STOICHIOMETRY AND CALCULATION

   

•     Let's start with how to say this word. Five syllables -STOY-KEE-AHM-EH-TREE.

• It's a big word that describes a simple idea. Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. You might be looking at the amounts of substances before the reaction. You might be looking at the amount of material that is produced by the reaction. Stoichiometry is all about amounts.

• Balanced chemical equations are required. Because they tell us the ratio of the molecules or moles of the substances in a chemical reaction.

Example

Propane (C₃H₈) is the fuel used in most gas grills. It burn according

 to the balanced equation

C₃H₈ + 5O₂ ---> 4H₂O + 3CO₂

If you burn 215 g of propane, how many grams of H₂O will be produced?

215gC₃H₈ x (1mol C₃H₈/44.0gC₃H₈) x (4mol H₂O/1mol C₃H₈) x (18.0gH₂O/1mol H₂O)=325gH₂O

Molarity and Stoichiometrty

Example

For the reaction: Zn + 2HCl ₂ ---> ZnCl₂ + H₂

How many mL of 0.500 M HCl are needed to react with 2.00 grams of Zn?

2.00gZn x (1mol Zn/654gZn) x (2mol HCl/1mol Zn)x(1L HCl/0.500mol HCl) x (1000mL HCl/1L HCl) = 122mL HCl

Gas Stoichiometry

Hydrogen gas (and NaOH) is produced when sodium metal is added to water. What mass of Na is needed to produce 20.0 L of H2 at STP?

First we need a balanced equation:

2Na(s) + 2H2O(l) ----> H2(g) + 2NaOH(aq)

Then we can calculate the mass of Na:

20.0L H2 * 1mole/22.4L*2mole Na/1mole H2*23.0g/1mole Na=41.1gNa

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Endothermic and Extothermic Reactions (Continue)

The energy absorption or release can be placed directly in the equation.



An exothermic reaction releases energy. The energy change that accompanies a reaction can be written in the equation. The reaction takes less energy to break the bonds than it gives off to form bonds.

An endothermic reaction uses energy as a reactant.
The energy change that accompanies a reaction can be written in the chemical equation. The reaction requires the addition of energy to the reactants to form the bonds in the products. 
 

Enthalpy of a reaction or energy change of a reaction DH, is the amount of energy or heat absorbed in a reaction. If the energy is required, DH is positive, and if energy is released, the DH, is negative.







_________ Products
    ­
    |
    | DH, positive for
    |         endothermic reaction
    |
__|______ Reactants

DH = SH_products - SH_reactants

Energy Calculations 

The return of the MOLE!!!
EXAMPLE
What is the energy change when 5 moles of oxygen and 5 moles of nitrogen react?
Use the energy change per mol from the equation
N2(g) + O2(g) + 43 kJ -----> 2 NO(g) .
Check the balanced equation mole ratio for moles of N2 to moles of O2.
mole ratio N2 to O2 is 1 mole N2 / 1 mole O2
You can verify the number of moles of N2
moles N2 = [ 5 moles O2] [1 mole N2 / 1 mole O2] = 5 moles N2 
Determine the kcal change.
[ 5 moles N2 ][43 kcal /mole N2 ] = 215 kJ
The reactants in this reaction must take energy from the surroundings to form products.

 

Endothermic and Exothermic Reactions

Endothermic reactions 
the reactants have less potential energy than do the products. Energy must be input in order to raise the particles up to the higher energy level.

Exothermic reactions

the reactants have more potential energy than the products have. The extra energy is released to the surroundings.



Enthalpy (H)
The heat contained in the system.
ΔH = H product - H reactant

Activation Energy
The Energy required to initiate a chemical reaction. Both endothermic and exothermic reactions require activation energy.

                                   ENERGY DIAGRAM



Lab 5B

LAB 5B Types of Reactions

Objective
1.to observe a vareity of chemical reactions
2.to interpt and explain observations with balanced chemical equations
3.to classify each reactions as one of the four main types

Reaction 1


Using crucible tongs, hold a 6cm length of bare copper wire in the hottest part of flame for few minutes.
The flame turns green when burnning the copper, and the color of copper changes.

