Transcript Chapter 8 Chemical Reactions
1 Unit 9 Chemical Equations CP Chemistry
2 Describing Chemical Change
OBJECTIVES:
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Write equations describing chemical reactions, using appropriate symbols
3 Describing Chemical Change
OBJECTIVES:
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Write balanced chemical equations, when given the names or formulas of the reactants and products in a chemical reaction.
4 All chemical reactions
have two parts:
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Reactants - the substances you start with
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Products - the substances you end up with
The reactants turn into the products.
Reactants
Products
5 Symbols in equations
the arrow separates the reactants from the products
Read “reacts to form” or “yields”
The plus sign + “and”
(s) after the formula = solid
(g) after the formula = gas
(l) after the formula = liquid
6 Symbols used in equations
(aq) after the formula - dissolved in water, an aqueous solution.
used after a product indicates a gas (same as (g))
used after a product indicates a solid (same as (s))
7 Symbols used in equations
indicates a reversible
reaction (more later)
, shows that heat is supplied to the reaction
is used to indicate a catalyst is supplied, in this case, platinum.
8 What is a catalyst?
A substance that speeds up a reaction, without being changed or used up by the reaction.
Enzymes are biological or protein catalysts.
9 Exothermic and Endothermic
Exothermic Reactions give off heat
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Ex: explosions
Endothermic Reactions absorb heat and feel cold
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Ex: ice packs from the nurse
In a chemical reaction
The way atoms are joined is changed
Atoms aren’t created or destroyed.
Can be described several ways: 1. In a sentence Copper reacts with chlorine to form copper (II) chloride.
2. In a word equation Copper + chlorine
copper (II) chloride
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DO NOT FORGET DIATOMIC MOLECULES!!!
BrINClHOF
Never travel alone!!
Convert these to equations
Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.
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Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.
13 Now, write these:
Fe(s) + O 2 (g)
Fe 2 O 3 (s)
Cu(s) + AgNO 3 (aq)
Ag(s) + Cu(NO 3 ) 2 (aq)
NO 2 (g) N 2 (g) + O 2 (g)
14 Balancing Chemical Equations
15 Balanced Equation
Atoms can’t be created or destroyed
All the atoms we start with we must end up with
A balanced equation has the same number of each element on both sides of the equation.
16 Rules for balancing:
Assemble, write the correct formulas for all the reactants and products
Count the number of atoms of each type appearing on both sides
Balance the elements one at a time by adding coefficients (the numbers in front) - save H and O until LAST!
Check to make sure it is balanced.
Never change a subscript to balance an equation.
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If you change the formula you are describing a different reaction.
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H 2 O is a different compound than H 2 O 2
Never put a coefficient in the middle of a formula
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2 NaCl is okay, Na2Cl is not.
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Example
H 2 + O 2
H 2 O
18 Make a table to keep track of where you are at
Example
H 2 + O 2
H 2 O R 2 2 H O P 2 1
Need twice as much O in the product 19
Example
H 2 + O 2
2 H 2 O R 2 2 H O P 2 1
Changes the O 20
Example
H 2 + O 2
2 H 2 O R 2 2 H O P 2 1 2
Also changes the H 21
Example
H 2 + O 2
2 H 2 O R 2 2 H O P 2 1 4 2
Need twice as much H in the reactant 22
Recount 23 Example
2 H 2 + O 2
2 H 2 O R 2 2 H O P 2 1 4 2
Example
2 H 2 + O 2
2 H 2 O 4 R 2 2 H O P 2 1 4 2
24 The equation is balanced, has the same number of each kind of atom on both sides
25 Example
2 H 2 + O 2
2 H 2 O 4 R 2 2 H O P 2 1 4 2
This is the answer Not this
26 Balancing Examples
_ AgNO 3 + _Cu
_Cu(NO 3 ) 2 _Mg + _N 2 _P + _O 2
_Mg _P 4 O 3 10 N 2 _Na + _H 2 O
_CH 4 + _O 2
_H 2 _CO 2 + _NaOH + _H 2 O + _Ag
27 Balancing Examples
2AgNO 3 + 2Cu
Cu(NO 3 ) 2 3Mg +N 2 CH 4 + 2O
4P + 5O 2
P 4 O 10 2Na + 2H 2 O
H 2 2 Mg 3 N 2
CO 2 + 2NaOH + 2H 2 O + 2Ag
28 Types of Chemical Reactions
OBJECTIVES:
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Identify a reaction as combination, decomposition, single-replacement, double replacement, or combustion
Types of Chemical Reactions 29
OBJECTIVES:
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Predict the products of combination, decomposition, single-replacement, double replacement, and combustion reactions.
Types of Reactions
There are millions of reactions.
Can’t remember them all
Fall into several categories.
We will learn 5 major types.
Will be able to predict the products.
