Chapter 7: Reaction in Aqueous Solutions Reaction Types.

Chapter 7: Reaction in Aqueous Solutions

Reaction Types

Section 7.1

Predicting whether a Reaction will occur

7.1 Predicting whether a reaction will occur

• What causes a reaction to “want” to occur?– Forming a solid– Forming water– Forming gas– Transferring electrons

• (Note: color change, energy released, etc. are SIGNS that a reaction has occurred, but do not say WHY a reaction would want to occur)

Section 7.2

Reactions that Form a Precipitate

Aqueous Solutions• Aqueous Solutions: • When compounds are dissolved in water they

form aqueous solutions• What does the word “dissolve” mean?– On a macroscopic level, when something

dissolves, it looks like it disappears– What is happening on a molecular level depends

on whether we’re talking about ionic or molecular compounds

Aqueous Solutions with MOLECULES

• MOLECULES (nonmetals only) do not break up into their elements when dissolved in water

• Example: sugar dissolves in water, but it DOES NOT break up into C, H, and O, it breaks up into individual C6H12O6 molecules

Aqueous solutions with IONIC COMPOUNDS:

• IONIC compounds break up into the cation and the anion when they are dissolved in water

• The water contains the separated ions• When it breaks up into its ions, the dissolved compound

can conduct electricity• An ionic compound that dissolves in water (not all ionic

compounds do) it is called an electrolyte• Example: NaCl in water

Example:• Write the formula for barium nitrate: Is the compound ionic??

• When barium nitrate dissolves in water, what will the solution look like? How many of each type of ion will barium nitrate produce?

• Write the formula for sodium carbonate. What will the solution look like after sodium carbonate dissolves? How many of each type of ion will be produced?

Sections 7.2 and 7.3

Double Replacement Reactions

Let’s say a chemist begins a reaction by dissolving Ba(NO3)2 and K2CrO4 into water to

form an aqueous solution… • Are these compounds ionic? If so, what ions will the

solution contain before the reaction?

• Draw a picture of the solution containing these ions below:

• If these two compounds react, what products do you think are likely to form from the reaction?

Think of the ions that are present in the solution. How could they get

together to form new substances?

Think of the ions that are present in the solution. How could they get together to form new substances?

– The metals (cations) will be attracted to the nonmetals (anions) because opposite charges attract

– Ba+2 and two NO3- ions could get together, but this would

reform the reactants – If one Ba+2 and one CrO4 -2 get together to form a

compound, and one K+ and one NO3- get together to form a compound, then we will have NEW products

– Write the ionic formulas for the products (CRIS-CROSS the charges):

– Write and balance the equation between these two reactants. Be sure to account for the fact that this reaction takes place in water:

This reaction is called a

double replacement reaction… • Double Replacement Reaction: Occurs when atoms or ions from

two different compounds replace each other.• Format:

AX + BY AY + BX(A and B “switch partners”)

*Do practice problems on double replacement reactions• Predict the products for the following double replacement

reactions:1. sodium hydroxide + iron (III) chloride

2. Al2(SO4)3 + Mg(ClO3)2

Section 7.3

Net Ionic Equations

Consider the balanced double replacement reaction that you wrote above between Ba(NO3)2 and K2CrO4 . Remember that many ionic compounds dissolve in water, and the compounds participating in the reaction can be written as ions.

• The equation you wrote is considered a molecular equation for the reaction

• Molecular Equation: shows the complete formulas of all the reactants or products

• Instead, we can write the ionic equation to get a more accurate picture of what is happening in the solution:

• Ionic Equation: the ions of the reactants and products are shown instead of the complete formulas

Write the ionic equation for the reaction between barium nitrate and potassium chromate:

– Notice that in this equation, nothing really happens. The ions in the products look just like the ions in the reactants (maybe we wrote them in a different order, but nothing has changed).

When we get two compounds AX and BY together that dissolve in water, the ONLY ways

to get a reaction are…1. If a cation and an anion get together and they do

NOT dissolve in water, then a precipitate is formed– Precipitate: solid that forms as a result from a

reaction2. If a cation and an anion get together and form

water (like H+ and OH-)3. If a cation and an anion get together and form a

gas that exits the solution

When barium ions meet up with chromate ions in the solution, they form a solid.

• So, when a barium ion and a chromate ion find each other, a precipitate forms and the potassium ions and the nitrate ions are left behind in the solution

• Consider the ionic equation that you wrote above for the reaction between barium nitrate and potassium chromate. Because the product barium chromate is a solid, it will not exist as separate ions at the end of the reaction.

• Write the ionic equation above to account for the new solid being formed:

Notice that in your equation, the potassium and nitrate exist as ions on both the reactant side and the product

side of the reaction. These are spectator ions.• Spectator ions: ions that do not participate in a reaction• Just like in math class, when compounds exist on both sides

of the equation, they can be “canceled out”. After canceling out the spectator ions, you can rewrite the net ionic equation for the reaction.

