Notes unit six

Unit 7 – Chemical Reactions Chemical equations, Energy of Reactions, Rates of reactions, Limiting reactants

Intro Vocabulary
Chemical reaction is the changing of substances to other substances by the breaking of bonds in reactants and the formation of bonds in the products
- when some chemicals come into contact, they break apart, join, or rearrange to form new chemicals (always to become more stable)
Chemical equations are shorthand representations of chemical reactions.

Some more vocabulary
Reactants are the elements or compounds that enter into a reaction
Products are the elements or compounds that are formed as a result of a chemical reaction
Arrow (  ) means yields, produces or forms
Reactant(s)  product(s)

Al 2 (SO 4 ) 3 + Ca(OH) 2  Al(OH) 3 + CaSO 4
Skeleton equation - Consists of symbols and subscripts (An unbalanced formula equation.)
Symbols : element, yield (  ), combining (+)
Subscript : small number found below the element symbol representing the number of atoms of each element present

Name the reactants (everything left of arrow)
Aluminum sulfate & Calcium hydroxide
Types & number of atoms in each reactant
Aluminum sulfate
Al = 2 S = 3 O = 12
Calcium hydroxide
Ca = 1 O = 2 H = 2

Name the products (everything right of arrow)
Aluminum hydroxide & Calcium sulfate
Types & number of atoms in each reactant
Aluminum hydroxide
Al = 1 O = 3 H = 3
Calcium sulfate
Ca = 1 S = 1 O = 4

Practice Naming & Counting
Li 2 S
-Lithium Sulfide Li = 2 S = 1
CoF 2
-Cobalt (II) fluoride Co = 1 F = 2
MgSO 4
-Magnesium sulfate Mg=1 O=1 S=4
Be 3 (PO 4 ) 2
-Beryllium phosphate Be=3 P=2 O=8
CF 4
-Carbon tetrafluoride C = 1 F = 4

Chemical equations can be shown by
1. Word equation
Carbon dioxide gas reacts with dihydrogen monoxide gas to form solid carbonic acid
2. Formula equation
CO 2(g) + H 2 O (g)  H 2 CO 3(s)

Let’s start with a formula equation
H 2 + O 2  H 2 O
How many hydrogens & oxygens in reactants?
H = 2 O = 2
How many hydrogens & oxygens in products?
H = 2 O = 1
What is wrong with this chemical equation?
Two oxygen atoms in reactants and only one oxygen is in the product!!
So we must…
BALANCE the chemical equation!!

Warm up
aluminum phosphate(PO 4 3- ) reacts with potassium oxide to produce aluminum oxide and potassium phosphate.

zinc chloride reacts with lithium fluoride to produce lithium chloride and zinc fluoride

iron (II) oxide reacts with calcium sulfate to form iron (II) sulfate and calcium oxide.

Balancing equations why & how
Why do we balance equations?
Law of conservation of mass
-atoms are not created or destroyed in an ordinary chemical reaction, just rearranged to form new substances
Whatever goes into a reaction must come out.

Activity
Follow instructions as given.
When finished with procedure answer the post activity questions on the sheet.
Notebook paper is fine
If time allows:
Create your own procedure to prove the LCM using a flask, water, effervescent tablet, and balloon.

Think about this…
You want to make a bicycle out of the following parts: frame, wheel, handlebar, pedal, seat
Write the word equation for making a bicycle:
frame + wheel + handlebar + pedal + seat  bicycle
Is the equation balanced?
No
frame + 2 wheels + handlebar + 2 pedals + seat  bicycle

Counting molecules/compounds
How many molecules of each of the following compounds are present in this equation?
Al 2 (SO 4 ) 3 + 3Ca(OH) 2  2Al(OH) 3 + 3CaSO 4
Reactants:
Al 2 (SO 4 ) 3 = 1 (when only 1 = no number)
Ca(OH) 2 = 3 (large 3 in front)
Products:
Al(OH) 3 = 2 (large 2 in front)
CaSO 4 = 3 (large 3 in front)

Steps to Balancing Equations
1. Determine the number of each element in reactants and in products
2. Balance
A. Polyatomic ions (if same poly. ion on both sides  balance as a chunk)
B. Metals
C. Nonmetals
D. “O” & “H”
3. Recheck your count!!!

