Regents Chemistry

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Regents Chemistry

• Chemical Reactions An Introduction

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Physical vs. Chemical Properties

• Substances are described by their physical and
  chemical properties
• Physical properties describe how it looks, smells
  and feels. No change in composition occurs!
• ex color, odor, volume and state of matter
• Chemical properties describe the substances
  ability to form new substances
• ex ability of wood to burn, metal to rust,
  food to digest

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Examples of Properties

• Classify each as a physical or chemical property
• 1. Boiling point of ethanol
• 2. The hardness of a diamond
• 3. Sugar ferments to form alcohol
• 4. A metal wire conducts electricity

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Physical vs. Chemical Changes

• A physical change is a change in one or more
  physical properties, but no change in the
  fundamental components that make up the
  substance. Most common are changes of state
• A Chemical change is a change in fundamental
  components, a change into a new substance.
  Called reactions!

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Examples

• Classify each of the following as a physical or
  chemical change
• Melting of ice into boiling of water
• Electrolysis of water
• Iron metal is melted
• Tarnishing of silver
• Breaking a rock into pieces

? Worksheet
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Regents Chemistry

• Chemical Reactions

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Whats a Chemical Reaction?

• Chemical reactions are part of our everyday life

Natural gas is burned to heat houses
Steel on cars rusts
Plastic is made for all kinds of uses
Mini-hot packs are used to keep Us warm outdoors!
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How Do We Know a Reaction Occurs?

• Chemical reactions often give visible clues
• Demos

silver nitrate and potassium chloride
Methane gas and oxygen
Mini-heat packs
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Evidence of a Reaction

• Some of the BIG clues..
• 1. The color changes
• Bunsen Burner
• 2. A solid forms
• silver nitrate and potassium chloride
• 3. Bubbles form
• zinc metal and HCl
• 4. Heat and/or a flame is produced, or heat is
  absorbed
• Mini-heat packs

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Whats involved in a reaction?

• Chemical reactions have (2) parts
• The Reactants and the Products

Reactants are on the left side of the equation
and shows what goes into the reaction!
Products are on the right side of the equation
and shows what comes out of the reaction!
Example
K H2O ? H2 KOH
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Reactants/Products Have Physical States!

• We look at our previous example

K H2O ? H2 KOH
(s)
(l)
(g)
(aq)

• We place the abbreviation for the state
• After each chemical formula
• Symbol State
• (s) solid
• (l) Liquid
• (g) gas
• (aq) aqueous (dissolved in water)

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Writing Unbalanced Chemical Equations

• We will start by writing basic chemical equations
  from word problems
• Example Solid carbon reacts with gaseous oxygen
  to form gaseous carbon dioxide

Reactants solid carbon gaseous oxygen
Products gaseous carbon dioxide
C(s) O2(g) ? CO(g)
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Example 2

• Solid zinc metal reacts with hydrochloric acid to
  produce bubbles of hydrogen gas and aqueous zinc
  chloride

Zn(s) HCl(aq) ? H2(g) ZnCl2(aq)
You must use the naming compound rules To figure
out how to write the chemical formulas
Remember- some gases are always diatomic H2 , O2
, Cl2
worksheet
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Exothermic and Endothermic Processes

• Exothermic processes release energy
• Example Burning of wood
• Endothermic processes absorb energy
• Example Cold packs!

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Regents Chemistry

• Balancing Chemical Equations

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BALANCING CHEMICAL EQUATIONS
RECAP All forward equations are written with
reactants of the left an products on the
right Example 2H2 O2 ? 2H2O
4 atoms of H in product
Coefficient means 2 molecules of H2, 4
atoms total
Subscript means 2 atoms of H
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BALANCING CHEMICAL EQUATIONS

• Atoms are neither created or destroyed
• so all equations must be balanced!
• RULES FOR BALANCING EQUATIONS
• Cannot change subscripts
• 2. Can change coefficients

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BALANCING CHEMICAL EQUATIONS
Examples Balance this equation Na(s)
H2O(l) ? NaOH(aq) H2(g)

• 2Na(s) 2H2O(l) ? 2NaOH(aq) H2(g)

Worksheet
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Regents Chemistry

• Writing and Balancing Equations

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Regents Chemistry

• Types of Reactions

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Types of Reactions

• Although we cannot classify all reactions into
  distinct categoriesthere are four major types of
  reactions you should know
• Synthesis (combination) Reactions
• Decomposition (analysis) Reactions
• Single Replacement Reactions
• Double Replacement Reactions

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Synthesis (combination) Reactions

• A reaction is classified as a synthesis reaction
  when two or more reactants combine and a single
  product is formed

B G ? BG
4Fe(s) 3O2(g) ? 2Fe2O3(s)
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Decomposition (analysis) Reactions

• Decomposition reactions are the reverse of
  combination reactions
• Occur when one substance breaks down into simpler
  substances

BG ? B G
CaCO3(s) ? CaO(s) CO2(g)
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Regents Chemistry
Single and Double Replacement Reactions
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Single Replacement Reactions

• Involves an element and a compound
• Occurs when a metal displaces the metal in a
  compound

B2 B1G ? B2G B1
Cu(s) 2AgNO3(aq) ? Cu(NO3)2(aq) 2Ag(s)
Not all metals will displace (react with) a
metal in a compound..so how do we know if a
reactions will occur? we use our Table J in our
reference tables!
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Predicting if a Reaction Will Occur

• Table J (Activity Series) on p. 4 is arranged so
  that a metal listed on the table will react with
  the compound of a metal that is below it
• For Example

Zn will react with a compound of copper
Zn(s) Cu(NO3)2 ? Cu(s) Zn(NO3)2(aq)
But because Cu is below Zn on the table, it will
not react with compounds of Zn
Cu(s) Zn(NO3)2 ? no reaction
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So..

