Predicting Chemical Reactions

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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.

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

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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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• 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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• 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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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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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 now make sure that you balance the equation 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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When a strong acid reacts with a strong base, the ionic equation is…


H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)

Na+ (aq) + Cl- (aq) + H2O (l)

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When a strong acid reacts with a strong base, the net ionic equation is…


H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)

Na+ (aq) + Cl- (aq) + H2O (l)

H+ (aq) + OH-(aq) H2O (l)

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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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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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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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• 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)

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Predicting Chemical Reactions