Feb 21-- Energy Calculation

△H: the energy change in a reaction 

Unit: expressed in kilojoules per mole (kJ/mol) of one of the chemicals

Eg: 2C(s) + 3 H2 (g) + 0.5O2 (g) →C2H5OH (l) +235kJ

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

-235kJ/2 mole C =117.5kJ/1 mole of C             

-235kJ/3 mole H2 = 78.3kJ/1 mole of H2

-235kJ/0.5 mole O2=470kJ/ 1 mole O2

Similarly for the products:

-235kJ / 1 mole C2H5OH              

We can see the value of △H depends on which chemical you are referring to (each chemical has different △H)

Eg: Calculate the energy released when 0.50 moles of H2O are produced

CH4 +2O2→CO2 +2H2O +812kJ

0.50 mole H2O * 812kJ/2 mole H2O =203kJ energy released (-203kJ)

Eg:How many moles of CH4 are needed to produce 2500kJ of energy?

CH4 + 2O2→CO2 + 2H2O +812kJ

2500kJ * 1mole CH4/812kJ =3.1mole of CH4

Eg: Calculate how many grams of O2 would be needed to produce 2000KJ of energy

（same reaction as above）

-2000kJ * 1mole O2/-406kJ =4.9moles of O2

4.9 moles O2 * 32g/1mole =158g of O2

Eg: Calculate how many atoms of O2 would be needed to produce 2000kJ of energy.

-2000kJ * 1mole O2/-406kJ =4.9 moles of O2

4.9 MOLES * 6.022*1023 / 1mole =2.95 * 1024 molecules of O2  2.95 * 1024  molecules * 2atoms of O2 / 1 molecule= 5.9 * 10 24   atoms O2

Lab 5B--Types of Reactions

Materials and Equipments

- refer to page 33 in the Lab Text book

Procedures

- refer to page 33 in the Lab Text book

Objectives

To observe the different types of reactions and write their chemical equations.

 

Results

1. Take a 6cm of copper and hold in the flame. The copper reacts with oxygen in the air.

2. Take an iron nail and placed into a test tube. Add copper sulfate into the test tube. Let sit for a while , the iron nail turned pink with rust

3. Take another test tube. Add copper sulfate pentahydrate and heat it over a bunsen burner for 5 to 10 mins. The color turned white. 

4. After the experiment 3 cool down. Add water into it. the result turned back to the regular color and stuck on the bottom of the test tube.

5. Take 2 unused test tube. #1 add calcium chloride. #2 add sodium carbonate. Then mix them together. It formed a milky white solution. 

6. Add mossy zinc with hydrochloric acid. Formed bubbling for half hour

7. Add hydrogen peroxide and manganese oxide. Placing a glowing splint into the test tube, the splint was rekindled.

Feb.16--Endothermic and Exothermic Reactions

All chemical reactions involve changes in energy.

Endothermic Reaction: reactions that release energy to the surroundings

Exothermic Reaction: reactions that absorb energy

Ex. 1. Instant ice packs absorb energy and are endothermic reactions.

       2. Explosions release energy and are exothermic reactions.

Molecules are held together by chemical bonds.

- to break bonds: add energy

- to join together: give off energy

* Exothermic: if a reaction takes more energy to break bonds than it gives off to form bonds

* Endothermic: if a reaction takes less energy to break bonds than it gives off to form bonds

ENERGY DIAGRAMS

- shows the potential energy of the chemicals as they change from reactants to products

Reactants start with a certain amount of energy. Energy is added to start the reaction and then is released as the reaction proceeds. The relative amounts of energy determines if a reaction is endothermic or exothermic.

Potential Energy Diagram #1 (Endothermic)

Potential Energy Diagram #2 (Exothermic)

1) energy of reactants = total potential energy of all reactants in the reaction

2) energy of products = total potential energy of all products in the reaction

3) energy of the activated complex = potential energy of the “transition state” between

  reactants and products

4) activation energy = the energy that must be added to get the reaction to progress

5) △H (change in enthalpy ) = the change in potential energy during the reaction

                        = energy of products－energy of reactants

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

Ex. CH₄ + 2O₂ --> CO₂ + 2H₂O + 812 kJ

     higher energy    lower energy

Exothermic reactions have the energy term on the right hand side and a negative △H.

Endothermic reactions have the energy term on the left hand side and a positive △H.

Jan 27---Type of Reactions

 Six generalized types of reactions:

- Synthesis

- Decomposition

- Single Replacement

- Double Replacement

- Combustion

- Neutralization

Synthesis

-two or more reactants to form one product        

 Formula: A +B→C

Eg： Mg + Cl2 →MgCl2

Decomposition

-one reactant breaks down into two or more products

Eg: MgCl2 → Mg + Cl2

Single Replacement / Displacement

-an element replaces an ion in an ionic compound: 

Metal elements replace positive ions (cations), non-metal elements replace negative ions (anions)

Formula: A + BC →AC + B (A is metal)

              A + BC → BA + C (A is non-metal)

Eg: 2Li + MgCl2 →2LiCl + 2Al

      Cl2 +2 KF → F2 + 2KCl               

Predicting Single Replacement Reactions:

Some metals are more reactive than other metals, also there are some non-metals are more reactive     

“Active Series” help us to decide whether the equation can react or not.

An element higher up on the series replaces the ion below it on the table.

  

Feb.2--Types or Reactions (Part 2)

Double Replacement: a reaction between two ionic compounds (usually in solution).

AB + CD --> CB + AD

Note: Remember that a metal always combine with a non-metal!

Ex. Na₂CO₃ + CaCl₂ --> CaCO₃ + 2 NaCl

To determine if a double replacement reaction actually occurs, use your “Table of Solubility” to check if the reactants change state. If there is no change of state, the reaction won’t occur.

How To Use the Table of Solubility

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

Step 2. Find your cation (positive ion) in the second column.

Step 3. Check to see if the compound is soluble or not.

Step 4. If soluble, the compound is (aq); if insoluble, the compound is (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

Ex. 2 Na₃PO₄ + 3 Ca(NO₃)₂ --> 6 NaNO₃(aq) + Ca₃(PO₄)₂(s)

   net ionic equation: 3 Ca²⁺(aq) + 2 PO₄^3-(aq) --> Ca₃(PO₄)₂(s)

Combustion: a reaction where burning in air is involved. The reactants are the chemicals to be burned (organic compound) and the oxygen that it reacts with. They usually produce carbon dioxide and water.

AB + O2 --> AO + BO

Ex. C₄H₈ + 6 O₂ --> 4 CO₂ + 4 H₂O

Neutralization: acid react with base to produce water and an ionic salt. (Double Replacement)

Note: Both acid and base should be aqueous solutions!

HA + BOH --> H₂O + BA

Ex. 2 HBr(aq) + Sr(OH)₂(aq) --> SrBr₂(aq) + 2 H₂O(l)

Jan 25---Balancing Equations

To balance a chemical equation,

number of atoms on the left side = the number of atoms on the right side.

Ex.

             2 ScCl + 3 F2  --  2 F3Sc + 3 Cl2

Step 1. Balance the atoms first

Step 2. Balance the whole group

Step 3. Be systematic---in order

Step 4. elemental form ( K2,Cl2)