Evidence & Types Evidence & Types of Chemical of Chemical ReactionsReactions9/30/109/30/10

Part I: Describing Chemical Reactions•chemical reaction = the process by which one or more

substances are changed into one or more different substances. •in any chemical reaction, the original substances are known as the reactants and the resulting substances are known as the products.

•according to the law of conservation of mass:

total mass of reactants = total mass of products •chemical reactions are described by chemical equations.

•chemical equation = represents, with symbols and formulas, the identities and molar amounts of the reactants and products in a chemical reaction. For example:•“The reactant ammonium dichromate yields the products nitrogen, chromium (III) oxide, and water.” is the same as:

•the equation above is UNBALANCED. We will balance it using coefficients later.

total mass of reactants = total mass of products

(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + H2O(g)

(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + H2O(g)

Part II: Evidence of Chemical Reactions• to know for certain that a reaction has taken place requires evidence

that one or more substances have undergone a change in identity. 1.Evolution of heat and light. A change in matter that releases

energy as both heat and light is strong evidence that a chemical reaction has taken place.

2.Production of a gas. The evolution of gas bubbles when two substances are mixed is often evidence of a chemical reaction.

• Ex: bubbles of CO2 gas form immediately when baking soda is mixed with vinegar.

3.Formation of a precipitate. Many reactions take place between substances that are dissolved in liquids. If a solid appears after two solutions are mixed, a reaction has likely occurred.

• precipitate = a solid that is produced as a result of a chemical reaction in solution that separates from the solution.

NaHCO3(s) + CH3COOH(l)

CO2(g) + NaC2H3O2(aq) + H2O(l)

• precipitate = a solid that is produced as a result of a chemical reaction in solution that separates from the solution. 4.Color change. A change in color is often an indication of

a chemical reaction. Ex: Bleach breaks down dye molecules attached to fabric, altering the wavelength of light they reflect, thus changing the color.

5.Evolution of an odor. Odors that suddenly appear indicate new, aromatic chemicals have been produced.

Part III: Types of Chemical Reactions• there are 5 distinct types of chemical reactions. Each has a

general formula that represents the types of substances that react within each type of reaction.1.synthesis2.decomposition3.single replacement4.double replacement5.combustion

The first 4 types of reactions often involve ionic compounds. It’s

helpful to think of the cations (A and B) as boys and the anions (X

and Y) as girls.

1. synthesis = combines 2 small compounds or atoms into one larger compound• the “marriage” reaction• Ex: Na + Cl2 NaCl K + CO3 K2CO3 Al + O2

Al2O3

2. decomposition = splits 1 compound into 2 or more smaller compounds or atoms • the “divorce” reaction• Ex: NaCl Na + Cl2 Cr2O3 Cr + O2 N3S5 N2 +

S

3. single replacement = Replaces cation or anion of a compound with another free cation or anion• the “one-cheater” reaction• Ex: NaCl + K KCl + Na Al2(CO3)3 + Li Li2CO3

+ Al

NaCl + Br2 NaBr + Cl2 Mn3P2 + O2 MnO + P

A + X AX

AX A + X

AX + B BX + A

AX + Y AY + X

4. double replacement = the cation of one compound is combined with anion of the other compound, creating 2 new compounds• the “switching partners” reaction• Ex: NaCl + KBr NaBr + KCl Li2S + TiF2 LiF +

TiS

Sn3P4 + Zn(NO3)2 Zn3P2 + Sn(NO3)4

5. combustion = Hydrocarbon + O2 produces CO2/CO, H2O plus energy

• the “involves hydrocarbons” reaction (not ionic compounds)• sometimes results in CO2 as a product, other times CO

• Ex: C2H6 + O2 CO2 + H2O C7H14 + O2 CO2 + H2O

C10H22 + O2 CO2 + H2O C7H14 + O2 CO + H2O

• now let’s take a closer look at the single-replacement reaction again...

AX + BY AY + BX

CnHn + O2 CO2 + H2O

•let’s take a closer look at the single-replacement reaction...•the activity series helps determine whether or not one element can replace another in a single-replacement rxtn.

•if an element is above another, it can replace it. •so, can these reactions happen or not?•Ex: NaCl + K KCl + Na Al2(CO3)3 + Li Li2CO3 + Al

NaCl + Br2 NaBr + Cl2

Part IV: Symbols Found in Chemical Equations

LiRbKBaSrCaNa

MgAlMnZnCrFeCd

CoNiSnPb

H2

SbBiCu

HgAgPtAu

F2

Cl2

Br2

I2

(s) (g)

(l)2

atm

(aq) 0°C

MnO2

NaHCO3 (s) + CH3COOH(l) CO2 + NaC2H3O2(aq) + H2O(l)

Part IV: Symbols Found in Chemical Equations

Part V: Other Info About Chemical Reactions some elements occur only in diatomic form, meaning they are

bonded to another atom of that element. H2 N2 O2 F2 Cl2 Br2 I2 these gases are never found by themselves in any chemical formula or equation!!!

(s) (g)

(l)2

atm

(aq) 0°C

MnO2

NaHCO3 (s) + CH3COOH(l) CO2 + NaC2H3O2(aq) + H2O(l)

solid ( for precipitate)

gas ( for gaseous product)

liquid

in aqueous solution

yields or produces

reversible reaction

heat applied to reactants

specific pressure for rxtn

specific temp for reaction

catalyst needed for rxtn

Part V: Other Info About Chemical Reactions some elements occur only in diatomic form, meaning they are bonded

to another atom of that element. H2 N2 O2 F2 Cl2 Br2 I2 these gases are never found by themselves in any chemical formula or equation!!!

coefficients = large numbers in front of a chemical formula subscripts = little numbers within a chemical formula

5Al2(SO4)3 4Na2S 2MgCl2

we will learn about balancing equations (using coefficients) tomorrow!