Chemical Reactions

How do you know if a chemical reactions takes place?

•A color change takes place

•A solid (ppt) forms

•Bubbles are formed without heating

•A flame occurs

•Light or heat is given off/absorbed

•Water is produced

Chemical EquationsChemical Equations

HH22SOSO44 + 2 NaOH + 2 NaOH Na Na22SOSO44 + 2 H + 2 H22OOReactants on the left, products on the rightReactants on the left, products on the right ““” ” indicates a chemical reactionindicates a chemical reaction ““+” separates the substances+” separates the substances ““2” coefficient - how many of that 2” coefficient - how many of that

substance (big number out front)substance (big number out front) ““4” subscript – how many of the preceding 4” subscript – how many of the preceding

atom (little numbers)atom (little numbers)

Law of Conservation of MassLaw of Conservation of Mass

Mass can not be created or destroyed in a Mass can not be created or destroyed in a chemical reaction. It can only change chemical reaction. It can only change form.form.

This means in a chemical reaction, there This means in a chemical reaction, there must be the same number of each type of must be the same number of each type of atom on both sides of the arrow.atom on both sides of the arrow.

Balancing EquationsBalancing Equations

Copy formulas correctlyCopy formulas correctly Subscripts can not be changedSubscripts can not be changed Change only the coefficientsChange only the coefficients Determine the most complicated compound. Determine the most complicated compound.

Calculate the number of each atom/ion in that Calculate the number of each atom/ion in that compoundcompound

Go to other side of reaction, use coefficients to Go to other side of reaction, use coefficients to get the same number of ionsget the same number of ions

Repeat until all ions are balanced.Repeat until all ions are balanced.

Sometimes it is easier to balance equations when water is written as HOH, sometimes it is easier when it is written as H2O.

The coefficient of “1” is not written, it is understood.

__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O

This is easier to balance if written as

__ HCl + __ Ca(OH)2 __ CaCl2 + __ HOH

2 HCl + Ca(OH)2 CaCl2 + 2 HOH

Balance the following reactions:

__Ba(NO3)2 + __NaCl __BaCl2 + __NaNO3

__ Al + __ O2 __Al2O3

__ C7H16 + __ O2 __ CO2 + __ H2O•Ba(NO3)2 + 2 NaCl BaCl2 + 2 NaNO3

•4 Al + 3 O2 2 Al2O3

•C7H16 + 11 O2 7 CO2 + 8 H2O

The 5 types of equations

1.Combination (or Synthesis) – A + B AB. This forms a new compound from two or more reactants.

2 H2 + O2 2 H2O

2. Decomposition – AB A + B. A single reactant breaks up into two or more products. This is the opposite of a combination reaction.

2 HgO 2 Hg + O

3.Single Replacement

A + BX AX + B or A + BX BA + X

A single element replaces an element in a compound. A metal replaces the metal, a nonmetal replaces the nonmetal in the compound.

Cl2 + 2 NaBr 2 NaCl + Br2

Cs + NaCl CsCl + Na

4. Double Replacement – AB + CD AD + CB

The positive ions in two different compounds are swapping places, or the

negative ions in two different compounds are swapping places.

H3PO4 + 3 NaOH Na3PO4 + 3 HOH

5. Combustion –

CxHy (CxHyOz) + O2 CO2 + H2O

“Burning” or the rapid oxidation of a carbon compound into carbon dioxide and water.

CH4 + 2 O2 CO2 + 2 H2O

2 CH3OH + 3 O2 2 CO2 + 4 H2O

Tips for recognizing and balancing reactions:

Combination : 2 reactants 1 product

Decomposition : 1 reactant 2 products

Single : element + cmpd element + cmpd

Double : elements in 2 cmpds are exchanged

Combustion : CH(O) + O2 CO2 + H2O

•If your first guess of coefficients doesn’t work, change your guess and start over.

•Coefficient x subscript = # of atoms/ions

•Be consistent – balance the reactions the same way each time.

•Be right – Know when to add numbers together, and when to multiply numbers together.

Predicting the products of reactionsPredicting the products of reactions

Identify the type of reaction: combination, Identify the type of reaction: combination, combustion or double replacement.combustion or double replacement.

Complete the reaction with the correct Complete the reaction with the correct formulas based on the type of reaction.formulas based on the type of reaction.

Balance the reaction once all of the Balance the reaction once all of the complete formulas have been written.complete formulas have been written.

To complete the reaction with correct formulas:

•Combination: the compound formed will be the one formed from the elements as “ions”

•Combustion: the products will always be CO2 and H2O

•Double replacement: switch the positive ions, get the correct formulas by “cross method”. Subscripts in the original compound don’t matter unless they are part of the actual ion (a polyatomic ion).

Changing from word problems to Changing from word problems to equationsequations

HOFBrINCl or Lucky 7HOFBrINCl or Lucky 7

Solid (s), liquid (l), gas (g), aqueous (aq) Solid (s), liquid (l), gas (g), aqueous (aq)

Write all of the correct formulas, reactants Write all of the correct formulas, reactants on the left and products on the right.on the left and products on the right.

Balance the reactionBalance the reaction