CHEMICAL REACTIONS

SO FAR . . . So far we have looked at the

properties of individual atoms and molecules Atomic number Atomic mass Ions Lewis dot structures Molecular shapes Naming Moles

HOW DO THESE ATOMS AND MOLECULES INTERACT?

The way that these atoms and molecules interact with each other is through chemical reactions

Chemical reaction: Change of one or more substances into a different substance.

BEFORE WE BEGIN Important concept (by Antoine

Lavosier):Conservation of mass: Matter is

neither created or destroyed in a chemical reaction

In other words, you must have the same components in the beginning and end or a chemical reaction

GENERAL TYPES OF REACTIONS

1. Gas evolution reactions: these are reactions that occur in liquids and produce a gaseous product

For example – Alka Seltzer in H2O

2. Oxidation-reduction reactions: electrons are transferred from one substance to another

For example – rust formation, batteries

GENERAL TYPES OF REACTIONS

3. Precipitation reactions: reactions that occur in liquid that produce a solid substance

IN MORE DEPTH . . .

We will discuss these reactions more in depth at a later time

HOW DO WE KNOW THAT A CHEMICAL REACTION HAS TAKEN

PLACE?

Many chemical reactions produce easily detectable changesColor changeFormation of a solid Formation of a gasHeat absorption or emission (uses

up or produces heat)Light absorption or emission

HOW DO WE KNOW THAT A CHEMICAL REACTION HAS TAKEN

PLACE?

The previous are only INDICATIONS of a chemical reaction

Chemical analysis is needed to verify if a reaction is actually occurring. For example: Boiling water looks like a

gas is being produced, but this is a physical change, NOT A CHEMICAL CHANGE

H2O(l) H2O(g) NO NEW SUBSTANCE

SHOWING CHEMICAL REACTIONS

Chemical equations are a way that chemists summarize a chemical reaction that takes place

Reactions take the following general format

Reactants ProductsA set of chemicals react to form

a set of products

REACTIONS AND ENERGYChemical reactions either absorb

energy/heat from their surroundings or they release energy/heat:

EXOTHERMIC: a chemical reaction that produces heat (ie. Burning gas in your car)

ENDOTHERMIC: a chemical reaction that absorbs heat (ie. Icy hot)

SHOWING CHEMICAL REACTIONS

In chemical reactions, the state of matter is usually described:

1. Solid (s)2. Liquid (l)3. Gas (g)4. Aqueous (aq)

PRACTICE For each of the following, translate

the sentence into a chemical reaction:

1. Solid zinc is mixed with aqueous hydrochloric acid to form zinc chloride and hydrogen gas

2. Aqueous sulfuric acid is combined with solid gold to produce aqueous gold(I) sulfate and hydrogen gas

ANSWER

1. Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)

2. H2SO4(aq) + Au Au2SO4(aq) + H2(g)

HOW DO WE APPLY CONSERVATION OF MATTER TO CHEMICAL

REACTIONS?

C3H8(l) + O2(g) CO2(g) + H2O(l)

Let’s examine this reaction closer: How many atoms of each element are on

each side of the reaction?Reactants Products

C CH HO O

BALANCING THE REACTION

According the law of conservation of matter, you must have the same number of each atom on each side

NO MATTER is created or destroyed

BALANCING THE REACTION

Therefore, we must BALANCE the reaction to get the same number of atoms on the reactant side and the product side

To balance we can add more MOLECULES, but you cannot change the molecule

BALANCING THE REACTION

C3H8(l) + O2(g) CO2(g) + H2O(l)

Therefore, we CAN add more C3H8

For example: we can have 3 C3H8

But we CANNOT change the molecule itself Cannot make C3H8 into CH8

TRY TO BALANCE THE EQUATION

C3H8(l) + O2(g) CO2(g) + H2O(l)

ANSWER C3H8(l) + 5O2(g) 3CO2(g) + 4H2O(l)

Let’s look at each atom on the reactant and product side:

Reactant ProductC 3 C 3H 8 H 8O10 O 10 Same number on each side =

BALANCED

WHAT ARE THE NUMBERS IN FRONT OF EACH

COMPOUND? C3H8(l) + 5O2(g) 3CO2(g) + 4H2O(l)

The numbers in the front of each compound tells you how many MOLES you combine.

How many moles of each compound do you have in this reaction?

BALANCING TIPS Just balancing the equation is

fairly difficultTherefore there are some tips to

help you balance the equation

TIP #1 The first thing to do is to write the

skeletal reaction with just the compounds involved:

For example: Aqueous sodium hydroxide and aqueous

sulfuric acid mix to form liquid water and aqueous sodium sulfate

TIP #2 H2SO4(aq) + NaOH(aq) H2O(l) +

Na2SO4(aq)

If an element occurs in only 1 compound on both sides of the reaction, balance it first. Otherwise, balance metals before nonmetals

TIP #3 H2SO4(aq) + 2NaOH(aq) H2O(l) +

Na2SO4(aq) Not balanced yet

Save diatomic molecules and/or water for last H2SO4(aq) + 2NaOH(aq) 2H2O(l) +

Na2SO4(aq)

SIDE NOTE SIDE NOTE: Most times, when you have a

polyatomic ion, do not break it up. Think of the polyatomic ion as one atom for balancing. It will save you some grief.

