Unit 7 Chemical Reactions. Physical Vs. Chemical Change Physical changes= changes in form (phase), but not the identity of the substance ex: H 2 O (s)

Unit 7

Chemical Reactions

Physical Vs. Chemical Change

• Physical changes= changes in form (phase), but not the identity of the substance

ex: H2O (s) H2O (l)

(* Starts as H2O and ends as H2O)

• Chemical Change = Covalent/ ionic bonds break between reactants, producing new products.

ex: H2O (l) H2 (g)+ O2 (g)

Bonds: Chemical Vs. Physical

Physical Change

Strong bonds (covalent) between atoms break & recombine in new combinations

Weak bonds (hydrogen) break between molecules

Chemical Change

Indicators of a Chemical Rxn

1) Light/ heat are produced

2) Color Change

3) Formation of a precipitate (solid)

4) Production of a gas

Importance of a Chemical Equation

• Ex: HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

Symbols used in Chemical Equations(s) = solid (l) = liquid (g) = gas

(aq) = aqueous, a.k.a. dissolved in water

→ = yields/ results ↔ = reversible reaction = Reactions are heatedThere are many more symbols used too!

Reactants Products

It’s a chemical sentence describing a chemical or physical change

The charcoal used in a grill is basically carbon.

The carbon reacts with oxygen to yield carbon dioxide.

C + O2 CO2,

Naming & Formula Writing for Ionic and Covalent compounds

Pgs 203-215

Writing & Naming Ionic Formulas1) Write the charges above the symbols for the ions

2) “Swap & Drop” the charges

3) Formula = MgCl2

Name: Magnesium Chloride

(first element’s name + second element’s name with ending changed to “ide”

Mg12+ Cl2-1

(use periodic table to determine the charge or reference sheet on back of table)

ex: Mg2+ Cl-1 (Write the cation first)

Dealing with Transition Metals

• Many transition metals form more than one ion (see reference sheet on back of periodic table for this information)

Fe2+ = iron (II) Fe3+ = iron (III)

ex: Fe2+ & Cl-1 = FeCl2 Name= Iron (II) chloride

Ex: Fe+3 & Cl-1 = FeCl3Name: Iron (III) chloride

Formulas & Naming (w/ Polyatomic Ions)

• Show formula for: Al bonded with SO4

Al+3 SO4-2

Formula = Al2(SO4)3

Name: Aluminum sulfate

(SO4) = polyatomic ion

-acts as an ion, but has more than one atom

Formula Writing & Naming Covalently Bonded Substances

Prefix Meaning

mono 1

di 2

tri 3

tetra 4

penta 5

hexa 6

hepta 7

octa 8

nona 9

deca 10

Prefixes used in Covalently bonded molecules

Naming Rules

Example: N2O5

dinitrogen pentoxide

• Less electronegative element first– Use prefix if more than one atom

• More electronegative element second– Prefix of the number of atoms

• Drop (o) or (a) at end of prefix if element name begins with a vowel

– Root name– Ending –ide

Summary: Naming/ formulas NOTES: Type I Type IIa Type IIb Type IIc

Who’s Bonding?

2 non-metals Metal + non-metalTransition metal +

non-metal Metal (usually) +

Polyatomic Ion

Example of formula & name

N2O3

Dinitrogen trioxide

CaCl2Calcium chloride

CuBr2

Copper (II) Bromide

Ca(NO3)2

Calcium Nitrate

Name 1st element?

Prefix (except if only one) + full name

Full NameFull name + roman

numeral

Full name + roman numeral (if need)

Name the 2nd element? Prefix + -ide ending

+ -ide ending Root + -ide ending

Name polyatomic based on back of

periodic table

Special rules for this type?

Use pre-fixes:1: mono- 2: di- 3: tri- 4: tetra 5: penta 6: hexa- 7:hepta-8: octa-9:nona-10: deca-

“Swap & Drop”-Write Ion charges

-Swap & drop the numbers

- Write as a formula:

Ca Cl2

- Same as Ia, except:

- Transitional metals can have more than one cationEX: Cu 1+ or Cu 2+

(See back of Periodic Table to know)

- Use roman numerals to indicate the cation

- Same as Ia & Ib except:

-Some groups of elements form “polyatomic ions”

-NO3 = nitrate

-The whole unit has a charge: (NO3) 1-

Diatomic Molecules• Some elements are never found as a

single atom, they bond with themselves.

