Physical Physical ScienceScience

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

It’s like printing a word in a different font, it’s It’s like printing a word in a different font, it’s the same word just looks different!the same word just looks 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 oxygen into a new substance i.e. hydrogen and oxygen combine to form water.combine to form water. Chemical reactions take place when chemical bonds are either formed Chemical reactions take place when chemical bonds are either formed

or broken.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 chemical bonds are formed or brokenbroken 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 ReactionsWriting Chemical Reactions 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 ReactionsWriting Chemical Reactions Chemical formulas show the ratio of elements found in Chemical formulas show the ratio of elements found in

molecules and compoundsmolecules and compounds a. a. Subscript Subscript numbers designate how many atoms of each element are numbers designate how many atoms of each element are

present: Hpresent: H22OO22 ; 2 Hydrogen atoms and 2 Oxygen atoms are present in ; 2 Hydrogen atoms and 2 Oxygen atoms are present in

this moleculethis molecule b. When no subscript number is shown: it is understood that there is b. When no subscript number is shown: it is understood that there is

only one atom present: Honly one atom present: H22O = 2 Hydrogen atoms and only one Oxygen O = 2 Hydrogen atoms and only one Oxygen

atom are present in this moleculeatom are present in this molecule

C2H

6C2H

4C2H2

H2

O

Structure of an Chemical Equation:Structure of an Chemical Equation: Conservation of Mass Conservation of Mass - Matter cannot be created nor - Matter cannot be created nor

destroyed so there must be the same number of atoms on each destroyed so there must be the same number of atoms on each side of the equation 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 ReactionsClassifying Chemical Reactions

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

2H2H22 + O + O22 2H2H22OO

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

2H2H22O O 2H2H22 + O + O22

ReplacementReplacement: : When one element replaces another or When one element replaces another or when two elements in different compounds change places:when two elements in different 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 ReactionsControlling Chemical Reactions 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 holding the Some reactions absorb energy & the container holding the reaction gets colder to the touch (reaction gets colder to the touch (endothermicendothermic))

Getting Reactions StartedGetting Reactions Started The The activation energyactivation energy is the energy is the energy

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

Enough is Enough is enough….enough….

Stop already!!Stop already!!

OKOK