Chemical & Physical Changes

Chemical & Physical Changes

Definitions

• Physical Property:Physical Property: a property that a property that can be seen or measuredcan be seen or measured without without changing the identity of the substancechanging the identity of the substance– Examples: color, size, density, melting ptExamples: color, size, density, melting pt

• Chemical Property:Chemical Property: the ability or the ability or inability of a substance to inability of a substance to react withreact with or or change into other substanceschange into other substances– Examples: flammability, reactivityExamples: flammability, reactivity

5 Signs of a Chemical Change

1. Light being given off2. Drastic color change3. Precipitate being formed

• Precipitate: solid produced during a chemical reaction in a solution

4. Gas given off5. Change of temperature

Reactions & Equations

I. How do I tell if a reaction has occurred?

– A sign of a chemical must take place• Temperature change• Release of light• Color change• Appearance of precipitates (solids)• Gas/odor released

II. How do I represent a chemical rxn?

Reactant 1 + Reactant 2 Reactant 1 + Reactant 2 Product 1 + Product 1 + Product 2Product 2

ReactantsReactants are starting substances. are starting substances.ProductsProducts are substances formed. are substances formed.ArrowArrow shows direction of progress. shows direction of progress.A A “+” “+” sign is placed between reactants and sign is placed between reactants and products to separate them.products to separate them.Symbols are used to show physical state of Symbols are used to show physical state of reactants and products – reactants and products – (s) solid, (l) liquid, (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.(g) gas, (aq) dissolved in water.

Word & Skeletal Equations

A) Word equations:A) Word equations:• reaction written with words instead of reaction written with words instead of

chemical symbols.chemical symbols.

Ex. Ex. Magnesium (s)Magnesium (s) + + Oxygen (g)Oxygen (g) Magnesium oxide Magnesium oxide (s)(s)

Magnesium reacts with oxygen to produce magnesium Magnesium reacts with oxygen to produce magnesium oxide.oxide.

B) Skeleton Equations:B) Skeleton Equations:• Use chemical formulas and symbols Use chemical formulas and symbols

instead of words – instead of words – not balancednot balanced..

Ex. Ex. Mg(s) + OMg(s) + O22(g) (g) MgO(s) MgO(s)

Chemical Equations

C) Chemical equation:C) Chemical equation:• Like skeleton equation but shows Like skeleton equation but shows

conservation of mass – conservation of mass – balanced.balanced.

Ex. Ex. 2Mg(s) + O2Mg(s) + O22(g) (g) 2MgO(s) 2MgO(s)

III. Balancing Chemical Equations

• We use We use coefficientscoefficients in front of reactants and in front of reactants and products – usually whole numbers – and tells you products – usually whole numbers – and tells you the smallest # of particles involved in the the smallest # of particles involved in the reaction.reaction.

Steps:Steps:1.1. Write Write skeleton equationskeleton equation..2.2. Count the # of elements in reaction.Count the # of elements in reaction.3.3. Change the Change the coefficientscoefficients to make the # of atoms of to make the # of atoms of

each element equal on both sides of the equation.each element equal on both sides of the equation.4.4. Write the coefficients in their Write the coefficients in their lowestlowest possible ratio.possible ratio.5.5. Check your work.Check your work.

IV. Conservation of Mass

--matter is neither matter is neither created created or or destroyeddestroyed in a in a chemical reaction.chemical reaction.

--number of atomsnumber of atoms in reactant in reactant equalsequals number number of atoms in of atoms in productproduct..

-sum of mass in reactant equals sum of mass -sum of mass in reactant equals sum of mass in product.in product.

Ex. Ex. 2H2O2 2H2O + O2

68.0g 32.0g + _?_ 68.0g – 32.0g = 36.0 g

Types of Chemical Types of Chemical ReactionsReactions

There are five types:There are five types:

1)1) CombinationCombination

2)2) DecompositionDecomposition

3)3) Single-replacementSingle-replacement

4)4) Double-replacementDouble-replacement

5)5) CombustionCombustion

1. 1. SynthesisSynthesis

• Generally two or more substances Generally two or more substances combine to combine to form a single form a single substancesubstance..

• General equation: General equation: A + B A + B ABAB

• EX. Iron and chlorine combine to produce Iron chlorideEX. Iron and chlorine combine to produce Iron chloride..2Fe (s)2Fe (s) + Cl+ Cl22 (g) (g) 2FeCl 2FeCl33 (s) (s)

a. Pb(N03)2 + K2Cr04 PbCr04 +

KN03

b. C12 + KI KCl + I2

c. C3H6 + O2 CO2 + H2O

d. Al(OH)3 Al2O3 + H2O

e. Li + O2 Li2O

f. HCl + Fe203 FeC13 + H2O

g. MgC03 MgO + CO2

h. Ba(CN)2 + H2SO4 BaS04 +

HCN

2. 2. DecompositionDecomposition

• A A singlesingle compound is broken down into compound is broken down into two or moretwo or more substances. substances.

• Opposite of synthesis.Opposite of synthesis.

• General equation: General equation: AB A + BAB A + B

• EX. Hydrogen peroxide decomposes into water and oxygenEX. Hydrogen peroxide decomposes into water and oxygen..2H2H22OO22 (aq) (aq) O O22 (g) + 2H (g) + 2H22O (l)O (l)

a. Pb(N03)2 + K2Cr04 PbCr04 + KN03


c. C3H6 + O2 CO2 + H2O


e. Li + O2 Li2O


g. MgC03 MgO + CO2

h. Ba(CN)2 + H2SO4 BaS04 + HCN

3. 3. Single replacementSingle replacement

• OneOne elementelement replaces a second replaces a second element in a compound.element in a compound.