Here is the list of the 7 diatomic molecules:

H2, N2, O2 (the ”gens”)

F2, Cl2, Br2, I2 (the aka halo”gens”)Ex: hydrogen reacts with oxygen to produce water

Formula: H2 + O2 H2O

(notice it’s not H + O H2O)

Types of Chemical Reactions

Types of Chemical Reactions

There are 5 types of chemical reactions :1. Synthesis reactions2. Decomposition reactions3. Single replacement reactions4. Double replacement reactions5. Combustion reactions

You need to be able to identify the typeof reaction and predict the product(s)

Synthesis

A + B AB

Ex: 4Fe + 3O2 2Fe2O3

Decomposition

AB A + BExample:

CaCO3 Ca + CO3

Single Replacement

A + BC AC + B

Example: Mg + 2HCl MgCl2 + H2

Double ReplacementAB + CD AD + CB

Example: Pb(NO3)2 + 2 KI PbI2 + 2 KNO3

Combustion Reactions

CxHy + O2 CO2 + H2O + Energy

Example:CH4 + 2O2 CO2 + 2H2O + Energy

Balancing Chemical Equations

Law of Conservation of Matter

Matter can not be created or destroyed during chemical

reactions“what goes in must come out”

Law of Conservation of Matter

# of atoms that go in a chemical reaction MUST come out”

Unbalanced EX: C6H12O6 + O2 CO2 + H2O6 C’s go in & only 1 C comes out (H’s and O’s don’t add up either!) Not possible!

We balance chemical equations to adhere to the Law of conservation of matter.

EX: C6H12O6 + 6O2 6CO2 + 6H2O

(now it’s balanced)

-

Balancing Equations (Coefficients vs Subscripts)

When balancing, you can only add coefficients in front of the compounds (NEVER change the subscripts)

3H20Coefficient (3)tells the number of

molecules

Subscript (2)tells the number of atoms of each element

What this says: 3 molecules of water

6 atoms of hydrogen

3 atoms of oxygenChanging the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Steps to balancing chemical equations:

1) Balance one element at a time. Working left to right, find the number of atoms for each element on the left side and compare those against the number of the atoms of the same element on the right side.

Helpful hint: Never start by balancing the H’s or O’s. Save H for next to last and O until last.

2) Place coefficients in front of formulas so that the left side has the same number of atoms as the right side.

Helpful hint: Polyatomic ions that appear on both sides of the equation should be balanced as independent units

3) Double check your math to make sure you’ve balanced everything correctly.

Balancing EquationsEx 1: ___ H2O(l) ---> ___ H2(g) + ___ O2(g) 2 2

Ex 2: __C2H4(g) + __ O2(g) __CO2(g) + __H2O(g)2 22

Ex 3: ___ Al(s) + ___ Br2(l) ---> ___ Al2Br3(s)3 24

Ex 4: __ ZnCl2 + __Fe2(SO4)3 __ FeCl3 + __ZnSO43 32

Ex 5: __ Ca + __H2O __ Ca(OH)2 + __H22

Ex 6: CH4O + O2 CO2 + H2O

Predicting Products & Will Reaction Work?

Steps to Predicting Products1. Identify the type of reaction2. Write down all the atoms and their charges

below the problem (do not write down subscripts unless it’s a polyatomic ion. Subscripts change from reactants side to products side)

3. Predict the product (s) using the type of reaction as a model and write the products with the charges.

4. Do swap and drop to find formulas of products (Don’t forget about the diatomic elements! For example, Oxygen is O2 as an element.)

5. Balance it

Synthesis

A + B ABExample:

Fe + O2 -Write charges for each reactant: Fe+3

O2-

-Write product, swap and drop: Fe2O3

-Balanced equation:4Fe + 3O2 2Fe2O3

Decomposition

AB A + BExample:

CaCO3 -Break up the reactant into the 2 ions

that make it up: Ca2+ CO32-

-Write them as products-Balance equation:

CaCO3 Ca + CO3

Single ReplacementA + BX AX + B

Example: Mg + HCl -Write the charges for each ion

Mg+2 H+ Cl-

-If there is a lone cation, swap it with the cation in the compound.