H 2 + O 2  H 2 O
Only 1 oxygen atom “out” (2 “in”)
 place 2 in front of H 2 O
H O Only 2 hydrogen “in” (4 “out”)  place 2 in front of H 2 Recount total number of each type of atom 2 2 * Only add coefficients, NEVER Δ subscripts Can you ever make just 1 molecule of water? coefficient subscript

NaOH  Na 2 O + H 2 O
Only 1 sodium atom “in” (2 “out”)
 place 2 in front of NaOH
Recount total number of each type of atom 2 Na O H

Fe + O 2  Fe 2 O 3
Only 1 iron atom “in” (2 “out”)
 place 2 in front of Fe
Recount total number of each type of atom 2 Fe O Only 2 oxygen atom “in” (3 “out”)  place 2 in front of Fe 2 O 3 and place 3 in front of O 2 Only 2 iron atom “in” (4 “out”)  change 2 in front of Fe into a 4 2 3 4

Using state symbols
When writing chemical equations, the state of each product or reactant may be labeled with the following abbreviations
(s) = solid
(g) = gas
(l) = liquid
(aq) = aqueous (solid dissolved in a liquid, usually water)
NOTE: If the states of matter are not included, you will NOT need to include them. If the states of matter are present, you MUST include them!

7 Diatomic Molecules
7 elements can not exist as single elements – must exist in pairs if it is JUST that element
H O N Cl Br I F
These 7 are always H 2 , O 2 , N 2 , Cl 2 , Br 2 , I 2 , F 2
Never just write H, O, N, Cl, Br, I, F without being bonded to another element.
H 2 0 is okay – WHY?
Because O is bonded to another element

Steps to using word equation to form formula equations:
1) Write formulas / symbols
2) Check for diatomic molecule
3) Add state symbols (if given)
4) Balance (if can’t balance, then recheck formulas!!)

Writing formula equation from word equations
Solid sodium bromide reacts with chlorine gas to yield solid sodium chloride and bromine gas.
Na +1 Br -1 Na +1 Cl -1 NaBr + Cl  NaCl + Br Write formulas & element symbols Check for diatomics (HONClBr I F) Add state symbols Balance Recheck 2 2 (s) (s) (g) (g) NaBrCl 2 2

Another word equation
Solid aluminum metal reacts with oxygen gas to form solid aluminum oxide.
Al +3 O -2 Al + O  Al 2 O 3 Write formulas & element symbols Check for diatomics (HONClBr I F) Add state symbols Balance Recheck 2 (s) (s) (g) AlO 2 2 3 4

Writing Word Equations
Na 2 O (s) + CO 2(g)  Na 2 CO 3(s)
Solid sodium oxide combines with (reacts with / and) carbon dioxide gas to form (yields/produces) solid sodium carbonate.
NaCl (s) + AgNO 3(aq)  NaNO 3(aq) + AgCl (s)
Solid sodium chloride and (combines with / reacts with) aqueous silver nitrate forms (yields / produces) aqueous sodium nitrate and solid silver chloride.

5 Basic Types of Reactions
Synthesis Reaction
Two or more substances combine to form a single substance.
Also known as a combination reaction.
A + B  AB
always forming 1 product
Example: 2K + Cl 2  2KCl

Decomposition Reaction
A single compound is broken down into two or more products.
AB  A + B
always having 1 reactant
Example: CaCO 3  CaO + CO 2

Single Replacement (Displacement) Reaction
one element replaces another element in a compound (also called single displacement)
AB + C  AC + B
Always a compound + element as reactants
Example:
Mg + Zn(NO 3 ) 2  Mg(NO 3 ) 2 + Zn
(Mg is Cation so replaces the cation in the compound)

Double Replacement (Displacement) Reaction
the positive ions are exchanged between two reacting compounds (also called double displacement)
AB + CD  AD + CB
Always a compound + compound as reactants
Example: BaCl 2 + K 2 CO 3  BaCO 3 + 2KCl
(Ba & K are the cation that switch places forming the new compounds)

