Evidence & Types of Chemical ReactionsGPS 2a & 2bFall 2011

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.

steel wool + oxygen (reacting) rust Fe O2 Fe2O3

reactants

productClick for Bill Nye Video

Part I: Describing Chemical Reactions• according to the law of conservation of mass:

total mass of reactants = total mass of products

223.38 g Fe + 96.00 g O2 287.38 g Fe2O3

• 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.

“The reactant ammonium dichromate yields the products nitrogen, chromium (III) oxide, and water.” is the same as:

(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + H2O(g)[This equation is UNBALANCED. We will balance it using

coefficients later.]

Example of a chemical equation for the breakdown of ammonium dichromate.

Click Image for

Cool Video

Part II: Evidence of Chemical ReactionsTo 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. An example would be the decomposition you just viewed.

2.Temperature Change. If a reaction gets hot, we say it is exothermic (releasing heat energy to the environment). However, a reaction can absorb energy from the environment (get cold); this is called an endothermic reaction.

Check out this “cool” reaction!

3. 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; the classic middle school volcano experiment.

(baking soda) (vinegar) (carbon dioxide)

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

Try this one for yourself!(Click the image

to watch the video.)

4. 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. This solid is called a precipitate.

precipitate = a solid that is produced as a result of a chemical reaction in

solution & it separates from the solution.

Another video:

This one you can’t try at home. The precipitate, lead iodide, is toxic but still usedin the paint that lines the roads we travel.

5. 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.

6. Evolution of an odor. Odors that suddenly appear indicate new, aromatic chemicals have been produced. Examples would be synthesis of perfumes & flavorings. (Sorry, hard to demo this online! )

Check out these awesome color-changing reactions!

Part III: Types of Chemical ReactionsThere 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. Synthesis

2. Decomposition

3. Single replacement

4. Double replacement

5. 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

A + X AX

Synthesis Explained!!

Making Salt

2. Decomposition = splits 1 compound into 2 or more smaller compounds or atoms

• the “divorce” reaction

Ex) NaCl Na + Cl2 H2O2 H2O + O2

hydrogen peroxide breaks down

into water & oxygen gas

AX A + X

Watch this video!!

3. Single replacement = Replaces the cation or anion of a compound with another free cation or anion; the “one-cheater” reaction.

Ex) Cation Replacement

NaCl + K KCl + Na Al2(CO3)3 + Li Li2CO3 + Al

AgNO3 + Cu Ag + Cu(NO3)2

Click image to watch video

of this reaction.

AX + B BX + A

Ex) Single Replacement Rxns

Anion Replacement

NaBr + Cl2 NaCl + Br2 NaI + Cl2 NaCl + I2

Watch the video of

these reactions.

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• are often precipitation reactions

• Ex: NaCl + KBr NaBr + KCl Li2S + TiF2 LiF + TiS

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

AX + BY AY + BX

You’ve seen this one before!

What is th

e yellow stuff?

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

• This reactions “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

CnHn + O2 CO2 + H2O

Burn, baby, burn!

Some of these demos could be classified as synthesis also. Watch closely & see if you can find them in the video!

For more help with types of reactions, watch the tutorial below:

Tutorial Lesson

More Demos!

• 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