H2SO4(aq) + 2NaOH(aq) 2H2O(l) + Na2SO4(aq)

The SO4 in the reactant is balanced by the SO4 in the product. Don’t balance all the S and the O

TIP #4 If a balanced equation contains

coefficient fractions, clear these by multiplying the entire equation by the appropriate number

C8H18 + O2 CO2 + H2O C8H18 + O2 8CO2 + 9H2O C8H18 + (25/2)O2 8CO2 + 9H2O 2C8H18 + 25O2 16CO2 +18 H2O

TIP #5 Check to make sure the equation is

balanced by summing the atoms on each side of the reactionReactant Product

C 16 C 16H 36 H 36O 50 O 50

TRY THESE1. Solid aluminum and aqueous sulfuric

acid combine to form aqueous aluminum sulfate and hydrogen gas

2. Solid iron combines with gaseous oxygen to form solid iron(III) oxide

3. Liquid ethanol (C2H6O) combines with gaseous oxygen to form gaseous carbon dioxide and liquid water

ANSWER1. 2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) +

3H2(g)

2. 4Fe(s) + 3O2(g) 2Fe2O3(s)

3. C2H6O(l) + O2(g) 2CO2(g) + 3H2O(l)

SPECIAL NOTE All reactions are reversible. They can

go forwards or backwards. For example:

4Fe(s) + 3O2(g) 2Fe2O3(s)

2Fe2O3(s) 4Fe(s) + 3O2(g)

To show this, we write reactions with double arrows 4Fe(s) + 3O2(g) 2Fe2O3(s)

SPECIFIC TYPES OF CHEMICAL REACTIONS

There are 4 specific types of chemical reactions.

1. Synthesis (combination) reactions2. Decomposition reactions3. Displacement reactions4. Double displacement reactions5. Combustion reactions (special)

SYNTHESIS REACTIONS A synthesis reaction is when you take

simpler molecules and combine them to form more complex molecules

They have the following general set up A + B AB Example: 2Na(s) + Cl2(g) 2NaCl(aq)

DECOMPOSITION REACTIONS These are the opposite of the synthesis

reactions. A more complex compound breaks down to form simpler compounds.

They have the following general set up AB A + B Example: 2H2O(l) 2H2(g) + O2(g)

DISPLACEMENT REACTIONS (SINGLE DISPLACEMENT)

One element displaces (or takes the place of) another element in a compound.

They have the following general set up A + BC AC + B Example: Zn(s) + CuCl2(aq) ZnCl2(aq) + Cu(s)

DOUBLE DISPLACEMENT REACTION

Two elements, in separate compounds, displace (or switch places with) each other.

They have the following general set up AB + CD AD +BC Example: AgNO3(aq) + NaCl(aq) AgCl(s) +

NaNO3(aq)

COMBUSTION REACTIONS When a compound combines with

oxygen to form carbon dioxide, water and heat

They have the following general set up Molecule + O2 CO2 + H2O + heat

Example: 2C2H6 + 7O2 4CO2 + 6H2O + heat

TRY THESE Translate, balance and tell what type

of reactions the following are:1. Chlorine gas combines with

sodium bromide to form sodium chloride and bromine gas.

2. Mercury(II) oxide breaks apart to form mercury and oxygen gas.

ANSWER1. Cl2 + 2NaBr Br2 + 2NaCl

The chlorine displaces the bromine in sodium bromide

This is a SINGLE DISPLACEMENT reaction

2. 2HgO 2Hg + O2

The mercury(II) oxide breaks apart into mercury and oxygen

This is a DECOMPOSITION reaction

GENERAL TYPES OF REACTIONS (EXPANDED)

Before we can describe the general types of reactions, in detail, we have to visit the concept of solubility

Soluble: the substance in question will dissolve in water

Insoluble: the substance in question will not dissolve in water

WHY SOLUBILITY If you know what compounds are

soluble, you can figure out if a reaction is: Gas evolution reaction Precipitation reaction

If soluble, the reaction will not form a precipitate or evolve a gas

SOLUBLE (VERY IMPORTANT – MAKE SURE YOU COPY)

Soluble1. All acetates (C2H3O2

-) except Fe+3

2. All ammonium (NH4+) compounds

3. All bromides (Br-) except Ag+, Hg2+2,

and Pb+2

4. All chlorates (ClO3-)

5. All chlorides (Cl-) except Ag+ and Pb+2

SOLUBLE (VERY IMPORTANT – MAKE SURE YOU COPY)

6. All iodides (I-) except Ag+, Hg2+2, and

Pb+2

7. All nitrates (NO3-)

8. All perchlorates (ClO4-)

9. All sulfates (SO4-2) except Ca+2, Ba+2,

Pb+2, Sr+2, Hg2+2

INSOLUBLE Insoluble

1. Carbonates (CO3-2), except group 1 and

NH4+

2. Chromates (CrO4-2), except group 1 and

NH4+

3. Hydroxides (OH-), except group 1, Ca+2, Sr+2, and Ba+2

4. Oxalates (C2O4-2), except group 1 and

NH4+

INSOLUBLE5. Oxides (O-2), except group 1, Ca+2, Sr+2,

and Ba+2

6. Phosphates (PO4-3), except group 1 and

NH4+

7. Sulfides (S-2), except those of group 1, group 2, and NH4

+

SOLUBLE OR INSOLUBLE?1. H2SO4

2. Ca(C2H3O2)2

3. AgNO3

4. Na2O

5. Li2CO3

6. BaC2O4

7. Mg(OH)2

ANSWER1. Soluble2. Soluble3. Soluble4. Soluble5. Soluble6. Insoluble7. Insoluble

WHY DO WE CARE ABOUT SOLUBILITY?

