introduction to chemical reactions notes part ii · 2018-09-10 · the law of conservation of mass...
TRANSCRIPT
Science 1206 Unit 3: Chemical Reactions Page 1 of 15
Introduction to Chemical Reactions – Notes Part II
TEXT p. 218-219 (word equations)
There are many chemical reactions – too many to count in fact!
Like many other areas in science, chemical reactions have to be grouped to stay organized.
You know how we scientist types are about organization!
To group them – we must recognize patterns.
We have already grouped other things in chemistry:
Ionic vs. molecular
- Acid vs. base
Being able to recognize the patterns in chemical reactions will help us to:
Understand the reaction
Predict the products of similar reactions
Word Equations
- Used to easily represent a chemical reaction using words and symbols.
Generally:
all reactants all products
OR
reactant 1 + reactant 2 product 1 + product 2
Read as:
reactant one plus reactant two goes to product one plus product two
Examples:
Zinc and lead (II) nitrate react to form zinc nitrate and lead. Aluminum bromide and chlorine gas react to form aluminum chloride and bromine gas.
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Writing a Chemical Equation
If we are given a word equation
THEN
we are able to write a chemical equation by replacing the words with chemical formulas
(this is where all your naming practice comes in handy!)
State of matter
- We must also include the state for each compound in the reaction;
- It goes in brackets after each chemical formula
- Most of the time this information will be given in the word problem.
- We also need to know how to use a solubility table.
Solid (s) – all solids including metals (before they combine to make compounds, but
remember mercury is the exception – it’s a liquid at room temperature) and precipitates
(Precipitation is the creation of a solid from a solution. When the reaction occurs in a liquid
solution, the solid formed is called the 'precipitate'. The chemical that causes the solid to form
is called the 'precipitant'.)
Liquid (l) – for pure liquids such as water
Gas (g) – gaseous substances like many molecular compounds
Aqueous (aq) – compounds dissolved in water (solutions) including acids
SOLUBILITY TABLES:
- Contains general solubility guidelines for ionic compounds.
Can be used in just 2 easy steps!
Step 1: Look for the anion (negative ion) of the compound in the top of the columns.
Step 2: Look down the column to locate the cation (positive ion) and determine if it is in the…
- high solubility row - write (aq) after the compound in the chemical equation
- low solubility row - write (s) after the compound in the chemical equation.
o HINT: this is generally the precipitate in the products!
Science 1206 Unit 3: Chemical Reactions Page 3 of 15
TRY: Potassium chloride ____________________
Silver sulfate ____________________
Ammonium fluoride ____________________
Copper (II) acetate ____________________
Copper (I) acetate ____________________
Examples – Be sure to write the chemical formula and correct states for
each:
Solutions of sodium phosphate and calcium chloride are mixed and react to form a precipitate of calcium phosphate and sodium chloride. Copper metal and sulfuric acid react to form copper (II) sulfate, water and sulfur dioxide.
Extra Work
- Worksheet – Writing Chemical Equations (Unbalanced)
- Questions on Pg. 219 #’s 2 and 3
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The Law of Conservation of Mass
TEXT p. 222-223 (conserving mass)
Think about some of the chemical reactions you are familiar with like:
A nail becoming rusty – rust is added - nail gains mass
Wood burning in your woodstove – all that’s left is a few ashes - mass is lost
In fact, it seems like mass is gained or lost in many reactions.
However:
Through much experimentation,
scientists have discovered that
mass is NOT gained or lost in any chemical reaction.
Rather, it is conserved.
Every Single Time!
That is: Mass of the reactants = the mass of the products
ALWAYS!!
This has become known as:
The Law of Conservation of Mass
- In a chemical reaction the total mass of the reactants is always equal to the total mass
of the products
A woodstove example…
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And getting a little more technical…
This allows us to solve problems like:
Questions p. 223 #’s 4 and 5
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Balancing Chemical Equations
TEXT p. 226-229 (balancing chemical equations)
According to the Conservation of Mass, what does this tell us about the number and type of
atoms in the reactants versus the number and type of atoms in the products?
They MUST be the same – exactly the same…ALWAYS
During the chemical reaction the atoms become rearranged to form new products.
Consider the reaction:
H2 + O2 H2O
The atoms are not balanced.
How do we balance this equation?
What could we do to make it balanced?
Because the oxygen atoms are not equal on the product side, we add another water molecule.
Now the hydrogen atoms are not equal on the reactant side, so we add another hydrogen
molecule.
Look what we have:
- The numbers that we are placing in front of each compound to balance the equation are
called coefficients
- They do NOT change the chemical formula
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Let’s try this:
___CH4 + ___O2 ___CO2 + ___H2O
Check your answer:
Hints for balancing:
It doesn’t matter which side of the chemical equation you start on.