Combustion Reaction
an element or a compound reacts with oxygen often producing energy as heat and light
C x H y + O 2  CO 2 + H 2 O
Always has oxygen as a reactant
Is an exothermic reactions (gives off heat)
Example: CH 4 + 2O 2  CO 2 + 2H 2 O

Information from chemical formulas
The types of atoms that are bonded
the ratio of atoms in the compound or molecule
the “ formula mass ” of the compound or molecule (sometimes called molar mass)

Calculating Formula Mass
Formula mass can be calculated in amu’s or g’s of a substance by multiplying the number of atoms of each element by the mass in amu’s or g’s of the element. Then add the values together. (YES, sig figs COUNT!!!)
Example: CaSO 4
(# atoms each element x mass = total mass of element in compound)
1 Ca x 40.08g = 40.08 g
1 S x 32.06g = 32.06g
4 O x 15.999g = 63.996g
Then add masses of all elements together
+ + 136.14 g

The Mole
In chemistry one mole is equal to 6.022 x 10 23 particles (Avogadro’s number).
The gram formula mass of any compound is the mass of 1 mole of the compound in grams.
1 mole = 6.0022 x 10 23 is similar to
12 eggs = 1 dozen
52 weeks = 1 year
1 gross = 144

Percent Composition
The percent composition of a compound is the mass of each element in a compound relative to the total mass of the compound
Found by dividing the mass of the element by the mass of the compound and multiplying the answer by 100 percent
Example CaSO 4
Ca=40.08 g (40.08g/136.14g) x 100%= 29.44%
S =32.06g (32.06g/136.14g) x 100%= 23.55%
O =63.996g (63.996g/136.14g)x100%=47.007%
FM = 136.14g

Limiting Reactants
The limiting reactant is the reactant that determines the maximum amount of product that is formed.
The limiting reactant will be completely used up in a reaction and then the reaction stops.
The other reactant will have some unchanged so it is said to be the excess reactant .
For example, if you need to make 10 chicken sandwiches. You have 10 slices of bread and 10 pieces of chicken. If each sandwich requires 2 slices of bread and 1 piece of chicken, which is the limiting reactant? Excess reactant?

Rates of Reactions
The reaction rate is the change in concentration of reactants and products in a certain amount of time.
Rate at which the reactants disappear and the products appear.
Combining two substances (causing a reaction) means forcing their particles to hit, or collide with, one another
Collision Theory states that molecules must collide in order to react

Activation Energy
The activation energy is the energy needed to start the reaction.
When particles collide with sufficient energy – at least equal to the activation energy – existing bonds may be disrupted and new bonds can form
Endothermic reaction – the energy of the product is greater than that of the reactants (energy is absorbed into the reaction)
Exothermic reaction – the energy of the products is lower than that of the reactants (energy is released from the reaction)

Factors Affecting Reaction Rates
1. Nature of Reactants
Depends on the state of particular reactants and the complexity of the bonds that have to be broken and formed in order for the reaction to proceed
The more bonds to be broken then the longer the reaction takes
A reaction between two gases will be quicker than a reaction between two liquids or two solids.

2. Temperature
The higher the temperature at which a reaction occurs, the faster the particles will move and the more frequent the collisions
For example, food spoils faster at room temperature than when it is refrigerated.

3. Concentration
Deals with how many particles are there
An increase in concentration means that there are more particles within a given volume and thus smaller spaces between the reacting particles.
Thus, the higher the concentration of reactants, the greater the frequency of collisions among their particles.
For example, the more people there are in a room the more people you will bump into as you walk through the room.

4. Surface Area
Surface area deals with the number of particles that are exposed for reaction.
The larger the surface area the greater the number of particles that are exposed for reaction.
For example, many small pieces of coal will burn faster than a lump of coal (small pieces have more particles exposed to react with more oxygen particles)

5. Catalysts
A catalyst is a substance that increases the rate of the reaction without itself being used up in the reaction (doesn’t appear as a reactant or a product)
Catalysts lower the activation energy required for a reaction to occur.
Thus a catalyst creates a different pathway from reactants to products – one that requires less energy.
Catalysts in the body are enzymes – there to speed up reactions in the body that are essential to life.

Notes unit six

by Frederick High School

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Notes unit six

by Frederick High School

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Notes unit six Presentation Transcript

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