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Notes: Chemical Reactions (text Ch. 8)

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NOTE: This set of class notes is not complete. We will be filling in information in class. If you are absent, it is yourresponsibility to get missing information from a fellow classmate or the chemistry website:

A chemical reaction is the rearrangement of atoms to form new substance(s).Reactant(s) appear on the left and product(s) appear on the rig t.

Example: Bonds are broken in the diatomic molecules H2 and O2, and new bonds are formedbetween the hydrogen and oxygen atoms to form H2O. The SAME ATOMS are on both sides of theequation.

Diagram: CO00

reactants

2 H2 + O2product

2 H20 H-O

H- •• cft> + efto--«

5 >icjns of a chemical reaction (this is a review from Matter and Change unit):1. a gas is produced2. light is produced3. a solid (precipitate) is formed from 2 liquids4. temperature change

a. exothermic (hot- heat is released): Reactants - Products + HEATb. endothermic (cold- heat is absorbed: Reactants + HEAT -> Products

5. permanent color change

Balancing Equations

According to the Law of Conservation of Mass, matter is not created or destroyed in a chemicalreaction. So the mass of all reactants must equal the mass of all products, (this is a review)

total mass of reactants in the reaction below?

2 H2O Answer: _ 72 g /72 g

Example 1: What is the

02 H2 + 02ZJ

¦72.3Example 2: What is theAgNOs + NaCI (->)170 g 56 g

Example 3: What is the

Ca + ZnCOs40g 125g

total mass of products in the reaction below?AgCI + NaNOs Answer: 22(i> q

? 2

mass of zinc produced in the reaction below?

CaCOs + Zn Answer: _100 g ?

I 1 ' I I/(*£ Ib5 - I00 = 5

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Because the Law of Conservation of Mass always holds in chemical reactions, we must always besure we are working with a balanced equation. A balanced equation is one in which the number ofatoms of each element is the same on both sides of the arrow.

Example of an equation that IS NOT balanced:

Na + FeCIs NaCI + FeAtom inventory: j -Na- |

i -Fe- i Not alanc !_ l

Example of an equation that IS balanced:

Atom inventory:3 Na + FeCIs 3 NaCI + Fe

3-Na-3~

• -Fe- 1 Balanc !3 -ci- 3

The big numbers that are used to balance the equation are called coefficients.2 Fe(s) + 3 CL (g) - 2 FeCIs (s)

Coefficients

Coin BeChan el

Subscripts are the small numbers written in formulas; they stand for the number of atoms orpolyatomic ions immediately to their left. ubs ipts' Cftrtnob B Chdnij O !

Examples: FeCJs 3 is a subscript, meaning there are 3 Cl atoms-Mg(N03)2 3 is a subscript, meaning there are 3 O atoms; 2 is a subscript, meaning

there are 2 NOs (2 N atoms and 6 O atoms all together)N/03- N03 Mg-I = 0 (e

***When balancing equations, ONLY add coefficients to balance. NEVER alter a chemical formula inthe equation in order to balance (do not change SUBSCRIPTS!!).***

CaO + 5 C CaC2 +

I - O - i1 - 0 - t

3 - c - 3

2

COExample

Writing Chemical Formula Equations from Word Equations:Symbols Used in Chemical Reactions

(s) solid (1) liquid (g) gas

(aq) aqueous solution (t e substance is dissolved in water; example: NaCI(aq) is salt water)

+ separates 2 reactants or products, reads as reacts with on reactant side, reads as and onproduct side

- separates reactants from products; read as yields or produces

a heat added to reactants (triangle appears above arrow)

Catalysts are substances that increase the rate of the reaction but are not used up themselves. Theformula for the catalyst is written above the arrow: (example MnOa)

Catalysts are not used up inthe reaction so they ap earabove the arrow

2 KCIOs (s) Mn°2 2KCI(S) + 3 02(g)

***IMPORTANT to remember in writing chemical equations***

Diatomic elements: These 7 elements do not occur as single atoms. If they are not combined withanother element(s), they must always be written as pairs (diatomic):

l2, Bra, Cla, Fa, Oa, Na, Ha, NOT I, Br, Cl, F, O, N, H !!!!!

