Precipitation and Redox Reactions

How Reactions Occurwww.humbleisd.net/.../filedownload.ashx?...

TEK10 (H) understand and differentiate among acid-base reactions, precipitation reactions, and oxidation-reduction reactions;

OBJECTIVE 1: Precipitation Reactions

• Understand:• What a Precipitation Reaction is.• What makes a Precipitation Reaction Occur.• How the compounds swap Negative Ions.• Use Formula Writing and Solubility Rules to

predict products of a Precipitation Reaction

Precipitation Reactions

• What is a Precipitant?• A Product of a chemical reaction that is a:

• Solid• Gas

• Or Water (H2O) NOT AQUEOUS!!!!!!!!!!

• A Product must be INSOLUABLE.• How do you know if a product is insoluble?

• Look on your solubility chart.

Remember?

1. Chemical reactions start with REACTANTS and make PRODUCTS

2. Five Signs of a Chemical Change

1. Gas Given Off (Bubbles)

2. Color Change (BIGGIE!!!!!!)

3. Heat is Produced or Absorbed

4. Odor Change

5. Precipitate Forms (Solid is Made)

Precipitation Reactions

1. Also known as Double Replacement Reactions

2. Two or more aqueous Reactants are mixed to form one or more Products that at least one is:

1. A solid (insoluble)

2. A gas

3. Or WATER!!

How Do I Know the States of Matter1. Find the Ion that is in the

compound that is on the list in the first column.

2. Determine if that ion is normally soluble (aq) or Insoluble (s)

3. Look to see if the exception is present in the compound. If the exception is present then the opposite is true.

4. Example: 1. CaCO3: CO3

-2 is insoluble, Ca is not an exception so the compound is written CaCO3(s)

2. Na2CO3: CO3-2 is insoluble,

Na is an exception so the compound is written Na2CO3(aq)

Precipitation Reactions occur

1. when metals in two different compounds swap nonmetals.

2. Compound + compound compound + compound

AX + BY AY + BX

3. You MUST write NEW Formulas on every compound involved!

4. Determine if a product is a Solid, Gas or H2O

1. If all products are aqueous (aq) then NOTHING HAPPENS.

Let’s Review that Formula Writing Thing

• Copper (II) Chloride• Copper (II) means?• Copper has a charge of +2

• SO: Cu+2

• Chloride means?• Chlorine has a charge of -1• How do I know? Chlorine is in Group 17 and all of

them are -1.• It takes two Chlorines to cancel out Copper’s +2 charge:

• CuCl2• Remember: Drop and Swap the Numbers

Let’s look at a reaction

• Copper (II) Chloride is mixed with Silver Nitrate.• Again, Copper (II) Chloride is CuCl2 • Silver Nitrate is:

• Silver is always +1 so: Ag+

• Nitrate is always: NO3-1

• So: AgNO3

• AX + BY AY + BX• CuCl2 + 2 AgNO3 Cu(NO3)2 + 2 AgCl• Copper (II) Chloride + Silver Nitrate

Copper (II) Nitrate + Silver Chloride• The last names swap

Does this reaction happen (STATES OF MATTER – MATTER!!!!)

• Check your states of matter• CuCl2 is soluble so, CuCl2 (aq)• AgNO3 is soluble so, AgNO3 (aq)• Cu(NO3)2 is soluble so Cu(NO3)2 (aq)• AgCl is insoluble so AgCl (s)• CuCl2(aq) + 2 AgNO3(aq) Cu(NO3)2(aq) + 2 AgCl(s)• A solid formed so a reaction will occur• NOTICE: Cl had a 2 subscript in the reactants but not in

the products, why?• Every compound is different!! • You must Drop and Swap everytime!

Let’s try another

• Lead (II) Nitrate is mixed with Sodium Bromide• Write the correct formulas for the reactants:

• Pb(NO3)2

• NaBr• Swap last names:

• Lead (II) Bromide• Sodium Nitrate

• Write the correct formulas for the products:• PbBr2

• NaNO3

• Pb(NO3)2 + 2 NaBr PbBr2 + 2 NaNO3

Continue

• Pb(NO3)2 + 2 NaBr PbBr2 + 2 NaNO3

• Check your states of matter• WAIT A MINUTE, where did the in front 2’s come

from?????• BALANCING

• Pb(NO3)2 + 2 NaBr PbBr2 + 2 NaNO3

• All Nitrates are soluble• All Bromides are soluble except Ag+, Pb+2, Hg+2

• So,• Pb(NO3)2 (aq) + 2NaBr (aq) PbBr2 (s) + 2NaNO3 (aq)• A solid formed so a reaction occurs

Precipitation Reactions

• Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together

• Example:

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

Another example:

K2SO4(aq) + Ba(NO3)2(aq) 2 KNO3(aq) + BaSO4(s)

Practice

• Predict the products. Balance the equation

1. HCl(aq) + AgNO3(aq)

2. CaCl2(aq) + Na3PO4(aq)

3. Pb(NO3)2(aq) + BaCl2(aq)

4. FeCl3(aq) + NaOH(aq)

5. H2SO4(aq) + NaOH(aq)

6. KOH(aq) + CuSO4(aq)

Objective 2: Oxidation-Reduction Objective 2: Oxidation-Reduction (REDOX) Reactions(REDOX) Reactions

LEO SAYS GER

OBJECTIVES

1. Define oxidation and reduction in terms of the loss or gain of oxygen, and the loss or gain of electrons.

2. State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)

A process called “reduction” is the opposite of oxidation, and originally meant the loss of oxygen from a compound Oxidation and reduction always occur

simultaneously The substance gaining oxygen (or losing

electrons) is oxidized, while the substance losing oxygen (or gaining electrons) is reduced.

