CHAPTER 6

CHEMICAL REACTIONS AND EQUATIONS

Chemical reactions are responsible for just about everything that occurs around us all the time.

How do you know a chemical reaction has taken place when a cake is baked?

What are some other examples of chemical reactions in real life?

MACROSCOPIC CHANES INDICATE A MICROSCOPIC CHANGE

OR CHEMICAL REACTION HAS TAKEN PLACE.

INDICATIONS OF A CHEMICAL REACTION

1. Gas is released (bubbles)

2. Light is released or absorbed

3. Odor is formed

4. Color change

5. Heat is released or absorbed

6. Precipitate is formed (solid from mixed liquids)

• During a chemical reaction energy is either released or absorbed.

• Endothermic reaction: energy is absorbed

• Exothermic reaction: energy is released (explosions)

- seen as heat and or light

CHEMICAL REACTION (same as chemical change)change of one substance into another substance

• original substance can not be recovered

• proper amounts of starting materials are needed, and all must be used to make finished product

• all amounts going in = amounts coming out

ex: building a bicycle

BICYCLE MATERIALSFrame, bolts, screws, nuts, wheels, handlebars,

seat, gears, etc…..

Would the bicycle work properly if there were nuts and bolts left over when it was built?

In chemical reactions the LAW OF CONSERVATION OF MASS

holds true

C + O2 CO2

Same numbers of atoms must beon each side of reaction

MASS IS CONSERVED

activity

CHEMICAL EQUATION• method of using chemical symbols to represent a

chemical reaction on paper

• describes what goes on in a chemical reaction

• easier than writing out words

Ex: 2 mols. of + one mol. of becomes 2 mols. of

hydrogen oxygen water

2 H2 + O2 2 H2O

PARTS OF A CHEMICAL EQUATION reactants “goes to” product

2 H (g) + O (g) ----------------- 2 H2O “becomes”

coefficient physical state subscript

Reactant- chemicals that will react , left side of the arrow

Product- chemicals that are produced, right side of arrow

Physical state- solid, liquid, gas

Arrow- shows direction of reaction and what reactants become

Coefficient- large numbers in front of chemical formulas represent number of atoms of each element in

molecule

Subscript- small number at lower right of chemical symbols represent number of atoms of that element in

molecule

COUNTING ATOMSRules1. Multiply subscript x coefficient

2. If parenthesis is present multiplysubscript of each x subscript x coefficienteach element outside

inside parenthesis parenthesis

3. Add total atoms

COUNTING ATOMS2 H2O H 2 x 2 = 4

O 1 x 2 = 2total 6

3 H2SO4 H 2 x 3 = 6S 1 x 3 = 3O 4 x 3 = 12 total 21

5 C6H12O6 C 6 x 5 = 30

H 12 x 5 = 60O 6 x 5 = 30total 120

Mg(OH)2 Mg 1 x 1 = 1O 1 x 2 x 1 = 2H 1 x 2 x 1 = 2total 5

3 Na(PO4)3 Na 1 x 3 = 3

P 1 x 3 x 3 = 9

O 4 x 3 x 3 = 36

total 48

BALANCING EQUATIONS

When a chemical reaction occurs between reactants there must always be equal number of atoms of

reactants and products.

WHAT LAW DOES THIS DEMONSTRATE?

REACTANT S PRODUCTS

NUMBER = NUMBER

OF ATOMS OF ATOMS

BALANCING EQUATIONS Rules1. Write all the reactants on the left side of arrow,

products on right side of arrow

2. NEVER change subscripts (this will change the reactant substance), ONLY change coefficients

Ex: H2O water H2O2 hydrogen peroxide

3. Find numbers of atoms of each elementEx: 2 H2O O = 2

H = 4

BALANCING EQUATIONSRules, cont.

4. Make sure to add up all sources of same element on each side of arrow

ex: 2 H2O + C2H4O6 do same with products

C 2 = C 2

H 4 + 4 = H 8

O 2 + 6 = O 8

5. Balance equation so that numbers of elements are equal on each side of the arrow (conservation of mass)

6. Simplify (reduce) equation to lowest terms (find common denominator)

ex: 4 NH3 + 2 O2 4 NO3 + 6 H2O (all divisible by 2, so simplify)

2 NH3 + O2 2 NO3 + 3 H2O

STEPS FOR BALANCING EQUATIONS

H2 + O2 H2O 1. List out the atoms on each side of

equation H H O O

2. Count number of each type of atom in reactants

H 2 O 2

3. Count number of each type of atom in products

H 2

O 1

4. Note which atoms are not balanced

H2 + O2 H2O

H 2 H 2

O 2 O 1 5. Select one atom to balance - easiest to start with atom which is by itself on one side - order: metal, nonmetal, save H & O last

H2O + O2 2 H2O

6. update atom counts on each side of equation

H 2 H 4 O 2 O 1

7. continue to update atom counts until both sides have equal numbers of atoms of each element

2H2O + O2 2 H2O BALANCED EQUATION!!!

