The Calculation of Energy

Energy Calculation

--the return of mole

CH₄+2O₂àCO₂+2H₂O

△H for this exothermic reaction is expressed using the coefficients of the balanced equation:

-812KJ/1mol CH₄ or -812KJ/2mol O₂ = -406KJ/1mol O₂

So the value of △H depends on which chemical you are referring to

1.    The value of △H changes with different reactions; -812KJ is only specific to this reaction. Thus like molar mass is different for each compound, △H depends on the chemical reaction

2.    △H is not a constant like Avogadro’s number

Ex. How many moles of CH₄ are needed to produce 2100KJ of energy?

CH₄+2O₂àCO₂+2H₂O+812KJ

-2100KJ×-1mol CH₄/812KJ =2.6mol

TYPES OF REACTIONS (2)

Double Replacement
-A double replacement is a reaction between two IONIC compounds usually in solution. The ions switch partners like a dance. The (+) ions sitch places but remember to watch your charges.

General Formula
AB + CD→ CB+AD

Eg: CaCO3 + 2HCl → CaCl2 +  2H2

If a double replacement reaction actually occurs:

The reactants change state during reaction

(usually a precipitate occurring)

Use your "Table of solubilities" to determine the states.- (aq) or (s)

Net Ionic Equation:

-There is a net reaction when you have a precipitation that occurs

-(aq) ions that are the same on both sides get cancelled

Eg.

CaCl2 + Na2CO3 → CaCO3 + 2 NaCl

Net Ionic Equation： 2Cl^-  + 2Na⁺  → 2NaCl

Using the Table of Solubilities

1) Find your anion ( negative ion) in the left hand column.

2) Look for your cation (positive ion) in the list in the 2nd column.

3) Follow its presence or absence to the word "soluble" or "not soluble"

4) If soluble, the compound is (aq)

5) If insoluble, the compound is (s)

Combustion

-A combusion reaction is a reaction where burning in air is involved. The reactants are the chemical to be burned and the oxygen that it reats with. The oxygen atoms usually end up combined with more that one type of atom as products.

-General Formula

AB + O2 → AO + BO

Eg. CH4 + 2 O2 → 2 H2O + CO2

Neutralization

-A neutralization reaction is a sepcial double replacement reaction where acids reat with bases to produce water and an ionic salt as products.

-The acids have han H+ as the cation (+) and the bases have  OH- as the anion (-). Both should be aqueous solutions. (aq)

-General Formula

HA + BOH → H2O + BA

Eg. HCl + NaOH → NaCl + H2O

NOTE： For DR reaction (include Neutralization）, some ions participate in the reaction wile other ions do not participate.

Eg. KCl(aq） + AgNO3(aq) → KNO3(aq) + AgCl(s)

* Be sure to write both the total and net ionic equations!!!

Total  ionic equation:  K⁺_(aq)   + Cl^- _(aq) + Ag⁺_(aq) + NO₃^-_(aq） → K⁺_(aq)  + NO₃^-_(aq) + AgCl_(s)

Net Equation: Ag⁺_(aq) + Cl^-_(aq)  → AgCl_(s)

Endothermic And Exothermic Reactions

Introduction
-All chemical reactions involves △Energy
Some release energy.  exothermic
Some absorb energy. endothermic


Molecules are held together by chemical bonds.
-Add energy to break bonds
-Give off energy to join together

Takes MORE enerygy to break down than it gives off to form bonds→Endothermic

Takes LESS energy to break down than it gives off to form bonds→ Exothermic

Enthalpy, H, heat contained in the system.

Energy Diagrams
-Chart the potential energy of the chemicals as they change from reactants to products
-Reactants start with a certain amount of energym energy is added to start the reactions and then energy is released as the reaction proceeds.
-The relative amounts of energy determine: Endothermic & Exothermic.

Energy of reactants: total E of all reactants in the reaction
Energy of products: total E of all products in the reaction
Energy of activated complex: potentioal E of the "transition state" between reactants & products
Activation Energy: The E must be added to get the reaction to progress.
△H: the change in potential energy during the reaction.
△H= E of reactants - E of products

Potential Energy Diagram

E of reactants ＜ E of products                                               E of reactants  ＞E of products

△H is positive.                                                                        △H is negative.

Endothermic reaction                                                               Exothermic reaction.

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

Eg. CH4+2O2→CO2+2H2O+812KJ

      Higher E        Lower E

Exothermic reactions have the E term on the RIGHT side(RHS) -△H

Endothermic reactions have the E term on the Left side (LHS)    +△H

Eg.

E of Reactants= 150 KJ

E of Products= 100KJ

E of Activated complex= 300 KJ

△H=?      Ea=?

△H= E products- E reactants

      = 100KJ - 150KJ= -50KJ  ( exothermic)

Ea= E activated complex - E reactants

    = 300KJ - 150KJ= 150KJ

Eg.

E of Reactants （E）=200 KJ
△H（F）                =100 KJ ( Endothermic)
Ea(B)                        =300 KJ
E of activated complex (C) =?      E of Product (G)=?      

E of activated complex= Ea+ E of reactants= 200KJ+300KJ=500KJ
E of product= E of activated complex- E of reactants= 500 KJ- 200KJ= 300KJ

Lab 5B

Objectives:

1.       To observe a variety of chemical reactions

2.       To interpret and explain observations with balanced chemical equations

3.       To classify each reaction as one of the four main types

Material and Equipment:

Refer to page 33 in Health Lab Text, Lab 5B

Procedure:

  Refer to page 33 in Health Lab Text, Lab 5B

Data and Observation:

  Attached to the back.

Analysis of results:

1.       Copper reacts with oxygen in the air.

2.       The solution become lighter blue indicates the concentration of CuSO4 in the solution decreased.

3.       The colour changes in reaction 3&4, which means new substance was yielded.

4.       The precipitate was CaCO3.

5.       Put a glowing (not burning) splint into the mouth of the test tube, the splint will start a flame. That shows the identity of supporting combustion.

6.       A. has the identity of supporting combustion.

B. Water.

Follow-up questions

1.       CdSO4+ZnàZnSO4+Cd

2.       2H2Oà2H2+O2 (decomposition)

Conclusion

1.       2Cu+O2à2CuO (synthesis)

2.       2Fe+3CuSO4àFe2(SO4)3+3Cu (single replacement)

3.       CuSO4·5H2OàCuSO4+5H2O  (decomposition)

4.       CuSO4+5H2Oà CuSO4·5H2O (synthesis)

5.       CaCl2+Na2CO3àCaCO3+2NaCl (double replacement)

6.       Zn+2HClàZnCl2+H2 (single replacement)

7.       2H2O2+(MnO2)à2H2+O2+(MnO2*) (decomposition)  (*=catalyst)