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox) Today, many of these reactions may not

even involve oxygen Redox currently says that electrons are

transferred between reactants

Mg + S → Mg2+ + S2- MgO (s)

•The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg2+

•The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S2-

(MgS)

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)11

2

00

22

ClNaClNa

Each sodium atom loses one electron:

Each chlorine atom gains one electron:

eNaNa10

10 CleCl

How do I know if REDOX occurs

• If one (or more) element(s) in the reaction loses electrons (LEO) and one (or more) element(s) in the reaction gains electrons (GER) then REDOX has occurred.

• It is that simple:• Write the equation• Determine the charges on every element• See if LEO-GER has occurred

Assigning Oxidation NumbersAssigning Oxidation Numbers

• An “oxidation number” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

• Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative element

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

1)The oxidation number of any uncombined element is zero.

2)The oxidation number of a monatomic ion equals its charge.

11

2

00

22

ClNaClNa

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

3)The oxidation number of oxygen in compounds is -2, except in peroxides, such as H2O2 where it is -1.

4)The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.

2

2

1

OH

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

5) The sum of the oxidation numbers of the atoms in the compound must equal 0.

2

2

1

OH2(+1) + (-2) = 0 H O

2

122

)(

HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

6) The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its ionic charge.

3

2?

ONX + 3(-2) = -1N O

24

2?

OS

thus X = +5 thus X = +6

X + 4(-2) = -2S O

Reducing Agents and Oxidizing AgentsReducing Agents and Oxidizing Agents

•An increase in oxidation number = oxidation

• A decrease in oxidation number = reduction

eNaNa10

10 CleCl

Sodium is oxidized – it is the reducing agent

Chlorine is reduced – it is the oxidizing agent

LEO says GER :LEO says GER :

eNaNa10

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

10 CleCl Chlorine is reduced

LEO says GER :LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent. Mg(s) + S(s) → MgS(s)

Mg is oxidized: loses e-, becomes a Mg2+ ion

S is reduced: gains e- becomes a S2-

ion

Mg is the reducing

agent

S is the oxidizing agent

Identifying Redox EquationsIdentifying Redox Equations

In general, all chemical reactions can be assigned to one of two classes:

1) oxidation-reduction (REDOX), in which electrons are transferred:• Single-replacement – Where an element replaces another

element in a compound (AX + B BX + A)• Combination – Where two or more elements combined to

form a single compound (A + X AX)• Decomposition – Where a single compound breaks down into

it’s individual elements (AX A + X)• Combustion – Where something burns in the air.

Non REDOX

This second class has no electron transfer, and includes all others:• Precipitation Reactions (Double-replacement)

(AX + BY AY + BX)

• Acid-base reactions (HX + AOH AX + H2O)

REDOX Reactions - Single Replacement (AX + B BX + A)

• Copper (II) Chloride reacts with Lithium to produce Lithium Chloride and Copper

• +2 -1 0 +1 -1 0• CuCl2 (aq) + 2 Li (s) 2 LiCl (aq) + Cu (s)• You have to use the Activity Series on the back of your

Periodic Table to determine if this will happen.• If the element by itself is above the element in the

compound then the reaction will occur.• If the element in the compound is above the element by

itself, then no reaction will occur.• Remember, METALS REPLACE METALS,

NONMETALS REPLACE NONMETALS.

REDOX Reactions - Single Replacement (AX + B BX + A)

• Practice:• Solid Copper is mixed with aqueous Silver Nitrate • Copper is above Silver on the Activity Series, so a

reaction will occur (a copper (II) compound is formed)• Cu (s) + 2 AgNO3 (aq) Cu(NO3)2 + 2 Ag• Check• Reactants Cu = 0 Products Cu = +2

Ag = +1 Ag = 0N = + 5 N = + 5O = - 2 O = - 2

• Notice N and O didn’t change only Cu and Ag

ACTIVITY SERIES CHART

• In a Single Replacement reaction, you look at the metals in the Reactants.

• If the individual metal is higher on the Activity Series Chart then the reaction WILL Occur.

• If the individual metal is lower on the Activity Series Chart, then the reaction WILL NOT occur.

• I always have students say, what do you mean higher on the chart? I MEAN HIGHER, ABOVE, FIRST.

• Lithium is higher (above) Barium.