1 predicting chemical reactions exchange reactions (metathesis reaction)
TRANSCRIPT
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Predicting Chemical Reactions
Exchange Reactions
(Metathesis Reaction)
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Precipitation ReactionsPredicting Products
Consider the reaction between AgNO3 and NaCl in aqueous solution:
AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)
Notice that the anions and cations appear to have exchanged partners.
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Precipitation Reactions
• Metathesis Reactions– reactions in which the positive ions and negative ions
present in the reactants appear to exchange partners– also called exchange reactions
AX + BY AY + BX
AgNO3 + NaCl AgCl + NaNO3
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Precipitation Reactions
• How can you predict the products of a reaction between ionic compounds?– Identify the ions (including charge) present in the
reactants.
– Exchange the anions between the two cations and write the correct formulas (electrically neutral) for the possible products.
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Precipitation Reactions
• How can you predict the products of a reaction between ionic compounds?
-Determine if any of the products are insoluble in water.• If any are insoluble, the reaction will procede!
• Also, if possible products are in gas form (CO2) or are in the liquid (not aq) form, the reaction will take place.
• If none of the products are a solid, a gas, or a liquid (i.e. different physical state, physical state other than aq,) there is no reaction….nothing happens.
-Write a balanced equation including the physical states of reactants and products.
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Predicting Reactions
Example: Write the chemical equation for the aqueous reaction between Ba(NO3)2 and Na2SO4. Predict which product(s) will be insoluble.
Ions: Ba2+ , NO3 -,
Na+, SO42-
At this point, you don’t care how many of each ion you started with.
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Predicting Reactions
Possible Products:
Ba2+ and SO42- BaSO4
Na+ and NO3 - NaNO3
Make sure that you combine the ions so that
total positive charge = total negative charge
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Predicting Reactions
Determine solubility of possible products:
BaSO4
NaNO3
Insoluble = solid
Soluble = aqueous
(s)
(aq)
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Predicting ReactionsWrite a balanced equation:
Ba(NO3)2(aq) + Na2SO4(aq) BaSO4 (s) + NaNO3 (aq)
You must always make sure that your equation is balanced at this point.
unbalanced
Ba(NO3)2(aq) + Na2SO4(aq) BaSO4 (s) + 2 NaNO3 (aq)
balanced
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Practice
• Predict the identity of the precipitate that forms when solutions of Fe2(SO4)3 and LiOH are mixed.
• Ions present in solution:
Fe3+ SO42- Li+ OH-
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Practice (contd)
• Write a balanced equation for the reaction:
Fe2(SO4)3(aq)+ LiOH(aq) Fe(OH)3 (s) + Li2SO4(aq)Unbalanced!!
Fe2(SO4)3(aq)+ 6 LiOH(aq) 2 Fe(OH)3 (s) + 3 Li2SO4(aq)
Balanced
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Molecular Equation
The molecular equation lists the reactants and products in their molecular form.
AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq)
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Ionic Equation• In the ionic equation all strong electrolytes (strong
acids, strong bases, and soluble ionic salts) are dissociated into their ions.
• This more accurately reflects the species that are found in the reaction mixture.
Ag+ (aq) + NO3- (aq)
AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq)
+ K+(aq) + Cl-(aq) AgCl (s) + K+ (aq) + NO3- (aq)
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Net Ionic Equation• To form the net ionic equation, cross out anything
that does not change from the left side of the equation to the right.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3
-(aq)
The only things left in the equation are those things that change (i.e., react) during the course of the reaction.
Ag+(aq) + Cl-(aq) AgCl (s)
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Net Ionic Equation• To form the net ionic equation, cross out anything that does not change
from the left side of the equation to the right.• The only things left in the equation are those things that change (i.e.,
react) during the course of the reaction.
• Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3
-(aq)
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Writing Net Ionic Equations1. Write a balanced molecular equation.
2. Dissociate all strong electrolytes.
3. Cross out anything that remains unchanged from the left side to the right side of the equation.
4. Write the net ionic equation with the species that remain.
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Practice
• Write a net ionic equation for the precipitation reaction that occurs when aqueous solutions of silver nitrate and potassium phosphate are mixed.
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More practice
• Lab 8:
• Write molecular, as well as ionic and net ionic reactions (13 of them) if there is a reaction taking place.
• Reaction #1:
Sodium carbonate plus hydrochloric acid
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Acids and Bases
and Their Reactions
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Acids
• Arrhenius defined acids as substances that increase the concentration of H+ when dissolved in water.
• Brønsted and Lowry defined them as proton donors.
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Acids
Increase the H+ concentration in water:
Example: nitric acid
HNO3 (aq) H+ (aq) + NO3- (aq)
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Acids
• Examples of Acids:– HCl hydrochloric acid
– HNO3 nitric acid
– CH3COOH acetic acid
– H2SO4 sulfuric acid
– H3PO4 phosphoric acid
Note: Acids can form different numbers of H+ ions!
