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Warm-up. In your notes, define chemical reaction and chemical equations. Objectives. TSWBAT: define chemical reactions and distinguish between types of chemical reactions. Unit 7 – Chemical Reactions. Chemical equations, Energy of Reactions, Rates of reactions, Limiting reactants.

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In your notes, define chemical reaction and chemical equations.


TSWBAT: define chemical reactions and distinguish between types of chemical reactions.

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)

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 reactant

    • Aluminum sulfate

      Al = 2S = 3O = 12

    • Calcium hydroxide

      Ca = 1 O = 2H = 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 reactant

    • Aluminum hydroxide

      Al = 1O = 3H = 3

    • Calcium sulfate

      Ca = 1 S = 1O = 4

Practice time

  • Take out the whiteboard and marker that are inside your desk.

Practice Naming & Counting

  • Li2S

    -Lithium SulfideLi = 2S = 1

  • CoF2

    -Cobalt (II) fluorideCo = 1F = 2

  • MgSO4

    -Magnesium sulfate Mg=1 O=1 S=4

  • Be3(PO4)2

    -Beryllium phosphateBe=3 P=2 O=8

  • CF4

    -Carbon tetrafluorideC = 1F = 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 equation

  • H2 + O2 H2O

  • How many hydrogens & oxygens in reactants?

    H = 2O = 2

  • How many hydrogens & oxygens in products?

    H = 2O = 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!!

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

  • What is used to balance chemical equations?

    • Coefficients

      -the number before the chemical formula (the number is written normal size – not superscript or subscript)

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?


    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?

    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 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!!!



H2 + O2 H2O





Only 1 oxygen atom “out” (2 “in”)

 place 2 in front of H2O

Recount total number of each type of atom

Only 2 hydrogen “in” (4 “out”)

 place 2 in front of H2

* Only add coefficients, NEVER Δ subscripts

Can you ever make just 1 molecule of water?


NaOH  Na2O + H2O




Only 1 sodium atom “in” (2 “out”)

 place 2 in front of NaOH

Recount total number of each type of atom



Fe + O2 Fe2O3





Only 1 iron atom “in” (2 “out”)

 place 2 in front of Fe

Recount total number of each type of atom

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

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 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 / 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.





Check for diatomics (HONClBrIF)

Write formulas & element symbols

Add state symbols
















Another word equation

  • Solid aluminum metal reacts with oxygen gas to form solid aluminum oxide.



Check for diatomics (HONClBrIF)

Write formulas & element symbols

Add state symbols
















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 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 + Cl2 2KCl

5 Basic Types of Reactions

  • Decomposition Reaction

    • A single compound is broken down into two or more products.

    • AB  A + B

    • always having 1 reactant

    • Example: CaCO3 CaO + CO2

5 Basic Types of Reactions

  • 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(NO3)2 Mg(NO3)2 + Zn

      (Mg is Cation so replaces the cation in the compound)

5 Basic Types of Reactions

  • 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: BaCl2 + K2CO3 BaCO3 + 2KCl

      (Ba & K are the cation that switch places forming the new compounds)

Take out your packets

Warm-up 4/11/11

  • List the 5 types of chemical reactions that we have been learning about and describe how they work.


  • Students will be able to identify types of reactions.

  • Students will calulate formula mass.

  • Take out packets and prepare to take a few notes.

5 Basic Types of Reactions

  • Synthesis: A + B AB

  • Decomposition: AB A+B

  • Single Replacement: A + BC  AC + B

  • Double Replacement: AB + CD  AD + CB

  • Combustion: CxHY + O2 CO2 + H2O

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: 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 = 63.996g

      Then add masses of all elements together



136.14 g

The Mole

  • In 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 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 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 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.

Factors Affecting Reaction Rates

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.

Factors Affecting Reaction Rates

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.

Factors Affecting Reaction Rates

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)

Factors Affecting Reaction Rates

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.

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