Chemistry SM-1131Week 9 Lesson 1

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademyFall 2008

Class Today• Tests• Chemical Reactions• Types of Chemical Reactions• Balancing Chemical Reactions• Solubility• Precipitation Reactions

Tests

• There are still people needing to take tests.• They will hopefully be handed back on Friday• The range was huge. A handful of you really

aced that last test. There was a small section of Bs. There was a large section of 60-79, and then there were a handful of 10-30.

• We’ll go over it soon.• WIKI EXTENSION UNTIL NEXT WEDNESDAY

Chemical Reactions

• A chemical reaction is a rearrangement of atoms.

• Reactants Products• Once a reaction has happened chemists often

want to collect 1 of the products. Often times we’ll use solubility to help us… we’ll see how that works in a sec.

Evidence of a Chemical Reaction

• Heat and Light• Formation of solids• Gas emitting• Changing colors• Change in temperature

Some quick vocab

• (g) means the substance is a gas• (l) means the substance is a liquid• (s) means the substance is a solid• (aq) means the substance is aqueous• Aqueous means dissolved in water, which

does not necessarily mean the compound was a liquid. Ethanol and sugar both become aqueous, but only one of them was a solid at room temperature.

Rules about Chemical Equations

• Remember that law of conservation of mass?• Matter can neither be created nor destroyed.• We have to follow that!• So, whatever mass we started with on the

reactants side we’re going to have to have on the right side.

• We do this by making sure reactions are balanced. Mean we have the same number and types of atoms on both sides of the .

Counting molecules and atoms

• 2H2O

• 2 moles of water. To count atoms we’re going to use mutiplication. 2x2H= 4H, 2x1O= 2O

• 4Mg(NO3)2

• 4x 1Mg= 1Mg, 4x2N = 8N, 4x6O= 24O• The 2 and the 4 are called coefficients, just

like 5X in math where the 5 is the coefficient.

Types of Chemical Reactions

• There are 5 main types of reactions• Combination aka synthesis• Decomposition• Combustion• Single displacement• Double displacement• (Acid Base, gas evolution, precipitation,

oxidation and reduction aka redox)

Combination Reactions

• 2 things come together to make 1 thing.• Carbon and Hydrogen react to form the

compound methane.• C + H2 CH4

• This is called a skeleton equation since it’s not balanced. There is 1 C on the left, and 1C on the right, but there are 2H on the left and 4H on the right.

Balanced Chemical Reaction

• C + 2H2 CH4

• Note, when I’m talking about “things” I mean unique formulas. I recognize to balance this reaction it requires 3 moles or 3 molecules on the left, BUT, I’m only speaking about the “types of molecules,” and I’m not invoking the coefficients.

Combination Reaction 2

• Mg and oxygen react at high temperature what is formed?

• Mg + O2 MgO

• O2 is diatomic when written into reactions (HINClBrOF)

• (How’d I know MgO, well 4 steps: symbols, charges, switcheroo, reduce)

• To balance it…

Balancing Chemical Reactions

• ___ Mg + ___ O2 ___MgO

• 1- for all ionic compounds correctly establish their formula

• 2-List all elements in the rxn under the arrow• 3-Count the number of atoms of each type on

both sides of the equation• 4-Starting with metals change the coefficients

until both sides are balanced. DO NOT CHANGE CHEMICAL FORMULAS.

Balancing Chemical Reactions

• ___ Mg + ___ O2 ___MgO

___Mg___ ___O___• ___ Mg + ___ O2 ___MgO

1 Mg 1 2 O 1 NOT BALANCED• ___ Mg + ___ O2 _2_MgO

1 Mg 2 2 O 2 NOT BALANCED• _2_ Mg + ___ O2 _2_MgO

1 Mg 2 2 O 2 BALANCED

More examples

• N2 + 3H2 2NH3

• 2Al + 3F2 2AlF3

• P4 + 5O2 P4O10

• SO3 + H2O H2SO4

Decomposition

• When things decompose they break down. This reaction is where 1 molecule breaks down into several molecules.

• Iron(III) chloride decomposes at high temperature into it’s elements.