To help you remember the 7 diatomic elements: I Bring Clay For Our New Hut, HQBrFINCI, or 7 (yourteacher will show you the 7 on the periodic table). [N 0Example

Solid sodium metal reacts with water produce aqueous sodium hydroxide and hy rogen gas.1. Turn it into a formula equation first (don t forget to add (s), (I), (g), or (aq)), review

nomenclature notes if you need help with formula writing.2. Balance!

fCTr

1an H

2.NAC ) t Z.H20u) ZNJaOH ca + Mz Cp

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SeeI. S lypes of Chemical Reactions

5 in < x+Kt PacKt

We will learn 5 basic reaction types in this class. More types exist, but we will not go into them here.

1. Synthesis (also called Combination or Composition)

Synthesis means pu

2 or more elements/simple compounds combine to form 1 compound.

General form: A + B -> AB

Identifying feature: onl one p odvK-V

Analog : A boy and a girl come to the dance separately, but end up dancing together.

Example: 2 Mg + O2 - 2 MgO

2. Decomposition

Decomposition means apar-r •

1 compound is broken down into 2 or more simpler elements/compounds.

General form: AB - A + B

Identif ing feature: only one r fl d & l * (opposite of Synthesis!)

Analogy: A boy and a girl come to the dance together, but argue and stay apart the rest of the night.

Examples: 2 NaCI - 2 Na + CI22 KCIOs - 2 KCI + 3 O2

3. Single Replacement

1 element takes the place of another in a compound ( like replaces like )

A BC - B + ACD + BC - C + BD

General forms: (metal replacement)(halogen replacement)

Identifying feature: 1 element + 1 compound on each side of the arrow

Analo : A boy and a girl are dancing, but then another boy cuts in and dances with the girl, leavingthe first boy alone.Or a boy and a girl are dancing, but then another girl cuts in and dances with the boy, leaving thefirst girl alone.

Examples: |iK repla.C CS like.Metal replacement: 2 Na + CuCh - 2 NaCI + Cu

Halogen replacement: F2 + 2 KCI - 2 KF + C

BUT the boy/girl will not always be able to cut in.” Sometimes the other boy/girl will not let them!

We use an activity series to predict whether or not the replacement will occur.If an element is more reactive than (above) another element, it can replace that element.

Metal Activity Series

!!! ; gacK or P- *lS

Halogen Activity Series (same order as on PT!)F (most reactive)ClBrI (least reactive)

Can Al replace Li? 00

Can Cu replace Au?

Can Br replace I?

Can Cl replace F? HO

*Note: The activity s ries is only used for single r placement reactions.

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4. Double Replacement

Ions from 2 ionic compounds switch places.

General form: AB + CD - CB + AD

- A and C (the cations from each compound) switch places

Identifyin feature: 2 compounds on each side of the equation

Analo y: 2 boy-girl pairs are dancing, and they switch partners.

Example: FeCb + Na COs - 2 NaCI + FeCOs¦ -r ,

inside ou-KicKBo 60x

Note: An acid-base (neutralization) reaction is a special ty e reaction. Itoccurs when an acid and a base react to form an ion and

Examples: HCI + NaOH Ndd H2Q

5. Combustion

Also known as

HjSQ4 +2k|oh + |- O

Nr1® Cl ® = rJaCl

K S j® KzS0

HOHHOB is jT

Always follows the same form:

Compound containing C and H (& sometimes O) + O2 CO2 + H2O

Note: In a combustion reaction, the compound always burns in gas and always releasescarbon dioxide and water.

During incomplete combustion (a limited amt. of O2), carbon monoxide (CO) is also produced.

Examples: CFU + 2 O2 CO2 + 2 H2O

2 CaHe + 9 O2 6 CO2 + 6 H2O

***Trick for balancing tough combustion reactions:

Balance the H first by placing a coefficient in front of the water. If that coefficient is ODD, double itand proceed with balancing C, then O. Always balance oxygen last!!!