8. Reduce coefficients if possible

4 NH3 + 12 O2 4 NO3 + 6 H2O

ALL COEFFICIENTS ARE DIVISIBLE BY 2 SO SIMPLIFY

2 NH3 + 6 O2 2 NO3 + 3 H2O

Al + Fe2O3 Al2O3 + Fe Al 1 Al 2 unbalanced Fe 2 Fe 1 unbalanced O 3 Fe 1

2 Al + Fe2O3 Al2O3 + Fe Al 2 Al 2 Fe 2 Fe 1 unbalanced O 3 O 3

2 Al + Fe2O3 Al2O3 + 2 Fe

ALL ATOMS EQUAL ON BOTH SIDESEQUATION IS BALANCED !

Mg Cl2 + K2S MgS + KCl Mg 1 Mg 1 Cl 2 Cl 1 unbalanced K 2 Cl 1 unbalanced S 1 S 1

Mg Cl2 + K2S MgS + 2 KCl

Mg 1 Mg 1 Cl 2 Cl 2 K 2 K 2 S 1 S 1

ATOMS ARE EQUAL ON BOTH SIDESEQUATION IS BALANCED !

P4O10 + H2O H2PO4 P 4 P 1 H 2 H 3 O 11 O 4

P4O10 + H2O 4 H2PO4

P 4 P 4 H 2 H 12 O 11 O 16

P4O10 + 6 H2O 4 H2PO4

P 4 P 4 H 12 H 12 O 16 O 16

PbO2 + HCl PbCl2 + Cl2 + H2O Pb 1 Pb 1 Cl 1 Cl 4 H 1 H 2 O 2 O 1

PbO2 + 4 HCl PbCl2 + Cl2 + H2O

Pb 1 Pb 1 Cl 4 Cl 4 H 4 H 2 O 2 O 1

PbO2 + 4 HCl PbCl2 + Cl2 + 2 H2O

Pb 1 Pb 1 Cl 4 Cl 4 H 4 H 4 O 2 O 2

Example of Parenthesis

(NH4)2 Cr2 O7 + Cr2O3 + N2 + H2O

N 2 N 2 Cr 2 Cr 2 H 8 H 2 O 7 O 4

(NH4)2 Cr2 O7 + Cr2O3 + N2 + 4 H2O

N 2 N 2 Cr 2 Cr 2 H 8 H 8 O 7 O 7

EQUATION IS BALANCED !

Practice Problems

TYPES OF REACTIONS

Reactions are classified into types because it makes it easier to predict

what will happen during and at the results of that reaction.

Types of ReactionsSynthesis two or more substances or

combine to form a new single product- most exothermic

a + b ab 2H2 (g) + O2 (g) 2H2O

(g) element + element = new

cpd4 Fe (s) + 3 O (g) 2 FeO

(rust)

cpd + cpd = new cpd CO2 (g) + H2 (g) H2CO3

element + cpd = new cpdO2 (g) + CO (g) CO2 (g)

Types of ReactionsDecomposition:

compound breaks down into two or more simpler substances

ab a + b

2H2O (l) 2H2 (g) + O2

NH4NO3 (s) N2O (g) + 2 H2O

- most endothermic

Types of ReactionsSingle displacement:

- one element takes the place of another in a compound

- element can replace first or second part of a compound

- more reactive element will replace less reactive element

a + bc ac + b or

d + bc bd + c

Zn + Cu(NO3)2 Zn(NO3)2 + Cu

ele cpd new cpd new ele

Fe + CuSO4 FeSO4 + Cu

ele cpd new cpd ele

Cl2 + 2NaBr 2NaCl + Br2

Real Life Example of Single Displacement Reaction

Salicylic acid (C7H6O3) Aspirin

(C9H8O4)

Types of ReactionsDouble displacement

positive portions of 2 ionic compounds are interchanged

ab + cd ad + cbAgNo2 + NaCl AgCl +

NaNO3

cpd cpd cpd cpd

Pb(NO3)2 + 2 KCL PbI2 + 2 KNO2

Types of ReactionsCombustion:- Compound usually containing

carbon rapidly combines with oxygen to form one or more oxides (compound containing oxygen)

- called burning- heat is produced

ab + O2 oxide of a + oxide of b

CH4 + 2O2 CO2 + 2H2O

2C2H2 + 5O2 4CO2 + 2H2O + energy

Most reactions fall into these five categories, and some may fall into more than one .

