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Replacement Reactions

• Single Replacement– One species changing at expense of another– Zinc dissolving in Copper Sulfate solution

• Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

• Double Replacement– “Trading Partners”, ions exchange partners

– NaCl(aq)+AgNO3(aq) AgCl(s)+NaNO3(aq)

– Very common reaction type

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Why does replacement occur?• Formation of gas or solid

– Removes materials from solution, • 1-way street, removed material no longer available

• Chemical activity– Some materials more reactive than others

• Alkalai metals dissolve in water to make hydrogen• Gold, Silver, Copper found as elements in nature

– We make an empirical list of reactivity• Hydrogen chosen as reference point• Metals MORE active than hydrogen dissolve in water• Metals LESS active than hydrogen don’t dissolve in water

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Activity Series of Elements

• Oxidation rates differ greatly– Sodium reacts instantly & violently (Potassium more so)

• With water or any oxidizer (e.g. O2, Cl2)

– Iron rusts slowly but surely, no metallic iron in nature• >100 year old steel structures, autos, tools

– Gold does not oxidize at all• Native gold here after 4.5*10^9 years (age of earth)• Artifacts found >1,000 years old

• “Activity” is qualitative oxidation measure– Activity Series = relative reactivity

• Voltage is quantitative oxidation measure– Half Cells, referenced to Hydrogen

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• Chemical Activity– Elements are different

• High to low activity

– Index is acid reaction• Is hydrogen liberated?

– “Active” elements• Liberate H2 with acid• Replace less reactive

– Transition = hydrogen• Reference Activity point

– “Less Active” elements• No acid reactions• Replace less reactive

– A Qualitative concept• No measured values

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Activity Series of Elements

• Element higher in chart will replace the ones below– Iron replaces (reduces) Copper ion

• Fe0 + Cu++ Fe ++ + Cu0

• Iron is oxidized, Copper gets reduced

• Hydrogen defined as reference point – Elements above Hydrogen can liberate H2 gas

• Zn0 + 2H+ (acid ion) Zn++ + H2(g)

• Half Cells, referenced to Hydrogen

• Elements below Hydrogen don’t liberate H2 gas– No reaction with (non-oxidizing) acids

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Copper replacing Silvermore active copper dissolves, less active silver

precipitates, a SINGLE REPLACEMENT reaction, also an example of a REDOX reaction

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Single Replacement Examples

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Double Replacement Examples

• Na2SO4(aq) + BaCl2(aq) BaSO4(s) + 2NaCl(aq)– Barium Sulfate very insoluble, swallow for X-ray imaging

• AgNO3(aq) + NaBr(aq) AgBr(s) + NaNO3(aq)– Silver Bromide very insoluble but light sensitive, photo film

• HCl(aq) + NaOH(aq) NaCl(aq) + HOH + heat– Neutralization of acid + base creates water & heat

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Four Reaction Types

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Limiting Reactants

• Nature rarely provides 100% balance– Oxygen in air exceeds animal and fuel needs– Almost always an excess of all but 1 reactant– One which runs out first = “Limiting” reactant– Even with perfect balance of reactants

• Reaction may not end in timely fashion (or ever)

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Social analogy• Limiting reactants can be people, not chemicals

– Male/Female birth ratio starts out ≈ 50/50%• Chinese government limits Number of children

– Boy babies favored over girls, preserve family name– Intervention result is surplus of males over females

• Implication is more bachelor men without families• Women are the limiting reagent, all girls have husbands, • Run out of women first, men are in surplus

• Warfare effects mostly men– Mostly males volunteer (or are drafted) as soldiers – Soldiers kill each other, reducing male population– Result is surplus of females over males after war

• Implication is more women without men• Men are the limiting reagent, all men have wives, • Run out of men first, women are in surplus

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Hindenburg Hydrogen ExplosionAtmosphere 21% oxygen, Limited reactant was hydrogen,

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Stoichiometry Conversions

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Mole relationships

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Balancing Equations

• Same atoms before & after rearrangement– Mass not created or destroyed in reactions– Need to find correct multipliers (coefficients)– Reactants and products must be realistic

• Trading atomic partners is common– AgNO3 + KCl AgCl(s) + KNO3

– Parenthesis (s) indicates solid precipitate– Precipitates takes reactants out of solution,

typically makes reaction irreversible

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Chemical Equation Balancing

• Start with the elements which are changing form– Often the metals or carbon– Write the unbalanced input + output materials

