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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 vocabularyReactants 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)
Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4
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
Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4
Name the reactants (everything left of arrow)Aluminum sulfate & Calcium hydroxide
Types & number of atoms in each reactantAluminum sulfate
Al = 2 S = 3 O = 12Calcium hydroxide
Ca = 1 O = 2 H = 2
Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4
Name the products (everything right of arrow)Aluminum hydroxide & Calcium sulfate
Types & number of atoms in each reactantAluminum hydroxide
Al = 1 O = 3 H = 3Calcium sulfate
Ca = 1 S = 1 O = 4
Practice Naming & CountingLi2S
-Lithium Sulfide Li = 2 S = 1
CoF2
-Cobalt (II) fluoride Co = 1 F = 2
MgSO4
-Magnesium sulfate Mg=1 O=1 S=4
Be3(PO4)2
-Beryllium phosphate Be=3 P=2 O=8
CF4
-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
CO2(g) + H2O(g) H2CO3(s)
Let’s start with a formula equationH2 + O2 H2O
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(PO43-) 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 & howWhy 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/compoundsHow many molecules of each of the following compounds are present in this equation?
Al2(SO4)3 + 3Ca(OH)2 2Al(OH)3 + 3CaSO4
Reactants:
Al2(SO4)3 = 1 (when only 1 = no number)
Ca(OH)2 = 3 (large 3 in front)Products:
Al(OH)3 = 2 (large 2 in front)
CaSO4 = 3 (large 3 in front)
Steps to Balancing Equations1. Determine the number of each element in
reactants and in products2. Balance
A. Polyatomic ions (if same poly. ion on both sides balance as a chunk)
B. MetalsC. NonmetalsD. “O” & “H”
3. Recheck your count!!!
H2 + O2 H2O
Only 1 oxygen atom “out” (2 “in”) place 2 in front of H2O
H
OOnly 2 hydrogen “in” (4 “out”) place 2 in front of H2
Recount total number of each type of atom
22
* Only add coefficients, NEVER Δ subscripts
Can you ever make just 1 molecule of water?
coefficientsubscript
NaOH Na2O + H2O
Only 1 sodium atom “in” (2 “out”) place 2 in front of NaOHRecount total number of each type of atom
2Na
O
H
Fe + O2 Fe2O3
Only 1 iron atom “in” (2 “out”) place 2 in front of Fe
Recount total number of each type of atom
2Fe
O
Only 2 oxygen atom “in” (3 “out”) place 2 in front of Fe2O3
and place 3 in front of O2
Only 2 iron atom “in” (4 “out”) change 2 in front of Fe into a 4
234
Using state symbolsWhen 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 Molecules7 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 H2, O2, N2, Cl2, Br2, I2, F2
Never just write H, O, N, Cl, Br, I, F without being bonded to another element.
H20 is okay – WHY?Because O is bonded to another element
Steps to using word equation to form formula equations:1) Write formulas / symbols2) 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 + BrWrite formulas & element symbolsCheck for diatomics (HONClBrIF)Add state symbols BalanceRecheck
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 Al2O3
Write formulas & element symbolsCheck for diatomics (HONClBrIF)Add state symbols BalanceRecheck
2(s) (s)(g)
AlO
2 234
Writing Word Equations
Na2O(s) + CO2(g) Na2CO3(s)
Solid sodium oxide combines with (reacts with / and) carbon dioxide gas to form (yields/produces) solid sodium carbonate.NaCl(s) + AgNO3(aq) NaNO3(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 ReactionTwo or more substances combine to form
a single substance. Also known as a combination reaction.A + B ABalways forming 1 productExample: 2K + Cl2 2KCl
5 Basic Types of Reactions
Decomposition ReactionA single compound is broken down into
two or more products.AB A + B always having 1 reactantExample: CaCO3 CaO + CO2
5 Basic Types of ReactionsSingle Replacement (Displacement) Reactionone element replaces another element in a
compound (also called single displacement) AB + C AC + B Always a compound + element as reactantsExample:
Mg + Zn(NO3)2 Mg(NO3)2 + Zn
(Mg is Cation so replaces the cation in the compound)
5 Basic Types of ReactionsDouble Replacement (Displacement) Reactionthe positive ions are exchanged between two
reacting compounds (also called double displacement)
AB + CD AD + CB Always a compound + compound as reactantsExample: BaCl2 + K2CO3 BaCO3 + 2KCl
(Ba & K are the cation that switch places forming the new compounds)
5 Basic Types of ReactionsCombustion Reactionan element or a compound reacts with
oxygen often producing energy as heat and light
CxHy + O2 CO2 + H2O Always has oxygen as a reactantIs an exothermic reactions (gives off heat)Example: CH4 + 2O2 CO2 + 2H2O
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 MassFormula 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: CaSO4
(# 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 = 63.996gThen add masses of all elements together
++
136.14 g
The MoleIn chemistry one mole is equal to 6.022 x 1023 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 1023 is similar to 12 eggs = 1 dozen52 weeks = 1 year1 gross = 144
Percent CompositionThe 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 CaSO4
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 ReactantsThe 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 ReactionsThe 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 EnergyThe 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 Rates1. 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 proceedThe more bonds to be broken then the
longer the reaction takesA reaction between two gases will be
quicker than a reaction between two liquids or two solids.
Factors Affecting Reaction Rates2. 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.
Factors Affecting Reaction Rates3. Concentration
Deals with how many particles are thereAn 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.
Factors Affecting Reaction Rates4. 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)
Factors Affecting Reaction Rates5. 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.
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