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Acids
• Monoprotic acids– have one H in the formula– form a single H+ ion when they ionize
HNO3 (aq) H+ (aq) + NO3- (aq)
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Acids
• Diprotic acids– have two H’s in the formula– can form two H+ ion when they ionize completely
H2SO4 (aq) 2H+ (aq) + SO42- (aq)
• Polyprotic acids:– Have two or more H’s in the formula– Form two or more H+ ions when they ionize
completely
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Bases• Arrhenius defined
bases as substances that increase the concentration of OH− when dissolved in water.
• Brønsted and Lowry defined them as proton acceptors.
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Bases
• Bases:– substances that accept (react with) H+ ions.– any substance that increases the OH-
concentration when added to water
Examples:– Hydroxide ion (OH-)
OH- (aq) + H+ (aq) H2O (l)
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Bases
• Examples (cont)– Common hydroxide containing bases:
• NaOH, KOH, Ca(OH)2
• Note: These are strong electrolytes!
• NaOH (aq) Na+ (aq) + OH- (aq)
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Bases
• Examples (cont):
– Ammonia (NH3)
• Does not contain OH-
• Accepts H+ ion from water and increases the OH- concentration in the water
NH3(aq) + H2O (l) NH4+ (aq) + OH- (aq)
NH3 is a weak electrolyte!! (double arrow!)
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Acids & Bases
• Strong Acid:– an acid that is a strong electrolyte
• ionizes completely in solution
• Weak Acid:– an acid that is a weak electrolyte
• an acid that does not ionize completely
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Acids & Bases
• Strong acids:– Know the names and formulas of the 7 common strong
acids:
• HCl (aq) hydrochloric acid• HBr (aq) hydrobromic acid• HI (aq) hydroiodic acid
• HClO3 chloric acid
• HClO4 perchloric acid
• HNO3 nitric acid
• H2SO4 sulfuric acid
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Acids & Bases
• Examples of Weak Acids
HF (aq) hydrofluoric acid
H3PO4 phosphoric acid
CH3COOH acetic acid
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Acids & Bases
• Strong Base:– a base that is a strong electrolyte– ionizes completely in solution
• Weak Base:– a base that is a weak electrolyte– does not ionize completely in solution
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Acids & Bases
• Strong Bases: Know the names and formulas of the strong bases
– Alkali metal (1A) hydroxides• LiOH lithium hydroxide• NaOH sodium hydroxide• KOH potassium hydroxide• RbOH rubidium hydroxide• CsOH cesium hydroxide
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Acids & Bases
• Strong bases to know (con’t):
– Heavy alkaline earth metal (2A) hydroxides• Ca(OH)2 calcium hydroxide
• Sr(OH)2 strontium hydroxide
• Ba(OH)2 barium hydroxide
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Acids & Bases
• Examples of Weak Bases:
ammonia (NH3)
sodium bicarbonate (NaHCO3)• baking soda• a component of Alka-Seltzer
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Acids + BasesGenerally, when solutions of an acid and a base are combined, the products are a salt and water.
CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l)
Neutralization Reaction
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Neutralization Reactions
When a strong acid reacts with a strong base, the molecular equation is…
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
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Neutralization Reactions
When a strong acid reacts with a strong base, the ionic equation is…
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)
Na+ (aq) + Cl- (aq) + H2O (l)
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Neutralization Reactions
When a strong acid reacts with a strong base, the net ionic equation is…
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)
Na+ (aq) + Cl- (aq) + H2O (l)
H+ (aq) + OH-(aq) H2O (l)
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Neutralization Reactions
The products of these reactions have very different properties than the reactants.
HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq)
Sharpsour
bitterslippery
salt
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Acid-Base Reactions
• Salt:– any ionic compound whose cation comes from a base
and whose anion comes from an acid– an ionic compound that is neither an acid nor a base
• In general,
acid + metal hydroxide a salt + water
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Acid-Base Reactions
• Neutralization reactions are a type of metathesis reaction.
• To predict the products:– identify the ions present– exchange anions– write the correct formulas for the products including
physical states– write a balanced equation
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Acid-Base Reactions
Example: Write the balanced equation for the reaction between HBr (aq) and Ca(OH)2 (aq).
Ions: H+ Br-
Ca2+ OH-
Possible Products:
H-OH = H2O
CaBr2
2HBr (aq) + Ca(OH)2 (aq) CaBr2 (aq) + 2H2O (l)
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Gas-Forming Reactions
• Some metathesis reactions do not give the product expected.
• In this reaction, the expected product (H2CO3) decomposes to give a gaseous product (CO2) and H2O.
CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)
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Gas-Forming Reactions
When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water.
CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)
NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l)
“H2CO3”
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Gas-Forming Reactions
Similarly, when a sulfite reacts with an acid, the products are a salt, sulfur dioxide, and water.
SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l)
“H2SO3”
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Gas-Forming Reactions
• This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular!
• But just as in the previous examples, a gas is formed as a product of this reaction.
Na2S (aq) + H2SO4 (aq) Na2SO4 (aq) + H2S (g)