It usually helps to start with an atom that there is the most of.
Gradually work your way back and forth from reactants to rpoducts side until all
atoms are balanced.
When balancing combustion reactions (hydrocarbon and oxygen) balance in the
order of C-H-O.
Examples:
1) ____ N2 + ____ H2 ____ NH3 2) ____ KClO3 ____ KCl + ____ O2
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3) ____ C3H8 + ____ O2 ____ CO2 + ____ H2O 4) ____ Pb(OH)2 + ____ HCl ____ H2O + ____ PbCl2 5) ____ AlBr3 + ____ K2SO4 ____ KBr + ____ Al2(SO4)3
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Types of Chemical Reactions
TEXT p. 230 (combustion)
p. 233-235 (synthesis and decomposition)
p. 240-241 (single/double replacement)
- All members of a chemical family react in a similar way
there are definite patterns in reactions
- Knowledge of patterns is useful because:
o We will have a better understanding of chemical reactions
o We can predict products of unknown reactions
There are 5 types of chemical reactions that we will be
studying:
1. Hydrocarbon Combustion - rapid reaction of a substance with oxygen to produce oxides
- Often called burning
- In complete combustion, the hydrocarbon will always react with
oxygen to produce carbon dioxide and water.
BASE FORMULA:
Hydrocarbon + oxygen carbon dioxide + water
Balancing tip: Remember to balance in the order of C-H-O
Example:
Butane + oxygen carbon dioxide + water vapor
2 C4H10 + 13 O2 8 CO2 + 10 H2O
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2. Synthesis - also called combination or formation reactions
- combination of smaller atoms or compounds to form 1 larger compound (product)
- Often elements combine to form compounds
BASE FORMULA:
A + B AB
Examples:
1. Hydrogen + oxygen water
2H2 + O2 2H2O
2. Hydrogen chloride + ammonia ammonium chloride
HCl + NH3 NH4Cl
3. Decomposition
- The splitting of one compound (reactant) into elements or smaller compounds
BASE FORMULA:
AB A + B
Example
Water hydrogen + oxygen
2 H2O 2 H2 + O2
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4. Single Replacement - Also called single displacement
- Involves one element and one compound as reactants
- One element replaces (displaces) the other “like” element in the compound
o Metal element replaces the metal element in the compound
o Non-metal element replaces the non-metal element in the compound
BASE FORMULA
Z + AB A + ZB (where Z and A are metals)
Y + AB B + AY (where Y and B are non-metals)
Example:
1. 2 FeI2 + Pb→ PbI4 + 2Fe (metal replaces metal)
2. FeI2 + Cl2→ FeCl2 + I2(non-metal replaces non-metal)
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5. Double Replacement - Also called double displacement
- Like elements in different compounds replace (displace) each other
BASE FORMULA:
AB + XY AY + XB
o Where A and X are metals
o Where B and Y are non-metals
EXAMPLES:
1. HCl (aq)+ NaOH (aq)→NaCl (s)+H2O (l)
2. CaCO3 (aq)+2 HCl (aq)→ CaCl2(s) + H2CO3(aq)
NOTE: Neutralization reactions between acids and bases are double replacement
reactions.
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To Summarize:
AND:
Hydrocarbon
Combustion
Occurs when burning
takes place.
Hydrocarbon +
oxygen carbon
dioxide + water
CH4 + O2 CO2 + H2O
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Chemical Reactions and Energy TEXT: p. 281 (exothermic and endothermic reactions)
Some chemical reactions seem to speed up as they progress
- A forest fire burns slowly at first and then may burn out of control.
What Happens:
- Temperature speeds up reactions rates because it increases the motion of particles
which in turn increases the collisions between particles that result in a chemical change.
- That means that the heat energy released by the forest fire actually works to increase
the rate of combustion.
Other reactions release energy (heat):
- Water freezing
- Rubbing alcohol evaporating
- Dynamite exploding
Chemical changes that release energy:
Exothermic
Chemical changes that require an input of energy:
Endothermic
Exothermic Reactions
Example: gasoline + oxygen carbon dioxide + water + heat energy
Some physical changes can be exothermic
Example: sulfuric acid (l) sulfuric acid (aq) + heat
An energy diagram can be used to
graph the energy changes in an
exothermic reaction.
A + B -> C + D + Heat
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Endothermic Reactions
Some reactions require heat energy on a continuous basis for the change to take place
- Heat can be absorbed from the surroundings
- These changes do not speed up because surrounding temperature drops as heat is
absorbed.
Example: water (s) + heat energy water (l)
Energy diagram for an endothermic reaction
Heat + A + B -> C + D