For some, we will be able to predict whether they will happen at all.
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Will recognize them by the reactants
#1 - Combination (SYNTHESIS)
Combine - put together
2 substances combine to make one
compound.
Ca +O 2
CaO
SO 3 + H 2 O
H 2 SO 4
We can predict the products if they are two elements.
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Mg + N 2
32 Write and balance
Ca + Cl 2 Fe + O 2
Al + O 2
Remember that the first step is to write the correct formulas
Then balance by using coefficients only
33 #2 - Decomposition Reactions
decompose = fall apart
one reactant falls apart into two or more elements or compounds.
NaCl Na + Cl 2
CaCO 3 CaO + CO 2
Note that energy is usually required to decompose
34 #2 - Decomposition Reactions
Can predict the products if it is a binary compound
Made up of only two elements
Falls apart into its elements
H 2 O
HgO
35 #2 - Decomposition Reactions
If the compound has more than two elements you will only be
asked to balance them __ NiCO 3
iO + ___ CO 2
__KClO 3 (aq) __ KCl +__ O 2
36 #3 - Single Replacement
One element replaces another
Reactants must be an element and a compound.
Products will be a different element and a different compound.
K + NaCl
F 2 + LiCl
Na + KCl LiF + Cl 2
#3 Single Replacement
Metals replace other metals (and hydrogen)
K + AlN
Zn + HCl
Think of water as HOH
Metals replace one of the H, combine with hydroxide.
Na + HOH
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#3 Single Replacement
We can tell whether a reaction will happen
Some chemicals are more “active” than others
More active replaces less active
There is a list on the last page of your packet called the Activity Series of Metals
Higher on the list replaces lower.
If something isn’t on the activity series you have, assume the one with the higher atomic number is higher on the list
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#3 Single Replacement
Note the * concerning Hydrogen
H can be replaced in acids by everything higher
Li, K, Ba, Ca, & Na replace H from acids
and water Al + HCl
Fe + CuSO 4 Pb + KCl
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40 #3 - Single Replacement
Nonmetals can replace other nonmetals
Limited to F 2 , Cl 2 , Br 2 , I 2 (halogens)
Higher replaces lower.
F 2 + HCl
Br 2 + KCl
#4 - Double Replacement
Two things replace each other.
Reactants must be two ionic compounds or acids.
Usually in aqueous solution
NaOH + FeCl 3
The positive ions change place.
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NaOH + FeCl 3 NaOH + FeCl 3
Fe +3 OH
Fe( OH) 3 + Na +1 + Na Cl Cl -1
42 #4 - Double Replacement
Has certain “driving forces”
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Will only happen if one of the products:
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doesn’t dissolve in water and forms a solid (a “precipitate”), or
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is a gas that bubbles out, or
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is a covalent compound (usually water).
43 Complete and balance
assume all of the following reactions take place: CaCl 2 CuCl 2 + NaOH + K 2 S
KOH + Fe(NO 3 ) 3
(NH 4 ) 2 SO 4 + BaF 2
44 How to recognize which type
Look at the reactants: E + E = Combination C = Decomposition E + C = C + C = Single replacement Double replacement
45 Examples
H 2 + O 2 H 2 O
Zn + H 2 SO 4 HgO
KBr +Cl 2 AgNO 3
+ NaCl
Mg(OH) 2 + H 2 SO 3
46 #5 - Combustion
Means “add oxygen”
A compound composed of only C, H, and maybe O is reacted with oxygen
If the combustion is complete, the
products will be CO 2 and H 2 O.
If the combustion is incomplete, the products will be CO (possibly just C) and H 2 O.
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C 4 H 10 + O 2
Examples
C 6 H 12 O 6 + O 2
C 8 H 8 +O 2
48 An equation...
Describes a reaction
Must be balanced in order to follow the Law of Conservation of Mass
Can only be balanced by changing the coefficients.
Has special symbols to indicate physical state, and if a catalyst or energy is required.
49 Reactions
Come in 5 major types.
Can tell what type they are by the reactants.
Single Replacement happens based on the activity series
Double Replacement happens if the product is a solid, water, or a gas.
Net Ionic Equations
Many reactions occur in water- that is, in
aqueous solution
Many ionic compounds “dissociate”, or separate, into cations and anions when dissolved in water
Now we can write a complete ionic equation
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Net Ionic Equations
Example:
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AgNO 3 + NaCl
AgCl + NaNO 3 1. this is the full equation 2. now write it as an ionic equation 3. can be simplified by eliminating ions not directly involved (spectator ions) = net ionic equation
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Predicting the Precipitate
Insoluble salt = a precipitate - note Figure 8.13, p.227
General rules: Table 8.3, p. 227, Reference p.7 (back of textbook), and in Lab manual p.338
Sample problem 8-11, p.228
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