and products
• Perform stoichiometric calculations involving mass, moles, and solution molarity
Figure 4.11 The flowchart depicts the various computational steps involved in most reaction stoichiometry calculations.
4.4 Reaction Yields
• Explain the concepts of theoretical yield and limiting reactants/reagents.
• Derive the theoretical yield for a reaction under specified conditions.
• Calculate the percent yield for a reaction.
Theoretical yield - the 100% yield
Actual yield - what you got (should never be greater than 100%!)
Limiting reagent - which reactant is used up first
Excess reagent - which reactant is left over
Figure 4.13 Sandwich making can illustrate the concepts of limiting and excess reactants.
The balanced stoichiometric equation is: 2 bread + 1 cheese --> 1 sandwich.
You are provided with 28 bread and 11 cheese.
Using all the cheese (limiting reagent), you can make 14 sandwiches (100% yield) leaving 6 bread (excess reagent).
4.5 Quantitative Chemical Analysis
• Describe the fundamental aspects of titrations and gravimetric analysis.
• Perform stoichiometric calculations using typical titration and gravimetric data.
Titration is a quantitative method of determining concentration of an analyte.
The analyte is the species whose concentration is to be determined.
The standardized solution is the reagent in the buret (also known as the titrant) whose concentration is known to some precision and undergoes a chemical reaction with the analyte.
The indicator is generally an organic compound that undergoes a color change at or near the equivalence point of the chemical reaction.
The equivalence point is where a stoichiometric amount of titrant has reacted with all the analyte.
The end point is the measured volume of titrant - and hopefully very close to the equivalence point.
Figure 4.16 (a) A student fills a buret in preparation for a titration analysis. (b) A typical buret permits volume measurements to the nearest 0.01 mL.
Titration cartoon
Gravimetric analysis: A type of analysis in which a sample is subjected to some treatment that causes a change in the physical state of the analyte that permits its separation from the other components of the sample.
Mass measurements of the sample, the isolated analyte, or some other component of the system, used along with the known stoichiometry of the compounds involved, permit calculation of the analyte concentration.
Commonly, the analyte is separated by subjecting it to a precipitation reaction.