Unit 6 – Chemical Reactions Chemical equations, Molar mass, Rates of reactions, Limiting reactants.

Unit 6 – Chemical Reactions

Chemical equations, Molar mass, Rates of reactions, Limiting

reactants

Intro Vocabulary

Chemical reaction is the changing of substances to other substances by breaking bonds in reactants and forming new 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)-produce new substances with new

propertiesChemical 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) yield product(s)

A + B AB

Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4

Skeleton equation (Unbalanced equation)

Consists of symbols and subscriptsSymbols: 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

Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4

Notice that the number of atoms is not the same on both sides of the equation.

Al = 2 S = 3 O = 14 Ca = 1 H = 2

On the reactant side

Al = 1 S = 1 O = 7 Ca = 1 H = 3

On the product side

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

What is used to balance chemical equations?Coefficients

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

Steps to Balancing Equations1. Determine the number of atoms of each

element in reactants and products2. Balance by adding coefficients

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

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)

2H20 2H2 + O2How many molecules of oxygen are formed when 2 molecules of water are broken down?one

How many molecules of water would be required to form 3 molecules of oxygen?Six

The number of molecules does not have to be the same!2 molecules in, 3 molecules out.

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 elements7 elements can not exist as single elements – must exist in pairs if it is JUST that element

HONClBrIF

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 elements3)    Add state symbols (if given)4) Balance (if can’t balance, then

recheck formulas!!)

Writing chemical 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 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 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

Calculating Gram 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 40.078g = 40.078 1 S x 32.066g = 32.066 4 O x 15.9994g = 63.9976Then add masses of all elements together

++

136.1416 = 136.142

The MoleTo make a quantity that is measurable in lab, we use 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 dozen52 weeks = 1 year1 gross = 144

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. This makes the reaction stop.

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 RatesNature of ReactantsDepends 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 RatesTemperatureThe higher the temperature at which a

reaction occurs, the faster the particles will move

The faster the particles move the higher the frequency of collisions

For example, food spoils faster at room temperature than when it is refrigerated.

Factors Affecting Reaction RatesConcentrationDeals 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 RatesSurface AreaSurface 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 RatesCatalystsA 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.