• (unbalanced) Na0(s) + H2O NaOH(aq) + H2 (g)

– Make sure same number of metal atoms both sides• (1) Na0 (1) Na+• This atomic ratio must remain constant (in = out)

– Oxygen and Hydrogen are balanced last • Not enough hydrogen on right side with only 1 water• Need to double other materials for hydrogen to balance

• (balanced) 2Na0(s)+2H2O 2NaOH(aq) + H2 (g)

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Quantitative Relationships

• Coefficients in a reaction = quantity• Reactions occur in Mole multiples

– Moles are key reaction quantities– Mass to moles of reactants– Reaction product back to mass if need be

• Percent Yield– Actual / theoretical = yield

• Limiting Reactants– One reactant (almost) always in surplus

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Sequence of events

• Convert mass to moles (or molecules)– Cannot balance an equation with grams– Atoms weigh different amounts

• Balance equation of reactants + products– Mass Balance + Charge Balance– Ratio multipliers refer to moles or molecules

• Convert moles back to mass– Answers often desired in grams– We weigh in grams, calculate in moles

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Example #1

• Glass powder is reacted with 727kg of impure charcoal which is 88.9% carbon. How much SiC is produced?– Get grams into moles, – consider reaction based on moles– Convert product moles back to grams

• SiO2(s) + 3 C(s) SiC(s) + 2 CO(g)– Amount of pure carbon 0.889*727kg = 646kg– 646kg C / 12.01 g/mol = 53,790 moles Carbon– Reaction ratio is 1/3 mole SiC per mole Carbon– Moles of SiC = 53,790 / 3 = 17,930 moles SiC– 17,930 moles SiC * 40.1gm/mole = 719 kg SiC

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Thermite in action, another classic single replacement reaction

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How to obtain molten iron!Mix powdered aluminum and iron oxide (rust), then ignite

Applications include welding railroad tracks, military sabotage

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Example #2

• Thermite is a mixture of iron oxide and aluminum powder, when ignited produces liquid iron and aluminum oxide.

• Fe2O3(s) + 2 Al(s) 2 Fe(s) + Al2O3(s) + heat

• How much aluminum to mix with a pound of iron oxide?, and how much iron is produced?– Get pounds of iron oxide into grams, then into moles, – consider reaction based on moles– Convert desired moles back to grams

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Example #2• Fe2O3(s) + 2 Al(s) 2 Fe(s) + Al2O3(s) + heat

• Amount of iron oxide involved?– Starting with 1 pound = 454 gm of iron oxide

– 454 gm iron oxide / 159.7 g/mol = 2.84 moles Fe2O3

– Reaction ratio is 2 mole of Al per mole Fe2O3

– 2.84 mole Fe2O3 * 2 (Al/Fe2O3) = 5.69 Moles of Al

– 5.69 mol Al needed * 26.98 gm/mol = 153.4 gm Al

• Amount of iron produced ?– 2.84 mole Fe2O3 * 2 (Fe/Fe2O3) = 5.69 Moles of Fe

– 5.69 moles Fe * 55.85 gm/mol = 317.5 gm Fe

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Now to today’s experiment

• More work with Stoichiometry– Key to quantitative measurements– Handout due next week

• S’mores analogy– Use everyday edibles as “elements”

• C=Chocolate, G=graham cracker, M=marshmallow– Combine elements into a “compound”

• Sm = S’mores compound (a sandwich) • C + 2G + M CG2M

– Determine reaction limitations• When some materials used up, reaction stops• Calculate leftovers

– OK to eat the experiment• Clean lab surfaces well before handling food• Also due next week

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What’s a S’more ?Girl Scout camping-out invention from 1927

Graham Crackers + chocolate + marshmallow

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Traditional Campfire Method

• Place chocolate bar on graham cracker

• Heat up the marshmallow over open fire

• Put hot marshmallow over chocolate

• Put another cracker on top

• Squeeze together, insides melt together

• Eat it

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Campfire CuisineToasted marshmallow melts the chocolate

Graham crackers top & bottom form a sandwich, makes it easier to handle the hot materials

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Laboratory Method• Start with fixed amounts of “elements”• Weigh each component

– Weigh more than 1, take an average

• Assemble S’mores “compound”– Weigh the compound– Demonstrates conservation of mass, in = out– Demonstrates Dalton’s law of simple multiples

• Have some fun– Bunsen Burner is our “campfire”– Does heating change the “compound” ?

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Experiment

• Lets do it