Physical Physical ScienceScience

Chapter 17Chemical Reactions

Changes in Matter Physical Change -A change that alters the form

of a substance but not the chemical makeup of the substance, a change of state Words like: crush, smash, tear, evaporate, slice,

breakdown, dissolve, absorb, swell, burst Chemical Change - One or more substances

combine or decompose to form a chemically different substance Words like: react, burns, forms, decomposed, rusting,

sours, rotting, digesting, cooked, molecular change

Matter & Its ChangesMatter & Its Changes Physical Changes Physical Changes – Alters form or – Alters form or

appearance but doesn’t change it appearance but doesn’t change it into another into another substance ie. Water substance ie. Water evaporates into water vapor, a rock is evaporates into water vapor, a rock is broken into piecesbroken into pieces

It’s like printing a word in a different It’s like printing a word in a different font, it’s the same word just looks font, it’s the same word just looks different!different!

stampedes stampedes stampedesstampedes

Matter & Its ChangesMatter & Its Changes Chemical changeChemical change- - changes the material changes the material

into a new substance i.e. hydrogen and into a new substance i.e. hydrogen and oxygen combine to form water.oxygen combine to form water. Chemical reactions take place when chemical bonds are either Chemical reactions take place when chemical bonds are either

formed or broken.formed or broken. Strong chemical bonds resist change: glassStrong chemical bonds resist change: glass Weak chemical bonds breakdown easily: woodWeak chemical bonds breakdown easily: wood

A A chemical change chemical change is like scrambling is like scrambling letters to form new wordsletters to form new words

made + steps made + steps stampedes stampedes

Observing Chemical Observing Chemical ReactionsReactions

Chemical reactions produce new substances that Chemical reactions produce new substances that can usually be detected by observing the can usually be detected by observing the evidence:evidence: Color changeColor change PrecipitationPrecipitation Temperature changeTemperature change Property changeProperty change Gas producedGas produced

Chemical ReactionsChemical Reactions Chemical reactions occur when Chemical reactions occur when

chemical bonds are formed or brokenchemical bonds are formed or broken 2. Strong chemical bonds resist change: glass2. Strong chemical bonds resist change: glass 3. Weak chemical bonds breakdown easily: wood3. Weak chemical bonds breakdown easily: wood

Writing Chemical Writing Chemical ReactionsReactions

Elements are represented by a one or two letter symbolElements are represented by a one or two letter symbol a. When symbol is a single letter: always capitalize: Hydrogen=Ha. When symbol is a single letter: always capitalize: Hydrogen=H b. When symbol is two letters, capitalize first letter & lower case b. When symbol is two letters, capitalize first letter & lower case

second letter: Sodium = Nasecond letter: Sodium = Na

Writing Chemical Writing Chemical ReactionsReactions

Chemical formulas show the ratio of elements found Chemical formulas show the ratio of elements found in molecules and compoundsin molecules and compounds a. a. Subscript Subscript numbers designate how many atoms of each numbers designate how many atoms of each

element are present: Helement are present: H22OO22 ; 2 Hydrogen atoms and 2 ; 2 Hydrogen atoms and 2 Oxygen atoms are present in this moleculeOxygen atoms are present in this molecule

b. When no subscript number is shown: it is understood that b. When no subscript number is shown: it is understood that there is only one atom present: Hthere is only one atom present: H22O = 2 Hydrogen atoms O = 2 Hydrogen atoms and only one Oxygen atom are present in this moleculeand only one Oxygen atom are present in this molecule

C2H

6C2H

4C2H2

H2

O

Structure of an Chemical Structure of an Chemical Equation:Equation:

Conservation of Mass Conservation of Mass - Matter cannot be created - Matter cannot be created nor destroyed so there must be the same number nor destroyed so there must be the same number of atoms on each side of the equation of atoms on each side of the equation

Beginning materials are Beginning materials are reactantsreactants Ending materials are Ending materials are productsproducts Example of Chemical reaction:Example of Chemical reaction:

Reactant + Reactant Reactant + Reactant Product + ProductProduct + Product

Structure of an Equation:Structure of an Equation:

yields

Counting Atoms in an Counting Atoms in an EquationEquation

If no subscript present it is assumed to be 1 atomIf no subscript present it is assumed to be 1 atom If elements in brackets or parenthesis, treat If elements in brackets or parenthesis, treat

same as in math.same as in math. Coefficients multiple the entire molecule atomsCoefficients multiple the entire molecule atoms You must add all reactant molecules together & You must add all reactant molecules together &

compare w/ all molecules in the products compare w/ all molecules in the products Ca3(PO4)2

Ca=3P=2O=8

2Ca3(PO4)

2

Ca=6P=4

O=16

CaCl2

Ca=1Cl=2

It’s best to list the # of atoms under the molecules as we are doing in these examples

Balancing EquationsBalancing Equations Remember matter cannot be created Remember matter cannot be created

or destroyedor destroyed Therefore the # of reactant atoms must Therefore the # of reactant atoms must

equal the # of product atoms equal the # of product atoms

4 Reactant Hydrogens = 4 Product Hydrogens

2 Reactant Oxygens = 2 Product Oxygens

This reaction is balanced!

This reaction IS NOT balanced

Balance this…Balance this…

Step 1: Count the atoms on both sides of the equation & compare

Step 2: apply a coefficient to a molecule to balance an “easy” atom (in this case, we can add a 2 coefficient to the product water to balance the H’s)Step 3: Recount

** you can only add or change whole # coefficients to balance equations. Never change subscripts

Balance this… Balance this… (continued)(continued)

Step 4: apply a coefficient to a molecule to balance the next off balanced atom (in this case, the oxygens are not yet balanced. We can add a coefficient of 2 in front of the reactant oxygen to correct this)Step 5: RecountStep 6: Continue doing this until all atoms are balanced

BALANCED!!

Classifying Chemical Classifying Chemical ReactionsReactions

SynthesisSynthesis: : When two or more substances When two or more substances combine to form a more complex substancecombine to form a more complex substance

2H2H22 + O + O22 2H2H22OO

DecompositionDecomposition: When a complex substance is : When a complex substance is broken into two or more simpler substances:broken into two or more simpler substances:

2H2H22O O 2H2H22 + O + O22

ReplacementReplacement: : When one element replaces When one element replaces another or when two elements in different another or when two elements in different compounds change places:compounds change places:

2CuO + C 2CuO + C 2Cu + CO2Cu + CO22

SynthesisSynthesis Reactions Reactions

Decomposition ReactionsDecomposition Reactions

ReplacementReplacement Reactions Reactions 2 types: 2 types:

Single Replacement Single Replacement Double ReplacementDouble Replacement

Controlling Chemical Controlling Chemical ReactionsReactions Every chemical reaction involves a change in energy.Every chemical reaction involves a change in energy.

Some reactions release energy in the form of heat Some reactions release energy in the form of heat ((exothermicexothermic))

Some reactions absorb energy & the container Some reactions absorb energy & the container holding the reaction gets colder to the touch holding the reaction gets colder to the touch ((endothermicendothermic))

Getting Reactions Getting Reactions StartedStarted

The The activation energyactivation energy is the energy needed by is the energy needed by a system to initiate the reaction. It is the a system to initiate the reaction. It is the minimum minimum energyenergy needed for a specific chemical needed for a specific chemical reaction to occur. Once achieved, the reaction reaction to occur. Once achieved, the reaction continues until reactants are extinguished.continues until reactants are extinguished.

Enough is Enough is enough….enough….

Stop already!!Stop already!!

OKOK