• Net Ionic Equation: includes only those components that are directly involved in the reaction.

Write the net ionic equation for the reaction between barium nitrate and potassium chromate:

NOTE:

• If both products for a reaction are aqueous, after the “” write N.R. for NO REACTION. If every ion is the same on both sides of the equation, no reaction occurred.

Write the balanced net ionic equation for the reaction between aqueous potassium hydroxide and aqueous iron (III) nitrate. NOTE: the compound containing iron after the reaction does not dissolve.

Write the balanced net ionic equation for the reaction between aqueous sodium chloride and aqueous lithium nitrate. Both products are aqueous.

Write the balanced net ionic equation for the reaction between aqueous ammonium chloride and aqueous lead (II) nitrate react to form a solid containing lead.

Section 7.2

Solubility

The term “solubility” refers to how easily something dissolves in water:• Soluble solid: a solid that readily dissolves or

dissociates into its ions in water• Soluble solids are electrolytes in solution (form ions,

can conduct)• Insoluble solid (or slightly soluble solid): a tiny, tiny

amount dissolves in water, but this amount is INVISIBLE to the naked eye

• Can you think of anything familiar that is insoluble in water?

In order to predict whether a precipitate will form from a double replacement reaction, we use the following solubility rules:

Soluble (WILL dissolve) Exceptions (WON’T dissolve)

Compounds w/ …

Ionic compounds containing nitrate, chlorates, acetate

Na+, K+, and NH4+ salts

Cl-, Br-, I- salts Compounds w/ Ag+, Hg22+, and Pb2+

SO42- Compounds w/ Ba2+, Pb2+, and Ca2+

Insoluble (WON’T dissolve) Exceptions (WILL dissolve)

Compounds w/ …

S-2, CO3-2, PO4

-3 salts Compounds w/ H, Na, K, NH4+

OH- salts Compounds w/ group 1 metals, NH4+ Ca+2,

Ba+2

NOTE: in these rules, a “salt” refers to an ionic compound

Is it soluble??• Sodium hydroxide• Aluminum chloride• Silver nitrate• Silver bromide• Sodium carbonate• Barium sulfate• Barium hydroxide• sodium carbonate **soluble• potassium phosphate **soluble

Practice: For each of the following reactions…

A. Predict the products for the reactionB. Write the balanced net ionic equation

Aqueous calcium nitrate and potassium hydroxide react.

Aqueous barium hydroxide reacts with sodium sulfate.

Aqueous iron (III) chloride reacts with lead nitrate.

Aqueous magnesium nitrate reacts with ammonium sulfate

Aqueous calcium chloride reacts with phosphoric acid

Section 7.4

Reactions that form Water

Acid:– Produces H+ in water– Seven acids are STRONG electrolytes (separate into ions). To be

considered a strong acid, a compound needs to be a strong electrolyte.

– Seven strong acids: MEMORIZE!!• HCl• HBr• HI• HNO3

• HClO4

• HClO3

• H2SO4

– Weak acids do not completely dissociate (separate) into ions in solution

Base:– Produce an ion that accepts the H+ from an acid– Most common strong bases (strong electrolytes that

separate completely into ions) produce the hydroxide ion when dissolved in water

– Strong base examples:• LiOH• NaOH• KOH• Ba(OH)2

Let’s look at a reaction between a strong acid and a strong base:

• Write the MOLECULAR equation for the reaction between aqueous nitric acid and sodium hydroxide:

• Now write the ionic equation for the reaction:

Based on the products of your reaction above, we see that a reaction between a strong acid and a strong

base always produces liquid water and a salt.

• Salt = ionic compound• Strong acid/base reactions are also

considered to be double replacement reactions:– AX + BY AY + BX – Where “A” is hydrogen, “Y” is hydroxide, and

“AY” is water

Write the net ionic equation for the reaction between nitric acid and sodium hydroxide.

• NOTE: The hydrogen and hydroxide ion join together to form water… these ions will NOT be separated from each other in the solution. Because they’re not separated, we can’t cancel hydroxide and hydrogen ions out of our net ionic equation.

Practice– Write the balanced molecular equation for each

of the following reactions between acids and bases.

– Write the net ionic equation for each reaction.

Aqueous perchloric acid and potassium hydroxide react

Aqueous hydrochloric acid and barium hydroxide react

Aqueous sulfuric acid and sodium hydroxide react

Aqueous chloric acid and calcium hydroxide react

Section 7.5

Oxidation-Reduction Reactions

Red-ox reaction: • Red-ox reaction: involves the transfer of electrons• We will look at red-ox reactions with two different

formats…• FORMAT 1:– Write the balanced equation for the reaction between

sodium metal and chlorine gas to produce solid sodium chloride:

What is happening when this reaction occurs??