NH4NO3 (s) N2O (g) + 2 H2O

Combustion- two new products contain oxygen (oxides)

Decomposition- one reactant is broken down into two simpler substances

NATURE OF REACTIONS

Reversible reaction• reaction that changes direction based on energy flow

Moves right: more product is madeMoves left: more reactants are formed

• shown with double arrow each way or one arrow pointing both directions

• reactants do not become used up because as new product is formed

other new products break down and supply new reactants

• reactions will try to reach a state of equilibrium equilibrium – state where no net change in amount of

reactants or products (a stable state)

Equilibrium Example

equilibrium does not mean there are equal numbers of products and reactants, amount of material started with = amount of material in end

Equilibrium Example

Rechargeable Batteries

Dynamic Equilibrium:state where 2 exactly opposite chemical reactions are occurring at the same place, same time, and same rate (speed) where reactions continuously occur

• Two opposing forces are being exerted but they are in a state of balance

• Amounts of all chemical entities are constant but do not have to be the same

ex: tug of war where both teams are pulling eqally against each other

Dynamic equilibrium example:• person rowing boat upstream at exactly same speed

as current - if current speeds up, he has to speed up - if current slows, he has to slow down to

stay in same place

• NET RESULT: boat has NO net movement even though person is still rowing and stream is still flowing

Equilibrium does not mean there are equal numbers of reactants and products

It means:

amount of material = amount of material

started with in end

CaCO3 CaO + CO2

Double arrow shows that reaction can go in either direction

Process of Dynamic Equilibrium1. at first reactions goes to right more quickly until more of

thereactants are used up

2. as more product is formed , the reverse reaction starts to

occur more frequently

3. eventually reaction moves at same speed in either direction

4. EQUILIBRIUM has been achieved

• DYNAMIC because molecules are always moving back and forth

• reactions try to stay at equilibrium because this is a stable state

What happens if you add stress to a reaction?

• If one side pulls harder there is more stress• Other side has to react and pull harder in order to

keep stability (equilibrium)

** same is true of chemical reactions **

Le Chateliers PrincipleIf a change of condition (stress) is applied to a chemical system, the reaction will return to equilibrium by shifting to counteract the stress

3 stresses1. change concentration of reactants or

products2. change temperature3. change pressure in system containing

gases

FACTORS THAT AFFECT DIRECTION OF REACTION

1. addition or removal of reactants or products

• addition of reactants/ removal of products: pushes reaction to right

• addition of products/ removal of reactants:

pushes reaction to left

FACTORS THAT AFFECT DIRECTION OF REACTION

2. addition or removal of energy

- addition: endothermic reactions absorb energy so more product made (reaction moves to right)

- removal: exothermic reactions give off energy to make more reactant (reaction moves to left)

In order for any reaction to occur energy is needed.

Activation energy: amount of energy needed to make atoms collide

ex: H and Oboth highly reactive elementscan coexist in the same container for years if just sitting

quietlyadd spark (energy) and they will cause an explosion

Hindenberg- spark ignited H with O and exploded

Space shuttle- liquid O is fine to go into space, but exploded when heat energy was added due to loss of heat shield shingles

Reaction rate: how fast a reaction occurs

Rate of reaction = amt substance changed/ amount of time

or amt of substance made in an

amt of time

Importance of Reaction Rate• Industry• Kitchen

• Body processes• Nature

FACTORS THAT AFFECT REACTION RATE

1. Temperature

2. Concentration of reactants

3. Catalysts

4. Inhibitors

FACTORS THAT AFFECT REACTION RATE

1. Temperature temperature rate

temperature rate

2. Concentrationconcentration rateconcentration rate

** Higher amounts of substance and temperature will increase numbers of collisions **

limiting reactant: reactant which is depleated first (not the one with the lesser amount)

• concentration of reactants is important because ifwe run out of one of the reactants it can limit and stop

the whole reaction

ex: How many smores can be made with the following?

6 graham crackers3 marshmallows2 pieces of chocolate

Which ingredient is the limiting reactant?

How many bicycles can be made if the following is needed?

1 bicycle = 1 frame + 2 wheels

We have:100 frames120Wheels

Which component is the limiting reactant?

How many bikes can be made?

FACTORS THAT AFFECT REACTION RATE

3. Catalysts: - substance that speeds up a reaction without

being itself changed or used up

- lowers activation energy needed to start reaction

- most powerful catalysts found in nature

ex: enzymes

4. Inhibitors:Substance that slows down a reaction, doesn’t completely stop reaction

Ex: stabalizers in products, retardents

Study for the test !