By knowing the solubility rules, you will be able to do further analysis to determine if a chemical reaction will form a precipitate

If a chemical does not precipitate out of solution (have the product insoluble), then the reaction will not be a precipitation reaction

EXAMPLE Examine the reactions below,

determine which reaction produces a precipitate or not

1. Ba(OH)2 + K2O BaO + 2KOH

2. 2H3PO4 + 3BaO Ba3(PO4)2 + 3H2O

ANSWER1. Ba(OH)2(aq) + K2O(aq) BaO(aq) +

2KOH(aq) 1. BaO is soluble2. KOH is soluble3. Since both products are soluble, there is no

precipitate2. 2H3PO4(aq) + 3BaO(aq) Ba3(PO4)2(s) +

3H2O(l)1. Ba3(PO4)2 is insoluble.2. This means that as the reaction progresses,

the Ba3(PO4)2 is a precipitate that is produced.

GENERAL TYPES OF REACTIONS (REVISITED)

Precipitation reactions

Only nonsoluble compounds form precipitates

If any product is insoluble, it forms a precipitate

PRECIPITATION REACTION How to determine:1. Write the two compounds in question

as reactants1. Na2CO3 + CuCl2

2. Write the formulas of the potential products of the reaction

1. Na2CO3 + CuCl2 NaCl + CuCO3

PRECIPITATION REACTION3. Look at the solubility rules to

determine the solubility of the products

1. NaCl soluble2. CuCO3 insoluble

4. Write the equation with states of matter

1. Na2CO3(aq) + CuCl2(aq) NaCl(aq) + CuCO3(s)

PRECIPITATION REACTION5. Balance the equation

1. Na2CO3(aq) + CuCl2(aq) 2NaCl(aq) + CuCO3(s)

GAS EVOLUTION REACTIONS Whenever a reaction produces a gas,

that gas will leave the reaction (bubble forms)

If a product is a gas, then you have a gas evolution reaction HCl(aq) + NaHCO3(aq) H2O(l) +

NaCl(aq) + CO2(g) This is the reaction of Alka Seltzer with

your stomach acid

OXIDATION-REDUCTION REACTIONS

These reactions (redox) involve the transfer of electrons from one element to another

In order to understand these reactions better, we must look at the charges of the elements in a chemical reaction

DEFINITIONS Oxidation: when an element loses

electrons (the atom becomes +) Reduction: when an element gains

electrons (the charge is reduced, becomes -)

Oxidation and reduction must occur together so the electrons can travel from one element to another

OXIDATION-REDUCTION REACTIONS

Example 4Na(s) + O2(g) 2Na2O

4 Na (s) 4 Na+ + 4e- 4e- + 2O 2O-2

In this example, Na goes from a neutral charge to a +1, it needed to lose an electron

O goes from a neutral charge to a -2, it had to gain electrons

TRY THESE In each of the following reactions,

identify the element that is oxidized and the element that is reduced:

1. 2Na + Cl2 2NaCl

2. 4Fe + 3O2 Fe2O3

ANSWER1. 2Na + Cl2 2NaCl

1. 2Na 2Na+ + 2e- (oxidized)2. 2e- + 2Cl 2Cl- (reduced)

2. 4Fe + 3O2 2Fe2O3

1. 4Fe 4Fe+3 + 12e-2. 12e- + 6 O 6 O-2

SUMMARY There are 3 general types of reactions

Precipitation One of the products must be insoluble Use the solubility rules

Gas evolution If a gas is produced, it will come out of solution

Oxidation-reduction Involves the movement of electrons Something must be oxidized (lose electrons)

and something must be reduced (gain electrons)

SUMMARY There are 5 specific types of

reactions Synthesis

A + B AB Decomposition

AB A + B Displacement

A + BC AB + C Double displacement

AB + CD AD + BC Combustion

SUMMARY With any sort of chemical reaction, we

must follow the law of conservation of mass

Therefore, every chemical reaction must have the same number of atoms in the reactants and products

To balance, use the following tips

SUMMARY

1. Write the skeletal reaction with just the compounds involved.

2. If an element occurs in only 1 compound on both sides of the reaction, balance it first. Otherwise, balance metals before nonmetals

3. Save diatomic molecules and/or water for last

4. If a balanced equation contains coefficient fractions, clear these by multiplying the entire equation by the appropriate number

5. Check to make sure the equation is balanced by summing the atoms on each side of the reaction