MgCl2-If there is a lone anion, swap it with the anion in the compound-Write the new lone cation or anion

H2 (diatomic)-Balance the equation

Mg + 2HCl MgCl2 + H2

Hint: When H2O splits into ions, it splits intoH+ and OH- (not H+ and O-2 !!)

Double Replacement

Example: Pb(NO3)2 + KI

Pb+2 NO3-1 + K+ I-

#2 switch partners so that + and – are together

#1 Write the charges for each ion (not the subscript unless it’s a polyatomic)

#3 Swap and drop to get correct formulas for products: PbI2 KNO3

AX + BY AY + BX

#4 Write as products and balance equation

Pb 2+ I-1 + K+1 NO3-1

Example: Pb(NO3)2 + 2 KI PbI2 + 2 KNO3

Combustion

CxHy + O2 CO2 + H2O + Energy

Example:CH4 + O2

-The reactants are always a carbon, hydrogen compound and oxygen

-The products are always carbon dioxide, water and energy

-Balance equationCH4 + 2O2 CO2 + 2H2O + Energy

Predicting Products Practice

• Zinc metal reacts with aqueous hydrochloric acid

Zn(s) + 2 HCl(aq) ZnCl2 + H2(g)

Note: Zinc replaces the hydrogen ion in the reaction

NaCl(s) + F2(g) NaF(s) + Cl2(g)

Note that fluorine replaces chlorine in the compound

Will a Reaction Occur?

• 3 reactions ALWAYS work:– Combustion, Synthesis, and Decomposition

Single replacement & Double replacement reactions do not always work.

- You need more information for each type of reaction to know for sure if a reaction will take place.

Just because a reaction works out on paper, does not mean it would actually happen in a test tube.

Single replacement reactions and the Activity Series

You need the Activity Series chart to determine if a single replacement reaction will occur

Ex #1 : Will this reaction happen? Ex #1 : Will this reaction happen? MgMg + + HHCl Cl ???? ????

Mg must be able to replace H… can Mg must be able to replace H… can it?it? Find Mg and H on the chart

Mg is higher (Stronger) than H (weaker) and so Mg replaces H.

YES, Reaction will occur

* Elements on top can replace all those below in a reactionMg + 2HCl Mg + 2HCl MgCl MgCl22 + +

HH22

Single Replacement Reactions Practice Problems

Will this reaction occur?

Mg + NaCl ???

Mg is trying to kick out Na… can it?

-Na is higher on the chart than Mg so Na will not be kicked out by Mg.

We write:

Mg + NaCl NO REACTION

Will Double Replacement Reactions Occur?

• YES, if at least one of the products is a solid (aka precipitate). (or H2O)

AX + BY AY (s) + BX (s) YES

AX + BY AY (s) + BX (aq) YES

AX + BY AY (aq) + BX (aq) NO You need aYou need a “ “Solubility TableSolubility Table” ” to know what state to know what state thethe products will be in.products will be in.

Use solubility rules:Use solubility rules:Soluble Soluble – dissolved in water (aq)– dissolved in water (aq)InsolubleInsoluble - a solid precipitate (s) - a solid precipitate (s)

Solubility Table

The study of quantities of materials consumed and produced in chemical reactions.

Double Displacement Practice Problems

• Will this Reaction Occur? Pb(NO3)2 + 2KI PbI2 + 2KNO3

• Need to look up products on the chart to determine if either of them is a solid– PbI2 = solid (insoluble)– KNO3 = aqueous

Answer: Pb(NOAnswer: Pb(NO33))22 (aq)(aq) + 2KI+ 2KI (aq)(aq) PbI PbI22 (s)(s) + + 2KNO2KNO33 (aq)(aq)

Reaction will occur because 1 solid in productsReaction will occur because 1 solid in products

Reversible Reactions

• Ex: H2O + CO2 H2CO3

• Symbol for a reversible reaction• Reversible reactions can proceed in

both the forward and reverse directions

Equilibrium= When the forward and the reverse reactions occur at the same rate

funny