Chemical ReactionsChemical Reactions

Unit 8Unit 8

Chemical Reaction vs. Chemical Chemical Reaction vs. Chemical EquationEquation

A REACTION is the process where 2 or more atoms A REACTION is the process where 2 or more atoms or compounds rearrange themselves to form new or compounds rearrange themselves to form new substancessubstances..

+

An EQUATION is written to summarize what has happened in a reaction

2H2 + O2 2H2O

Signs of a Chemical Signs of a Chemical ReactionReaction

1.1. Precipitate formedPrecipitate formed

2.2. Permanent color Permanent color changechange

3.3. Temperature Temperature changechange

4.4. Gas producedGas produced

5.5. Light producedLight produced

Law of Conservation of MassLaw of Conservation of Mass

Mass is neither created nor Mass is neither created nor destroyed in a chemical reaction.destroyed in a chemical reaction.

Mass of the products equals the Mass of the products equals the mass of the reactants in a chemical mass of the reactants in a chemical reactionreaction

Atoms only rearrange themselvesAtoms only rearrange themselves

Practice ProblemsPractice Problems

Example 1: What is the total mass of reactants in the reaction below?2 H2 + O2 2 H2O Answer: __________g ? 72 g

Example 2: What is the total mass of products in the reaction below?

AgNO3 + NaCl AgCl + NaNO3 Answer: __________g 170 g 56 g ---------?----------

Example 3: What is the mass of zinc produced in the reaction below?

Ca + ZnCO3 CaCO3 + Zn Answer: __________g

40 g 125 g 100 g ?

72

226

65

Chemical EquationsChemical Equations

Reactants (2HReactants (2H22(g) and O(g) and O22(g))(g))

Products (2HProducts (2H220 (g))0 (g))

2H2H22((gg) + O) + O22((gg) ) 2H 2H22O(O(gg))

– Coefficient (The large number in front of HCoefficient (The large number in front of H22 and H and H220)0)

– Chemical formulaChemical formula

– Physical statePhysical state (s- solid; g- gas; aq- aqueous; l- liquid)(s- solid; g- gas; aq- aqueous; l- liquid)

Diatomic ElementsDiatomic Elements

These elements are never found by These elements are never found by themselves in nature. themselves in nature.

II22, Br, Br22, Cl, Cl22, F, F22, O, O22, N, N22, H, H22

To help you remember, memorize To help you remember, memorize this:this:

II BrBring ing ClClay ay FFor or OOur ur NNew ew HHutut

Some other stuff you may Some other stuff you may see:see:

Δ Δ heat added to reactants (triangleheat added to reactants (triangle

appears above arrow)appears above arrow) CatalystsCatalysts are substances that are substances that

increase the rate of the reaction but increase the rate of the reaction but are not used up themselves. are not used up themselves. The formula The formula for the catalyst is written above the arrow: (example MnOfor the catalyst is written above the arrow: (example MnO22))

MnO22 KClO3(s)2 KClO3(s) 2KCl(s) + 3 O2(g)

Catalysts are not used up in the reaction so they appear above the arrow

Balancing EquationsBalancing Equations

Copy down equation (leave space for Copy down equation (leave space for #’s)#’s)

Make an atom inventoryMake an atom inventory– List elements on both sides List elements on both sides

(reactants/products)(reactants/products)

– Count # of atoms on each sideCount # of atoms on each side Coefficient Coefficient subscript = # of atoms subscript = # of atoms

– Reduce if necessary!Reduce if necessary!– Double Double check your work!check your work!

Types of Chemical Types of Chemical ReactionsReactions

DisclaimerDisclaimer

The events depicted in this story are fictitious. Any similarity to any person living or dead is merely coincidental.

The Story

Begins…

2H2 + O2 → 2 H2O

3Mg + N2 → Mg3N2

Synthesis Reaction

A reaction in which two or more elements form a compound.

Synthesis ReactionSynthesis Reaction

+

AB A B

The Story Continues…

Decomposition Reaction

2H2O → 2H2 + O2

2HgO → 2Hg + O2

A reaction in which a compound is broken down into two or more elements.

Decomposition ReactionDecomposition Reaction

+

AB B A

The Story Continues…

Single Replacement Reaction

2KCl + F2 → 2KF + Cl2

Mg + 2HCl → H2 + MgCl2

A reaction in which one element reacts with one compound to form another element and another compound.

Single Replacement Single Replacement ReactionReaction

+ +

AB C CB A

The Story Continues…

Double Replacement Reaction

Pb(NO3)2 + K2CrO4 PbCrO4+ 2 K(NO3)

NaCl + AgNO3 AgCl + Na(NO3)

A reaction in which the metals present in two compounds change places to form two new compounds.

The End!But…

CombustionCombustion

When a carbon compound reacts When a carbon compound reacts with oxygen to make carbon dioxide with oxygen to make carbon dioxide and water.and water.– Hint:Hint:

It will It will alwaysalways have O have O22 on reactant side on reactant side

It will It will alwaysalways have CO have CO22 and H and H22O on product O on product sideside

Example:Example:CHCH44 + O + O22 → CO→ CO22 + H + H22OO

ExamplesExamples 2NaCl 2NaCl 2Na + Cl 2Na + Cl22

CC88HH1818 + O + O22 H H22O + COO + CO22

2HCl + F2HCl + F22 2HF + Cl 2HF + Cl22

HCl + NaOH HCl + NaOH HOH + NaCl HOH + NaCl

2Na + Cl2Na + Cl22 2NaCl 2NaCl

Decomposition

Combustion

Single Replacement

Double Replacement

Synthesis