• So..above it will bump it below it will not!
• The MORE ACTIVE will bump the LESS ACTIVE

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Predicting if a Reaction Will Occur

• There is one element on the table that is not a
  metalHydrogen ion (H)!
• All metals above hydrogen will react with acids
  to release hydrogen gas and produce a salt

Mg(s) 2HCl(aq) ? H2(g) MgCl2(aq)
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Predicting if a Reaction Will Occur

• In the 2nd column of Table J is a list of
  nonmetals
• A nonmetal will replace a less active nonmetal in
  a compound according to the equation

G1 BG2 ? BG1 G2
For example..Fluorine is listed as the most
active nonmetal, and it will replace chlorine,
bromine and iodine from other binary compounds
F2(g) 2NaCl(aq) ? Cl2(g) 2NaF(aq)
Cl is below F so it will not react!
Cl2(g) NaF ? no reaction
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Practice Problems

• Predict if reactions will occur and write the
  the products or NR for NO REACTION
• Ca(s) AgNO3(aq)
• Pb(s) Al(NO3)3(aq)
• Cr(s) Pb(NO3)2(aq)
• Co(s) HCl(aq)

--gt CaNO3(aq) Ag(s)
--gt NR
--gt Cr(NO3)2(aq) Pb(s)
--gt CoCl2(aq) H2(g)
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Double Replacement Reactions

• Double replacement reactions generally involve
  two soluble ionic compounds that react in
  solution to produce a precipitate, a gas, or a
  molecular compound such a water
• Represented by the equation

B1G1 B2G2 ? B1G2 B2G1
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Predicting if a Reaction Will Occur in a Double
Replacement Reaction

• Three general situations in which a double
  replacement reaction will occur
• 1. If a solid (precipitate) is formed
• We look at Table F (p.2) in the reference tables
  and check the solubility of the two compounds

Ex
AgNO3(aq) NaCl(aq) ? AgCl(s) NaNO3(aq)
The reaction will occur because AgCl is
insoluble meaning, it will not stay dissociated
in solution
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Soluble vs. Insoluble

• Soluble means the substance will stay as ions in
  the solution (just floating around!)no reaction
  will happen!
• Insoluble means it will form a solid, gas or
  molecular compound (such as water) and not stay
  as ionsa reaction will occur!

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Predicting a Reaction...

• 2. If a gas is formed
• Na2S(aq) 2HCl(aq) H2S(g)
  2NaCl(aq)
• If a molecular substance, such as H2O is formed
• NaOH(aq) HCl(aq) H2O(l)
  NaCl(aq)

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Practice Predicting..Will a Reaction Occur?
Ca(NO3)2(aq) KCl(aq)
CaCl2(aq) KNO3(aq)
NR
Pb(NO3)2(aq) Na2SO4(aq)
PbSO4(s) NaNO3(aq)
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REGENTS CHEMISTRY

• Determining Missing Mass in Equations

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The Law of Conservation of Mass

• The Law of Conservation of mass states that
  matter cannot be created nor destroyed
• This is very useful when considering chemical
  reactions
• WHY?
• When given a balanced equation in which either
  the reactant or product is missing, we can
  determine the formula of the missing
  substance..this we have done!

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Missing Formulas in Equations
K ___________ KCl
There must be a Cl in the reactants because it
appears in the products
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Missing Mass in Equations

• Just as the formula of a missing reactant or
  product can be determined, the mass of a missing
  substance can also be found!
• TOTAL MASS BEFORE TOTAL MASS AFTER
• HOW???? USING SIMPLE MATH!

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EXAMPLE

• If 103.0 g of potassium chlorate is decomposed to
  form 62.7 g of potassium chloride and oxygen gas,
  how many grams of oxygen are formed?

2KClO3(aq) --gt 2KCl(aq) 3O2(g)
1. Find the total mass of the reactants 2. The
total mass of the reactants must equal the
total mass of the products
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EXAMPLE cont..
2KClO3(aq) --gt 2KCl(aq) 3O2(g)
mass of KCLO3 mass of KCl mass of O2
103.0g 62.7g mass O2 mass O2 103.0g
- 62.7g mass O2 40.3g
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Example 2

• What mass of carbon dioxide will be produced if
  144g of carbon react with 384 g oxygen gas?

C(s) O2(g) --gt CO2(g)
Mass C mass O2 mass CO2 144g C 384g
O2 --gt mass CO2 528g mass CO2
worksheet
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Unknown Reactants and Products

• Just like we can find missing mass, we can also
  identify missing reactants or products in a
  formula
• Remember 2 steps
• Whatever is on the left side must also be on the
  right side..this tells you what us missing!
• Think about the type of formula..this will help
  you complete the equation!