2 Cu + O2 ---> 2 Cu2O

Reaction 2

Place the iron nail in a test tube and add copper sulfate solution so that one half of the nail is covered.
The solution turns green and copper is formed.

Fe + CuSO4 ---> Cu + FeSO4

Reaction 3

Heat the test tube which contains some solid copper sulfate pentahydrate.
There is smoke seen, and pentahydrate turens to white, then green, then green.

CuSO4·5H2O ---> CuSO4 + 5H2O
Reaction 4
Use a medicine dropper to add 2 or 3 drops of water to the test tube from reaction 4.
And the color of the pentahydrate turns to blue again.

CuSO4 + 5H2O ---> CuSO4·5H2O

Reaction 5

Fill a test tube one quarter full with CaCl2 solution, then fill another test tube with solution Na2CO3. Pour the CaCl2 soultion into the test tube contatining NaCO3 solution.
The white precipitate is formed during the reaction.

Na2CO3 + CaCl2 ---> CaCO3 + 2 NaCl

Reaction 6

Place a piece of mossy zinc in the test tube, then HCl acid solution into the test tube until the mossy zinc is completely covered.
There is bubble seena, the amount of zinc is decreased.

Zn + 2 HCl ---> H2 + ZnCl2

Reaction 7

Half fill a test tube with hydrogen penoxide, then add a small amount of manganese oxide.
There are bubble seen, the reaction is speeding up.

2 H2O2 ----> 2 H2O + O2

Types of Reactions (2)

Double Replacement
General Formula: AB+CD → CB+AD

AgNO₃ + KCl → KNO₃ + AgCl
BaCl2 + Na2SO4 → BaSO4 + 2NaCl
CaCO3 + H2SO4 → CaSO4 + CO2 + H2O
 Ag2O + 2HI → 2AgI + H2O
Ca(OH)2+ 2H3PO4 → Ca3(PO4)2  + 6H2O
K2CO3  + BaCl2  → 2KCl  + BaCO3
2Na3PO4  + (NH4)2S  → 3Na2S + 2(NH4)3PO4
H3PO4 + FeBr3 → FePO4  + 3HBr
AgNO3  + KCl  → AgCl + KNO3
Na2CO3  + H2SO4  → Na2SO4  + CO2 + H2O
H2SO4  + BaCl2  → BaSO4  + 2HCl

Combustion
General Formula: AB+O2→AO+BO

CH4 + 2O2 → CO2 + 2H2O

2H2 + O2 → 2H2ONa + H2O → NaOH + H2
















Neutralization


General Formula: HA+BOH→H2O+BA

¤HCl  +  NaOH à  H2O  +  NaCl

¤

H2SO4 + 2KOH à  2H2O +  K2SO4

H2SO4 + 2NaOH à  2H2O + Na2SO4

Types of Reactions (1)

Synthesis:
General Formula: A+B→AB

Mg + O₂ → MgO
H₂ + O₂ → H₂O
K + Cl₂→ KCl
Fe + O₂→ Fe₂O₃
CaO + CO₂ → CaCO₃
Na₂O + CO₂ → Na₂CO₃
KCl + O₂ → KClO₃
Ba(ClO₃)₂ → BaCl₂ + O<sub>2




Decomposition:</sub>
General Formula: BC→B+C

2HgO  → 2Hg + O2 

2Cl2O5  →2Cl2  + 5O2

Mg(NO3)2  → Mg(NO2)2  + O2 

2SO₃  →2SO₂  + O₂  

=

Single Replacement (A=mental)

General Formula: A+BC→AC+B

(A=non-metal)

A+BC→BA+C

Cu + AgNO₃ → Ag + Cu(NO₃)₂

Fe + Cu(NO₃)₂ → Fe(NO₃)₂ + Cu
Ca + H₂O → Ca(OH)₂ + H₂
Zn + HCl → ZnCl₂ + H₂

Cl₂ + NaBr → NaCl + Br₂
Br₂ + KI → KBr + I₂