Transcript Chemical Equations

Chemical Equations
&
Chemical Reaction Types
The Meaning of a Chemical
Equation
• Chemical equations give information in two major areas.
• First, they tell us what substances are reacting (those being
used up) and what substances are products (those being
made).
• Second, the coefficients of a balanced equation tell us in
what ratio the substances will react or are produced.
• Thirdly they also will represent the number of moles of
each substance reacting or produced.
Equation Breakdown
• The reactants are on the left side of a chemical
equation and the products are on the right side.
• However, you might ask, "On the left and right side
of what?"
• Answer - the arrow.
• 2 H2 + O2 ---> 2 H2O
• On the left side are the reactants - hydrogen and
oxygen. We will ignore the two in front of the
hydrogen for a moment.
• On the right side is the product - water. We will also
ignore the two in front of the water, but we will soon
return to it.
• Please be aware that there can be one, two, three, or
more substances on either side of the arrow, as in
this more complex equation:
• Ca(H2PO4)2 + CaSO4 + HF ---> Ca10F2(PO4)6 + H2SO4
• Typically the arrow is replaced with "produces" or "yields"
when the equation is said out loud.
• Coefficients are the numbers in front of the formulas.
• 2 H2 + O2 ---> 2 H2O
• Note the presence of a two in front of the hydrogen and also
the water.
• These are called the coefficients. These numbers give two
very important pieces of information about the equation.
• First: the coefficients give the number of molecules
(or atoms) involved in the reaction.
• Second: the coefficients give the number of moles of
each substance involved in the reaction.
• The point just made is CRITICAL.
• The coefficients of an equation tell us how many
moles of each reactant are involved as well as how
many moles of each product get produced.
• What is the relationship between molecules and
moles?
• Yes!, Avogadro's Number is the factor between the
two points above.
Quick Review
• There are three important things to remember about
reading an equation.
• One: Reactants are on the left and products are on
the right. Of What?
• Arrow
• Two: Coefficients are the numbers in front of each
formula. If no number is shown, a one is understood.
• Three: The coefficients tell us how many molecules
or moles of each reactant used and how many
molecules or moles of each product made.
Practice Problems
• Identify the reactants, the products and the
coefficients of these equations:
• What is the mass of each substance in the balanced
equation for #5.
1) Zn + 2 HCl ---> ZnCl2 + H2
2) 2 KClO3 ---> 2 KCl + 3 O2
3) S8 + 24 F2 ---> 8 SF6
4) 4 Fe + 3 O2 ---> 2 Fe2O3
5) 2 C2H6 + 7 O2 ---> 4 CO2 + 6 H2O
Balancing Chemical Equations
• Chemical equations do not come already balanced. This
must be done before the equation can be used in a
chemically meaningful way.
• All chemical calculations to come must be done with a
balanced equation.
• A balanced equation has equal numbers of each type of
atom on each side of the equation.
• Why?
• The Law of Conservation of Mass, Matter & Energy
“Mass, Matter & Energy are neither created nor destroyed."
Therefore, we must finish our chemical reaction with as many
atoms of each element as when we started.
Here is the example equation for this lesson:
• H2 + O2 ---> H2O
• In the example equation, there are two atoms of hydrogen
on each side, BUT there are two atoms of oxygen on the left
side and only one on the right side.
• Remember this: A balanced equation MUST have EQUAL
numbers of EACH type of atom on BOTH sides of the
arrow.
• An equation is balanced by changing coefficients in a
somewhat trial-and-error fashion. It is important to note that
only the coefficients can be changed, NEVER a subscript.
So, now to balancing the example equation:
• H2 + O2 ---> H2O
• The hydrogen are balanced, but the oxygens are not. We
have to get both balanced. We put a two in front of the
water and this balances the oxygen.
• H2 + O2 ---> 2 H2O
• However, this causes the hydrogen to become unbalanced.
To fix this, we place a two in front of the hydrogen on the
left side.
• 2 H2 + O2 ---> 2 H2O
• This balances the equation.
Balance this equation: H2 + Cl2 ---> HCl
•Remember that the rule is: A balanced equation MUST have
EQUAL numbers of EACH type of atom on BOTH sides of
the arrow.
•H2 + Cl2 ---> 2 HCl
•Balance this equation: O2 ---> O3
•3 O2 ---> 2 O3 The LCM tells you how many of each atom
will be needed. Least common multiple.
Reaction Types
• Single Replacement
• Double Replacement or Ionic
• Decomposition or Analysis
• Composition or Synthesis
• Combustion
Single Replacement
•
1.
