Chapter 4 Chemical Quantities and Aqueous Reactions
Patterns of Reactivity • Five basic types of reactions. 1 . Combination – two substances combine to make one new one. Generic: A + B C Ex) 2 Mg(s) + O2(g) 2 MgO(s) 2. Decomposition – one substance decomposes to several new ones. Generic: A B + C Ex) 2 NaN3(s) 2 Na(s) + 3 N2(g)
Patterns of Reactivity 3. Single Replacement – one element replaces the other. Generic: A + BC AC + B Ex) 2 AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2 Ag(s) 4. Double Replacement (aka “Metathesis”) – trading partners. Generic: AB + CD AD + CD Ex) Hg(NO3)2(aq) + 2 NaI(aq) HgI2(s) + 2 NaNO3(aq) 5. Combustion – a rapid reaction with O2(g) producing a flame. Ex) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)
Interpreting a Reaction • A simple reaction like: N2(g) + 3 H2(g) 2 NH3(g), can be interpreted on many levels. • Molecular Level: one molecule of N2 plus three molecules of H2 react to form two molecules of NH3
Interpreting a Reaction • For this reaction, we can establish that: 1 molecule N2 = 3 molecules H2 1 molecule N2 = 2 molecules NH3 3 molecules H2 = 2 molecules NH3 LEP #1
Interpreting a Reaction • The molecular level is really not practical as we cannot do reaction on this scale. • Rather, we can do them on a mole scale. • Thus: one mole of N2 plus three moles of H2 react to produce two moles of NH3. • This means our relations can be shortened to moles. LEP #1
Limiting Reactant • If given amounts of both reactants, we may run out of one of them first. This reactant limits how much can be made. • Analogy: Putting together a bicycle – parts on hand are 200 frames and 350 wheels. How many bicycles can you make? • Ex) 2 H2 + O2 2 H2O • Suppose a vessel contained 10 molecules of H2 and 7 molecules of O2. How many water molecules are possible?
Limiting Reactant • This also applies to mole amounts as well. • LEP #2
Stoichiometry • Pronounced: stoy-key-OM-uh-tree. • Relating quantities in chemical reactions – in particular – masses. • Cannot use mole-to-mole ratios to convert mass of one substance to mass of another by one single step. • A mass-to-mass conversion must be done in three steps.
Stoichiometry • Can be used to find a mass of another reactant or a product. • Can be part of a limiting reactant where amounts of both reactants are given. • Can also be asked to find a percent yield. • Where the Theoretical Mass is the maximum amount possible based on your limiting reactant. • LEP #3 and #4
Solution • A solution is a homogeneous mixture. • Consists of a solute and the solvent. • Ex) NaCl added to water • Chapter 12 will show us a wide variety of solutions. • Chapter 4 – only water is the solvent.
Solution Concentration • There are many methods for expressing a solution’s concentration and we will see more methods in Chapter 12. • Chemists typically use molarity (M). • Molarity =
Molarity • Calculations can be from a mass and volume – LEP #5. • Or from a molarity and volume – LEP #6. • Or involve a dilution – LEP #7.
Solution Stoichiometry • Reactions that take place in solution can be analyzed using molarity and volume. • Molarity can be used a conversion factor. • Can be calculation to find amount of other reactant (volume or mass). • Can be limiting reactant problem. • LEP #8, #9
Dissolving an Ionic Compound • When sodium chloride is added to water, the ions are pulled apart. • By itself, pure water is a poor conductor of electricity. • When an ionic compound dissolves, it produces ions. • These ions can carry a charge through the solution and are referred to as electrolytes.
