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Regents Chemistry
Chemical Reactions: An
Introduction
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
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
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!
Examples
Classify each of the following as a
physical or chemical change
1.
2.
3.
4.
5.
Melting of ice into boiling of water
Electrolysis of water
Iron metal is melted
Tarnishing of silver
Breaking a rock into pieces
 Worksheet
Regents Chemistry
Chemical Reactions
What’s 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!
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
Evidence of a Reaction
Some of the BIG clues..

1. The color changes


2. A solid forms


silver nitrate and potassium chloride
3. Bubbles form


Bunsen Burner
zinc metal and HCl
4. Heat and/or a flame is produced, or heat
is absorbed

Mini-heat packs
What’s 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
Reactants/Products Have
Physical States!
We look at our previous example…
K(s) + H2O(l)  H2(g) + KOH (aq)
We place the abbreviation for the state
After each chemical formula
Symbol
State
(s)
(l)
(g)
(aq)
solid
Liquid
gas
aqueous (dissolved in water)
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)
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
Exothermic and Endothermic
Processes
Exothermic processes release energy

Example: Burning of wood
Endothermic processes absorb energy

Example: Cold packs!
Regents Chemistry
Balancing Chemical Equations
BALANCING CHEMICAL
EQUATIONS
RECAP
All forward equations are written with reactants
of the left an products on the right
Example: 2H2 + O2  2H2O
Coefficient means
2 molecules
of H2, 4 atoms
total
4 atoms of
H in product
Subscript means
2 atoms of H
BALANCING CHEMICAL
EQUATIONS
Atoms are neither created or destroyed…
so all equations must be balanced!
RULES FOR BALANCING EQUATIONS
1. Cannot change subscripts
2. Can change coefficients
BALANCING CHEMICAL
EQUATIONS
Examples – Balance this equation
Na(s) + H2O(l)  NaOH(aq) + H2(g)
2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g)
Worksheet
Regents Chemistry
 Writing and Balancing Equations
Regents Chemistry
 Types of Reactions
Types of Reactions
 Although we cannot classify all reactions
into distinct categories…there are four
major types of reactions you should know
–
–
–
–
Synthesis (combination) Reactions
Decomposition (analysis) Reactions
Single Replacement Reactions
Double Replacement Reactions
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)
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)
Regents Chemistry
Single and Double Replacement
Reactions
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!
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
So..
So..above it will bump it…
below it will not!
The MORE ACTIVE will bump
the LESS ACTIVE
Predicting if a Reaction
Will Occur
 There is one element on the table that is not
a metal…Hydrogen 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)
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
Practice Problems
 Predict if reactions will occur and write the
the products or NR for NO REACTION:
 Ca(s) + AgNO3(aq) --> CaNO3(aq) + Ag(s)
--> NR
 Pb(s) + Al(NO3)3(aq)
 Cr(s) + Pb(NO3)2(aq) --> Cr(NO3)2(aq) + Pb(s)
 Co(s) + HCl(aq)
--> CoCl2(aq) + H2(g)
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
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
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 ions…a reaction will occur!
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)
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)
REGENTS CHEMISTRY
Determining Missing Mass in
Equations
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!
Missing Formulas in Equations
K + ___________ = KCl
There must be a Cl in the reactants
because it appears in the products
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!
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) --> 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
EXAMPLE cont..
2KClO3(aq) --> 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
Example #2
What mass of carbon dioxide will be
produced if 144g of carbon react with 384 g
oxygen gas?
C(s) + O2(g) --> CO2(g)
Mass C + mass O2 = mass CO2
144g C + 384g O2 --> mass CO2
528g = mass CO2
worksheet
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!
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