•
•
•
•
•
During single replacement, one element replaces another element in
a compound. There are two different possibilities:
One cation replaces another. Written using generic symbols, it is:
AX + Y ---> YX + A
Element Y has replaced A (in the compound AX) to form a new
compound YX and the free element A. Remember that A and Y are
both cations ( positively-charged ions) in this example.
Examples
Rule
Cu + AgNO3 ---> Ag + Cu(NO3)2
More active metal
Ca + H2O ---> Ca(OH)2 + H2
Metal replaces Hydrogen in Water
Zn + HCl ---> ZnCl2 + H2
Metal replaces Hydrogen in an Acid
2. One anion replaces another. Written using generic symbols,
it is:
A + XY ---> XA + Y
• Element A has replaced Y (in the compound XY) to form a
new compound XA and the free element Y.
• Remember that A and Y are both anions
(negatively-charged ions) in these examples.
Examples
Rule
Cl2 + NaBr ---> NaCl + Br2
More Active Halogen
Br2 + KI ---> KBr + I2
More Active Halogen
ZnS + O2 ---> ZnO + S8
More Active Non-metal
Reaction Types: Double Replacement
• During double replacement, the cations or anions of two
different compounds switch places.
• Written using generic symbols, it is:
AB + XY ---> AY + XB
• A and X are the cations (postively-charged ions) in this
example, with B and Y being the anions (negativelycharged ions).
• Examples
Rule
• KOH + H2SO4 ---> K2SO4 + H2O Water is a Product
• AgNO3 + NaCl ---> AgCl + NaNO3 A precipitate is formed
Use solubility table to see if a precipitate will form
Double Replacement
In order for a double replacement reaction to
occur, one of three things must show up as
a product.
1. Water
2. Precipitate
3. Gas (Non-metallic oxides, diatomic
substances)
Reaction Types
• Single Replacement
• Double Replacement or Ionic
• Decomposition or Analysis
• Composition or Synthesis
• Combustion
Reaction Types: Decomposition
• During decomposition, one compound splits apart into two (or more
pieces). These pieces can be elements or simpler compounds.
• Written using generic symbols, it is usually shown as:
• AB ---> A + B
• There are 6 general decomposition type reactions that can occur, the
1st 5 given here require heat being applied to the reactant side.
• 1. Metallic oxide when heated: form a metal and oxygen
•
Mercuric oxide  Mercury and Oxygen
• 2. Metallic carbonates when heated : form a metallic oxide and
carbon dioxide (gas)
•
Calcium carbonate  Calcium oxide and carbon dioxide gas.
Decomposition cont.
• 3. Many metallic hydroxides when heated form metallic
oxides and water vapor.
• Calcium hydroxide  Calcium oxide and water (gas)
• 4. Metallic chlorates when heated form metallic chloride
and oxygen (gas).
• Potassium chlorate  Potassium chloride and oxygen (g)
• 5. Some acids when heated decompose into nonmetallic
oxides and water.
• Carbonic Acid  Carbon dioxide and water
• 6. Decomposition by electrolysis (electricity)
• Water Hydrogen (g) and oxygen (g) or
• Sodium Chloride  Sodium (s) and chlorine (g)
Reaction Types
• Single Replacement
• Double Replacement or Ionic
• Decomposition or Analysis
• Composition or Synthesis
• Combustion
Reaction Types: Synthesis or Composition
• Synthesis are, at this introductory level, almost always the
reverse of a decomposition reaction. That means that two
pieces join together to produce one, a more complex
compound. These pieces can be elements or simpler
compounds.
• A + B ---> AB
Reaction Types: Combustion
•Combustion, at its most general, can mean the reaction of
oxygen gas (O2) with anything.
•However, we will understand combustion to mean the
reaction of oxygen with a compound containing carbon and
hydrogen. A common synonym for combustion is burn.
•Written using generic symbols, it is usually shown as:
•CxHy + O2 ---> CO2 + H2O
Combustion
• C6H12O6 + O2 ---> CO2 + H2O
• CH4 + O2 ---> CO2 + H2O
• Notice that some compounds contain carbon, hydrogen AND oxygen.
However, the products are all the same, in every reaction.
• We could vary it a bit by adding nitrogen (burns to form NO2) to the
compound formula or sulfur (burns to form SO2 or SO3). Like these:
• C21H24N2O4 + O2 ---> CO2 + H2O + NO2
• C2H5SH + O2 ---> CO2 + H2O + SO2
Phase Notation & Other forms of
notation
• (g) gas
(ppt) precipitate or (s) solid
• (l) liquid
(s) solid
• (aq) aqueous – dissolved in water or solution of
MnO2 shows as a catalyst
• heat is applied