• Na metal is GIVING its 1 valence electron to one of the Cl atoms in chlorine gas– Na Na+ + 1e-– Cl + 1e- Cl-

• Na+ and Cl- form the ionic compound NaCl

– Because electrons were GIVEN TO Cl, we say Cl was reduced (given negative charges)

– Because electrons were TAKEN from Na, we say Na was oxidized

FORMAT 2:

– Also called a single replacement reaction• A + BX AX + B• Called single replacement b/c “A” replaces “B” in

the compound

– Write the balanced equation between solid aluminum and solid iron (III) oxide:

What’s happening when this reaction occurs??

• Al metal has to LOSE 3 electrons to become part of the aluminum oxide ionic compound– Al Al+3 + 3e-– Al has been oxidized

• Fe has to GAIN electrons to go from Fe+3 in the ionic compound to just plain Fe– Fe+3 + 3e- Fe– Iron has been reduced

• Some oxidation reduction reactions form a gas that bubbles out of solution

• Anytime a HONClBrIF is formed from a reaction, it will escape the solution as a gas– Example: Write the balanced equation for the reaction

between hydrochloric acid and solid zinc:

Write the balanced equation for the reaction between solid sodium and hydroiodic acid.

NOTE:

• Some red-ox reactions involve electron transfer reactions between nonmetals (we won’t go over this in detail yet). For now know that the presence of oxygen as a reactant or product indicates that a red-ox reaction has occurred

Practice Problems

a. Predict the products for the following oxidation-reduction reactions

b. State which element was oxidized and which element was reduced for each example

Solid aluminum and iodine gas react

• Oxidized: Reduced:

Solid barium and cobalt (II) phosphate react

• Oxidized: Reduced:

Solid iron (III) and copper (II) chlorate react

– Oxidized: Reduced:

Solid iron (III) and oxygen gas react

• Oxidized: Reduced:

Solid sodium and nickel (II) phosphate react

• Oxidized: Reduced:

Summary:

Reactions we’ve Learned so Far

1. Double replacement reactions: AX + BY AY + BX

A. Either have format…i. A solid precipitate is formed when two aqueous ionic

compounds are mixedii. A strong acid and a strong base produce water and a salt

B. Need to be able to…i. Recognize the double replacement reaction and predict the

products• Recognize if the reaction produces a precipitate, liquid

water, or if no reaction occurs• Write the ionic and net ionic equation for the reaction

2. Single replacement reactions and Red-Ox Reactions:

a. Either have format…i. Metal + Nonmetal Ionic Compoundii. A +BX AX + B

b. Need to be able to…i. Recognize a single replacement reaction or a red-ox reaction

between a metal and a nonmetalii. Predict the products for the reaction

Predict the products for the following reactions and write the balanced chemical equation. If no reaction occurs, write N.R.

• NOTE: If a polyatomic ion in the example is NOT included on your solubility rules, assume that compounds containing that ion ARE soluble

Practice Problems: DR and SR reactions

1. aqueous calcium acetate reacts with aqueous sodium carbonate2. solid strontium reacts with iron (III) hydroxide3. sodium nitrate reacts with phosphoric acid 4. sulfuric acid reacts with sodium hydroxide5. aqueous barium hydroxide reacts with aqueous sodium phosphate6. solid magnesium reacts with nitrogen gas7. solid calcium reacts with solid copper (I) phosphate8. aqueous calcium acetate reacts with aqueous potassium carbonate 9. solid copper (II) chloride reacts with solid potassium10. aqueous calcium hydroxide reacts with aqueous calcium chlorate

Section 7.7

Other Reaction Types

A. Combustion ReactionsCombustion reactions: involve oxygen and produce energy (heat).

• Usually occurs between a hydrocarbon and oxygen, and the products are ALWAYS simply carbon dioxide and water

• Combustion reactions follow the format below:CxHy + O2 CO2 + H2O

CxHyOz + O2 CO2 + H2O

– In both these examples, the compound containing C and H is the hydrocarbon

– x, y, and z correspond to subscript numbers in the hydrocarbon

Write and balance the following equations:1. When we light our Bunsen burners, a reaction

occurs between methane (carbon tetrahydride) and oxygen in the air. Write the reaction for this process.

2. When you barbeque, the propane (tricarbon octahydride) in your gas tank reacts with oxygen in the air.

B. Synthesis

• Synthesis (Combination) Reactions: a given compound is formed from simpler materials. In synthesis reactions, two or more reactants combine to form one product.

• Format: A + B AB

Balance the following synthesis reactions

• 1. H2 + O2 H2O

• 2. N2 + H2 NH3

C. Decomposition Reactions• Decomposition Reactions: a single compound is

broken down into simpler compounds or its component elements

• Format: AB A + B• Usually accomplished by heating a compound or

passing an electric current through a compound• Opposite of a synthesis reaction

Predict the products and balance the following decomposition reaction:

• MgF2

• NO