Example: <3 C7H14 + o?] O2 iH CO2 + H H2O

EVEN/ #

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II. Predicting Products

For each of the following types of reactions:1. predict the products2. balance the equation using the lowest possible integers

Your teacher will work these examples with you.

1. Synthesis A + B AB PU+ -ItXje-Hne

Example 1: Na + Bra- N<x+ fi 1 s NaBr

Example 2: 2.AI +3012 AICI3 - ICI3

2. Decomposition AB A + B S i£.rx,K tX r +

+- 0 Example 1:2 AgaO -

Example 2: ZHgO + O2.

3. Single Replacement (check activity series!)A + BC B + ACD + BC C + BD

C ttK po piafomiCS A

wdE

(metal replacement)(halogen replacement)

Example 1: Cla + KF - HO ti x c+fDn !

Example 2: Ca + Aids 2AI + 3CaCI2

4. Double Replacement (includes acid-base reactions) £,3 mc+ icx IAB + CD CB + AD

Example 1: l- ci + NaOH - l\J( Cl +

Example 2: 2A rs + 3 (304 - >K AI2(S0 )

5. Combustion C and H (& sometimes O) compound + Oa

Example 1: CsHs +?0a

Example 2: C11H24 +I70a - J \ C 02 + |ZM20

COa + HaOI

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Redox Reactions

In redox reactions the oxidation number for an element changes in a chemical reaction.

Oxidation Number: A number assigned to an element, based on the distribution of electrons. Thesame element can have very different properties in different o idation states.

Rules for Assigning Oxidation #s Examples Oxidation #1 The oxidation number of any uncombined ELEMENT is 0 Na, 02 Na = ,02 =

2 The ox. # of an ION equals the charge of the ion CL Cl =

3 The ox. # of elements in COMPOUNDS typically, but not always(unless noted otherwise), follow a trend on the periodic table:Group 1 = +1 ~L ALWAYSGroup 2 =+2

LiF Li = -H

Group 13 = +3 CaCOa Ca = +2

Group 15 = -3 HF F = - I

Group 16 = -2 (0 usually -2)Group 17 = -1 (F ALWAYS -1) H2O H = +|

Transition metals AND Group 14 = multivalent4 The ox. # of HYDROGEN in most compounds is +1 unless it is with H2O H=

a metal, in which case it is -1 LiH H=

5 The sum of ox.#s of all atoms in a NEUTRAL COMPOUND is 0 LiF Li = -H+ F = -1

0

CaCOs Ca = +1C =

0-2(x3)=

The sum of ox. #s of all atoms in a POLYATOMIC ION equals the S042-

0

s = +(/charge of the ion +0 -2 (x4) = 8

-2

Note: Additional rules/exceptions to these rules do exist, but are beyond the scope of this course.

Oxidation is a reaction in which there is theloss of electrons.

0 +1Ex: Na -> Na+ + e-

Reduction is a reaction in which there is thegain of electrons.

0 -1Ex: Cl2 + 2e- 2CI-

LEO the lion says GER Loss of Electrons is Oxidation ORGain of Electrons is Reduction

OIL RIG Oxidation ]s Loss (of electrons)Reduction Js Gain (of electrons)

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Since oxidation is the loss of electrons and reduction is the gain of electrons, they must occursimultaneous} .

**Any chemical process in which elements undergo changes in oxidation number is anoxidation-reduction reaction, or redox reaction for short.**

Determine whether the following elements have been oxidized or reduced and label the reaction type.

Example 1:O o -2

4Fe + 302 2Fe203 TYPE of Reaction:(s)D/SR/DR

Element Ox.# Reactants side Ox.# Products side Lose/Gain e' Oxidized/Reduced

Fc 0 +3 loss oxidi ed

0 0 -2. g in &ed ctc

Example 2:+t -I +2 -1 o

Mg + 2HCI MgC + H2 TYPE of Reaction: S/D/@DR

Element Ox.# Reactants side Ox.# Products side Lose/Gain e' Oxidized/Reduced

M9 0 tz 1033 OXid \2-t

H -H 0 g in

Cl l -i -I -

*Note: ALL of the reaction types we have learned are redox reactions EXCEPT for doublereplacement reactions. ***

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