unit 7 reactions in solution chem i objectives describe the driving force for a chemical reaction. ...
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Unit 7 Reactions in Solution
Chem I ObjectivesDescribe the driving force for a chemical reaction.Use generalizations to predict the products of simple
reactions.Describe reactions in solutions by writing molecular,
complete ionic and net ionic equations.Identify the key characteristics of the reactions between
strong acids and strong bases.
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Will a Reaction Occur?• What causes reactions to occur?
Reactants → Products
• Driving forces that make reactants go in the direction of the arrow include:– Formation of a solid– Formation of a gas– Formation of water– Transfer of electrons
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Reactions which form solids• One driving force for a chemical reaction is the
formation of a solid, a process called precipitation.
• The solid formed is called a precipitate• The reaction is called a precipitation reaction
Example:Potassium chromate + barium nitrate
↑ Write equation for products
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Precipitation Reactions
• To predict the identity of the precipitate, consider the possible products
• First look at the nature of each reactant in an aqueous solution
• Barium nitrate contains Ba2+ and NO3- ions
• Typically, when a solid containing ions dissolves in water, the ions separate, moving around independently
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Precipitation Reactions
• Ba(NO3) 2 (aq) does not contain Ba(NO3) 2 units
• The solution contains Ba2+ and NO3- ions
• There are two NO3- ions for every Ba2+ ion
• When each unit of a substance that dissolves in water produces separated ions, it is called a strong electrolyte
• Ba(NO3) 2 is a strong electrolyte as is K 2CrO 4
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Ionic Compounds• When ionic compounds dissolve, the resulting
solution contains ions.• We usually write these reactants as
Ba(NO3)2 (aq) + K2CrO4 (aq) → Products• It is more appropriately shown asBa2+ (aq) + 2NO3
- (aq) + 2K +(aq) + CrO42- (aq)
→ Products• The mixed solution contains four types of ions.
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What products form?
• A solid compound must have a net zero charge• Must contain cations and anions• K+ and Ba2+ could not combine to form the solid• Most ionic materials contain only two types of
ions – one cation and one anion• Possible compounds are
K2 CrO4 Ba(NO3)2 K(NO3) BaCrO4
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Predicting Products• Need more information to determine which of
these is more likely to be the precipitateK(NO3) BaCrO4
• Potassium nitrate is a white solid. The chromate ion is yellow. So, the likely precipitate is BaCrO4
and the unbalanced equation is
Ba(NO3)2 (aq) + K2CrO4 (aq) →KNO3 (aq) + BaCrO4(s)
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Solubility Vocabulary• A soluble solid readily dissolves in water.
• An insoluble solid, or slightly soluble solid, means only a small amount of the solid dissolves in water.
• A salt means an ionic compound.
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Solubility RulesTable 7.1, Z page 178
1. Most nitrate (NO3-) salts are soluble.
2. Most salts of Na+, K+ and NH4+ are soluble.
3. Most chloride salts are soluble. Notable exceptions are AgCl, PbCl2 and Hg2Cl2.
4. Most sulfate salts are soluble. Notable exceptions are BaSO4, PbSO4 and CaSO4.
5. Most hydroxide compounds are insoluble. Notable exceptions are NaOH, KOH, Ba(OH)2 and Ca(OH) 2 .
6. Most sulfide (S2-), carbonate (CO32-) and phosphate
(PO43-) salts are insoluble.
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Predicting Products - Z page 180
Step 1 – Write the reactants as they actually exist before any reaction occurs.
Step 2 – Consider the various solids that could form. To do this, simply exchange the anions of the added salts.
Step 3 – Use the solubility rules to decide whether a solid forms and to predict the identity of the solid.
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Predicting Products
Guided exampleWhen an aqueous solution of silver nitrate is added to an aqueous solution of potassium chloride, a white precipitate is formed. Identify the solid and write a balanced equation for the reaction.
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Predicting ProductsWhen an aqueous solution of silver nitrate is added to an aqueous solution of potassium chloride, a white precipitate is formed.
AgNO3 (aq) + KCl (aq) → AgCl (?) + KNO3 (?)
Solubility rule 1 – NO3- is soluble
Solubility rule 2 – K + is solubleSolubility rule 3 – AgCl is insoluble
AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq)
Equation is balanced
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Predicting Products
In class examples1. KNO3 (aq) and BaCl2 (aq)
2. Na2SO4 (aq) and Pb(NO3) 2 (aq)
3. KOH (aq) and Fe(NO3) 3 (aq)
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Reactions in Aqueous Solutions
Returning to this equation, note that it is called a molecular equation.