Electrolytes • Strong Electrolyte – completely dissociates to produce 100% ions in solution. • Ionic compounds – must be soluble in water • Strong Acids (6) = • Strong Bases =
Electrolytes • Weak Electrolytes – partially ionize to produce a few ions. Solution is weakly conducting. • Weak acids – formula starts with “H”, but not on list of S.A. • Ex) HC2H3O2 , HF, HCHO2, etc. • Weak bases – ammonia or amines. • Ex) NH3, CH3NH2, CH3CH2NH2
Non-electrolytes • Molecular compounds that dissolve in water, but produce no ions. • Sugar molecules, Cn(H2O)n • Ex) C6H12O6, C5H10O5 • C1 to C4 Alcohols, Aldehydes, Ketones • Ex) CH3OH, CH3 C CH3 || O • LEP #10
Solubility Rules • Provides general solubility – only gives “black or white” details. • No detail on extent of solubility. • Ex) 34 g / 100mL for KCl • Some soluble compounds have low limit. • Ex) 0.17 g / 100mL for Ca(OH)2 • Some insoluble compounds may be slightly soluble. • Ex) 0.45 g / 100mL for PbCl2 • Insoluble compound = (s). • Soluble compound = (aq). • LEP #11, #12
Activity Series • Ranks the metals from most reactive to least reactive. • A metal HIGHER on the activity series will replace (react) any metal ion beneath it. • Will Mg(s) react with Cu+2(aq)? • Will Sn(s) react with Fe+2(aq)?
Writing a Reaction • A molecular equation shows all compounds written as if they were molecules – even substances that are known to exist as ions. • An ionic equation shows all aqueous compounds as ions in solution. • All aqueous compounds are broken apart.
Writing a Reaction AgNO3(aq) + NaCl(aq) • A Net Ionic equation removes all of the spectator ions. • Spectator ions are ones that do not change from reactants to products. • These are the aqueous ions.
Single Replacement • Writing a single replacement reaction from scratch. • A + BC • A and B trade places as long as predicted by Activity Series. • Watch out for charges! • Special treatment for H+ (acid) – will generate H2(g)! • LEP #13
Double Replacement • Writing a double replacement reaction involving a precipitation from scratch. • AB + CD • Trade partners – A goes with D and C goes with B. • Once again, charges MUST be observed when re-combining and formulas are written with cation first. • Reaction only happens if one of the two products is Insoluble (s). • LEP #14 a, b
Double Replacement • These reactions can also produce a gas like CO2 or H2S. • An acid plus a carbonate or bicarbonate • An acid plus any sulfide • An acid and a base react to form water – also called neutralization. • Acid plus a base form water plus a “salt”. • LEP #14 c, d, e
Oxidation – Reduction • Referred to as “redox” for short. • This process involves the loss and gain of electrons. • Corrosion of metals • Batteries • Oxidation = the loss of electrons. • Reduction = the gain of electrons. • OIL RIG =
Oxidation Numbers • An accounting method used to assign each element in a compound and oxidation state (number). • Rules: 1. The oxidation state of any element in its standard state is zero. Ex) Na(s), Cl2(g), P4(s), etc. 2. The oxidation state of a monoatomic ion is equal to its charge. Ex) Cu+2(aq) = +2, Cl-1(aq) = -1 3. The sum of all oxidation states in a compound should equal zero.
Oxidation Numbers 4. The sum of all oxidation numbers in an ion will equal the charge on the ion. 5. Group 1A metals = +1, 2A metal = +2, Al = +3. 6. Halogens: F = -1; Cl, Br, and I are usually -1 unless when O is present. Ex) NaClO 7. O = -2, unless in a peroxide like H2O2. S = -2 unless when O is present. 8. H = +1. LEP #15
Redox Reactions • Once all elements in a reaction have been assigned oxidation numbers, inspect to see if two elements have changed. • Cannot have oxidation without reduction! • Ex) 2 Cu(s) + S(g) Cu2S(s) • Ex) Zn(s) + 2 HCl(aq) ZnCl2(aq) + H2(g) • LEP #16
Redox Reactions • Just to make things a little more confusing… • The element or compound that was reduced = oxidizing agent. • The element or compound oxidized = reducing agent. • Always from the perspective of the REACTANTS.
Acid-Base Titration • These involve the neutralization reaction. • Endpoint = when all of the unknown solution has reacted. • Indicator = substance that changes color when the endpoint has been achieved.
Acid-base Titration • Delivery of the known solution is achieved using a buret. • Measures to nearest 0.05mL.
Acid-base Titration • Typically, the known solution is the base like NaOH. • LEP #17
Redox Titration • Assay analysis of an ore and many other applications. • Common oxidizing agent is KMnO4 • LEP #18