• FeCl3 Fe + Cl2

• Not balanced, again HINClBrOF

Decomposition

• ___ FeCl3 ___Fe + ___Cl2

___Fe___ ___Cl___ (count by atoms, not

Cl2)

• ___ FeCl3 ___Fe + ___Cl2

_1_Fe_1_ _3_Cl_2_

Decomp. Cont.

• _2_ FeCl3 ___Fe + _3_Cl2

_2_Fe_1_ _6_Cl_6_ • _2_ FeCl3 _2_Fe + _3_Cl2

_2_Fe_2_ _6_Cl_6_ • What’s the LCD of 3 and 2? 6 so Cl on both

sides needs 6. So how many FeCl3? Cl2?

Electrolysis of water• Electrolysis is when you put a current through water. Water

turns into it’s elements. Write and balance the reaction.• ___H2O ___H2 + ___O2

_2H 2 _1O_2 • _2_H2O ___H2 + ___O2

_4H_2 _2O_2• _2_H2O _2_H2 + ___O2

_4H_4 _2O_2

More examples of Decomposition

• CaCO3 CaO + CO2

• 2 HgO 2 Hg + O2

• 2 ClO3 2KCl + 3O2

Combustion Reactions

• Combustion means burning and fire. What two things does fire require? O2 and something to burn. We normally burn hydrocarbons (Hydro=H, Carbon = C therefore stuff made up of H and C).

• The products are always CO2 and H2O.

• Methane and Oxygen burn write the equation.• ___CH4+ ___O2 ___ CO2 + ___H2O

Balance a combustion

• ___CH4+ ___O2 ___ CO2 + ___H2O

__C __ __H __ __O __ (it comes from 2 places)

Balance this reaction now

Balanced Reaction

• _1_CH4+ _2_O2 _1_ CO2 + _2_H2O

1_C 1_ 4_H 4_ 4_O 4_ (it comes from 2 places)

Harder example

• C2H6 + O2 CO2 + H2O

2 C 1_ 6 H 2_ 2 O 3_• C2H6 + 3.5 O2 2 CO2 + 3 H2O

2 C 2_ 6 H 6_ 7 O 7_(no such thing as 0.5O2)

Double all the coefficients

• 2C2H6 + 7 O2 4 CO2 + 6 H2O

4 C 4_ 12 H 12_ 14 O 14_All combustion reactions will be just like one of

those two reaction.

Displacements

• These are the 2 hardest to tell apart when starting.

• Single displacements typically have 1 lone element on both sides of the reaction

• Double displacements look like the biggest reactions out there, and you’ll see that the two metals switch places

Single Displacement

• Magnesium metal starts making hydrogen gas when it’s dropped in aqueous hydrochloric acid. What’s the full reaction?

___Mg + ___HCl ___ H2+ __ ?

Single Displacement• ___Mg + ___HCl ___ H2+ __ MgCl?

• NO! 4 steps when writing ionic salts ALWAYS! So, it’s MgCl2

• ___Mg + ___HCl ___ H2+ __ MgCl2

1 Mg 1_ 1 H 2_ 1 Cl 2____Mg + _2_HCl ___ H2+ __ MgCl2

Single Displacement

• ___Mg + _2_HCl ___ H2+ __ MgCl2

• Notice: Mg is elemental on the left side, and Hydrogen is elemental on the right side.

• The single lone element was displaced by a different lone element.

More Examples

• 3AgCl + Al AlCl3 +3 Ag

• 2Na + H2O H2 + 2NaOH• Zn3N2 + 3Mg Mg3N2 + 3Zn

Double Displacementel double

• Silver nitrate and sodium chloride react to form silver chloride and sodium nitrate. Write and balance the reaction.

• KEY POINT: FIGURE OUT THE FORMULA FOR EACH INORGANIC PIECE AND DON’T MESS WITH THE FORMULA FOR THE REST OF THE TIME!

El Double

• AgNO3 + NaCl AgCl + NaNO3

• It’s already balanced.• The Ag and Na switched places.• That’s why it’s a double displacement.

El Double 2

• Barium nitrate and potassium sulfate reaction to form barium sulfate and potassium nitrate. Write and balance the equation.

Wiki Delay

• Due Wednesday of Next week. NO further extensions will be announced.

• Homework for chapter 7 is due Monday.