-the more -the more reactivereactive metal will replace metal will replace the least reactive metal.the least reactive metal.

-the more -the more reactivereactive nonmetal will nonmetal will replace the least reactive nonmetal.replace the least reactive nonmetal.

• General equation: General equation: A + BX AX + A + BX AX + B B

EX. 2AgNOEX. 2AgNO33 (aq) + Cu(s) (aq) + Cu(s) 2Ag(s) + Cu(NO 2Ag(s) + Cu(NO33))22 (aq) (aq)Silver nitrate reacts with copper to produce silver and copperSilver nitrate reacts with copper to produce silver and copper

a. Pb(N03)2 + K2Cr04 PbCr04 + KN03


c. C3H6 + O2 CO2 + H2O


e. Li + O2 Li2O


g. MgC03 MgO + CO2


4. 4. Double replacementDouble replacement

• Involves an Involves an exchangeexchange of positive ions of positive ions between two reacting compounds. One of between two reacting compounds. One of the following applies:the following applies:– One product One product precipitatesprecipitates from solution from solution oror– One product is a One product is a gasgas and bubbles out of and bubbles out of

solution solution oror– One product is a One product is a molecularmolecular compound compound

such as water.such as water.• AA++XX-- + B + B++YY- - A A+ + YY-- + B + B+ + XX--

Ex. AgNOEx. AgNO33 (aq) + NaCl (aq) (aq) + NaCl (aq) AgCl (s) + NaNO AgCl (s) + NaNO33 (aq) (aq)

a. Pb(N03)2 + K2Cr04 PbCr04 + KN03


c. C3H6 + O2 CO2 + H2O


e. Li + O2 Li2O


g. MgC03 MgO + CO2


5. 5. CombustionCombustion

• An element or compound reacts with An element or compound reacts with oxygenoxygen, often producing energy as heat , often producing energy as heat or light.or light.

• Important combustion reaction is burning Important combustion reaction is burning of of fossilfossil fuelsfuels which produce which produce carbon carbon dioxidedioxide and and water.water.

• Some synthesis (combination) reaction fall Some synthesis (combination) reaction fall under this category.under this category.

Ex. 2Mg(s) + OEx. 2Mg(s) + O22(g) (g) 2MgO(s) 2MgO(s)

a. Pb(N03)2 + K2Cr04 PbCr04 + KN03


c. C3H6 + O2 CO2 + H2O


e. Li + O2 Li2O


g. MgC03 MgO + CO2


Reactions in Aqueous Reactions in Aqueous solutionssolutions

- A solution contains a solute dissolved in - A solution contains a solute dissolved in water – water is the solvent in most water – water is the solvent in most cases.cases.

I.I. Aqueous solutions:Aqueous solutions:• Compounds that are soluble in water Compounds that are soluble in water

dissolved into their ions.dissolved into their ions.• NaCl(aq) NaCl(aq) Na Na++ + Cl + Cl- - see next slide.see next slide.

Water molecules Water molecules surrounding ions.surrounding ions.

Positive end of water (H)

Negative end of water (O)

II. Net ionic equations:II. Net ionic equations:

1.1. Write the chemical balanced equation.Write the chemical balanced equation.2.2. If there are ionic compounds look @ If there are ionic compounds look @ solubility solubility

rulesrules and determine which chemical compounds and determine which chemical compounds are either are either solublesoluble or or insoluble.insoluble.

3.3. If they are soluble you should write If they are soluble you should write (aq)(aq) after after them and if they are insoluble write them and if they are insoluble write (s).(s).

4.4. Rewrite the equation by breaking up the Rewrite the equation by breaking up the soluble soluble ionicionic compoundscompounds into their ions – remembering to into their ions – remembering to keep the number appropriate with coefficients in keep the number appropriate with coefficients in front. Do not break up front. Do not break up molecular compoundsmolecular compounds..

5.5. Cancel out the ions that appear on Cancel out the ions that appear on bothboth sides of sides of the product sign the product sign

6.6. Rewrite the equation again with only the ions Rewrite the equation again with only the ions that have not been canceled, the solids that have not been canceled, the solids (insoluble), and molecular compounds. (Net Ionic (insoluble), and molecular compounds. (Net Ionic Equation)Equation)

Example 1:Example 1:

silver nitrate + sodium chloride silver nitrate + sodium chloride silver chloride + sodium nitrate silver chloride + sodium nitrate

1.1. AgNOAgNO33 + NaCl + NaCl AgCl + NaNO AgCl + NaNO33

2.2. soluble soluble insoluble solublesoluble soluble insoluble soluble

3. AgNO3. AgNO33 ((aqaq)) + NaCl + NaCl ((aq)aq) AgCl AgCl (s)(s) + NaNO + NaNO33 (aq)(aq)

4 & 5. 4 & 5. AgAg++ + NO + NO33-- + Na + Na++ + Cl- + Cl- AgCl (s) + Na AgCl (s) + Na++ + NO + NO33

--

6. 6. Ag+ (aq) + Cl- (aq) Ag+ (aq) + Cl- (aq) AgCl (s) AgCl (s)

You Try……You Try……

• Barium chloride + sodium sulfate Barium chloride + sodium sulfate barium sulfate + sodium barium sulfate + sodium chloridechloride

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Chemical & Physical Changes