Ba(NO3)2 (aq) + K2CrO4 (aq) →2KNO3 (aq) + BaCrO4(s)
It shows the complete formulas of all products and reactants.This does not give a clear picture of what occurs in solution.
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Reactions in Aqueous Solutions
• Aqueous solutions of barium nitrate, potassium chromate and potassium nitrate contain the individual ions, not the complete molecules implied by the molecular equation.
• The complete ionic equation better represents the actual forms of reactants and products in solution. Ba2+ (aq) + 2NO3
- (aq) + 2K +(aq) + CrO42- (aq) →
BaCrO4 (s) + 2K +(aq) + 2NO3- (aq)
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Reactions in Aqueous Solutions• The complete ionic equation reveals that only
some of the ions participate in the reaction.• Ions which do not participate in the reaction,
such as K + and NO3- , are called spectator ions.
• The ions that participate in the reaction are Ba2+ (aq) + CrO4
2- (aq) → BaCrO4 (s)• This equation, called the net ionic equation,
includes only those components that are directly involved on the reaction.
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Equations7.3a) Molecular equationNaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Complete ionic equationNa+ (aq) + Cl- (aq) + Ag + (aq) + NO3
- (aq) →
AgCl (s) + Na + (aq) + NO3- (aq)
Net ionic equationCl- (aq) + Ag + (aq) → AgCl (s)
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Equations7.3b ) Molecular equation3KOH(aq) +Fe(NO3) 3(aq) → Fe(OH) 3(s) + 3KNO3(aq)
Complete ionic equation3K + (aq) + 3OH- (aq) + Fe 3+ (aq) + NO3
- (aq) →
Fe(OH) 3(s) + 3K + (aq) + 3NO3- (aq)
Net ionic equation3OH- (aq) + Fe 3+ (aq) → Fe(OH) 3(s)
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Reactions that Form Water• Acids – first associated with the sour taste of citrus
fruits. Dervied from Latin word acidus meaning sour
• Essential nature of acids discovered by Svante Arrhenius in the late 1800’s
• Arrhenius proposed that an acid is a substance that produces H+ ions when it is dissolved in water
• Studies show that when HCl, HNO3 and H2SO4 are placed in water nearly every molecule dissociates to give ions
• Because these substances are strong electrolytes that produce H + ions they are called strong acids
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Reactions that Form Water• Bases – characterized by bitter taste and
slippery feel• Arrhenius found that aqueous solutions that
exhibit basic behavior always contain hydroxide ions (OH-). He defined a base as a substance that produces OH- in water.
• Most common base in the lab is NaOH which dissolves in water to form Na+ and OH- ions
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Reactions that Form Water
• When strong acids and strong bases are mixed, the fundamental change that occurs is that H+ and OH- ions react to form water
H+ (aq) + OH- (aq) → H2O (l) • The tendency to form water is one of the
driving forces for chemical reactions
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Reactions that Form Water
The reaction between hydrochloric acid and sodium chloride shown as a molecular equation is:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
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Reactions that Form Water
Because HCl and NaOH exist as completely separated ions in water, the complete ionic equation can be written as:
H+ (aq) + Cl- (aq) + Na + (aq) + OH- (aq) → Na + (aq) + Cl- (aq) + H2O (l)
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Reactions that Form Water
Notice that Cl- and Na + are spectator ions so the net ionic equation is:
H+ (aq) + OH- (aq) → H2O (l)
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Acid-Base ReactionsIn class
Page 187 – Example 7.4HMolecular equation:
HNO3 (aq) + KOH (aq) → H2O (l) + KNO3 (aq)
Complete ionic equation:H+ (aq) + NO3
- (aq) + K + (aq) + OH- (aq) →
H2O (l) + NO3- (aq) + K + (aq)
Net ionic equation:H+ (aq) + OH- (aq) → H2O (l)
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Acid-Base ReactionsNote two things when looking at the reaction between HCl and NaOH and the reaction between HNO3 and KOH:
1. The net ionic equation is the sameH+ (aq) + OH- (aq) → H2O (l)
2. Besides water, the second product formed is an ionic compound called a salt. The salt may precipitate or remain in solution, depending on solubility. How could you recover the salt if it is soluble? If it is insoluble?