Enthalpy (Energy Calculations)

When converting from kJ-->mol or mol-->kJ, we use delta H to show us the change in energy.
*Remember sig figs apply to these conversions!

Eg. Calculate the energy  absorbed when 50g of BrCl is formed from Br2 and Cl2
      Br2 + Cl2 + 100 kJ----> 2BrCl
50g x1 mole of BrCl x   100kJ=
         ---------------     --------      21 kJ
          115.4g                2BrCl

When kJ is on the right side, it is an positive, endothermic reaction; when the kJ is on the left, it is an negative, exothermic reaction.


-Victoria


                 

Types of Reactions Continued

            There are 6 types of reactions, and 3 of them have already been covered (synthesis, decomposition, and single replacent). Now, we will look at double replacement, combustion, and neutralization.
  

Double Replacement:
Reaction between two ionic combounds. (AB+CD------> AD+CB)
All ions bond with other ions that they have nota been with yet; then balance the equation.
Eg:  2LiCl + SrO -----> Li₂O + SrCl₂    
Eg:   RbF + CsBr-----> RbBr + CsF

Combustion:
A combustion reaction between organic molecules  and an oxidant. The product will always contain water (H2O and Carbon Dioxide.
Eg: CH4 + 2O2 ----> CO2+ H2O

Neutralization:
A neutralization reaction is when an acid reacts with a base to form a salt. The product will always include H2O. Acids have H in the front, and bases have OH at the end.
Eg: HCl + NaOH---> NaCl+ H2O

* Look at the solubility sheet to see which compounds are soluble or not soluble.
Here is a link to rules of solubility tutorial!
http://www.ausetute.com.au/solrules.html                   
If all  the compounds are aq, there will be no reaction!

Net Ionic Equations:
Ions that are not useful in the reaction are reduced out of the equation.



-Victoria

Endothermic and Exothermic Reactions

 Endothermic Reactions: absorb energy

 Photosynthesis is an example of an endothermic reaction. The chlorophyll and pigments in the plant absorb the light energy from the sun's rays, as endothermic reaction is that in which energy is absorbed.


Exothermic Reactions: release energy to their surroundings
       - Chemical Equation =  reactants à products + energy 

         Explosions are examples of exothermic reactions as they release energy to their surroundings.


- Molecules are held together by chemical bonds

-         add energy to break bonds

-         give off energy to join together

- If a reaction takes more energy to break bonds than it gives off to form bonds à ENDOTHERMIC
- If a reaction takes less energy to break bonds than it gives off to form bonds à EXOTHERMIC

- Enthalpy (H) is the heat contained in a system

Energy Diagrams

-         We can chart 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 energy is released as the reaction proceeds.

-         The difference in energy between reactants and products determine whether a reaction is exothermic or endothermic

EXAMPLE 1.

Example 2:

Explanation of Terms:

Energy of reactants: total potential energy of all reactants in the reaction

Energy of Products: total potential energy of all products in the reaction
Energy of the activated complex: potential energy of the "transition state" between reactants and products
Activation Energy: the energy that must be added to get the reaction to progress (reactants to activated complex)
ΔH (change in enthalpy): the change in the potential energy during the reaction. It is the:
                                   energy of the products- energy of the reactants

 Why is this reaction EXOTHERMIC?

The value of ΔH is negative which means energy is given off (shown by the diagram) making this reaction exothermic.

Why is this reaction ENDOTHERMIC?

The value of ΔH is positive which means energy is absorbed making this reaction endothermic.

Energy in the Equation

- Exothermic reactions have energy term on right hand side and a negative ΔH

- Endothermic reactions have the energy term on the left hand side and a positive ΔH

Youtube TIME!!!
http://www.youtube.com/watch?v=XgiCn1IpvzM

Here is a video explaining the differences between exothermic and endothermic reactions.

By Candace

Next class we have a quiz. Here are some practice questions to help you get ready!

Identify the type of reaction taking place.

3AgNO3+K3PO4 -->Ag3PO4+3KNO3

C3H3+5O2-->3CO2+4H2O

Cu(OH)2+2HC2H3O2-->Cu(C2H3O2)2+2H2)

P4+5O2-->2P2O5

8Cu+S8-->8CuS

2NaClO3-->2NaCl+3O2

Ca+2HCl-->CaCl2+H2

2K+2H2O-->2KOH+H2

H2CO3-->H2O+CO2

2C2H2+5O2-->4CO2+2H2O

Ca(C2H3O2)2+Na2CO3+2NaC2H3O2

2C6H14+19O2-->12CO2+14H2O

Balance the following eq'ns:

SO2(g)+HNO3(aq)--> H2SO4(aq)+NO(g)

Al(s)+H2SO4(aq)-->Au(s)+I2(s)

H2O2(aq)+ClO4(aq)--> O2(g)+ClO2(aq)

Br2(aq)+OH(aq)-->O2(g)+ClO2(aq)


Good luck! :)

-Lauren

Metal and Metal Ion Reaction: A Laboratory Simulation

So last class we went to the computer lab to do a lab simulation on reactions between metals and metal ions.

Here are some of the things we learned:

-         Ratio of Ions in a given formula is the same as the ratio of ions in that solution.

o       For example in the formula Mg(NO₃)_{2 (aq)}  there are 1 Mg ion and 2 NO3 ions. In the solution shown in the lab there were 2 Mg ions and 4 NO3 ions. The solution become a multiple of the original formula.

-         Metals and their metal ions have different reactivity levels. 

o       Mg was much more reactive than Mg+2. 

§         Mg ²⁺ + 2e^- à Mg

-         When metals and metal ions reacted they lost or gained electrons.

o       Mg(s) + Cu²⁺ (aq) à Cu(s) + Mg²⁺ (aq)

§         When the Cu²⁺ ions were almost in contact with the Mg strip, electrons were transferred. The Cu²⁺ (aq) gained electrons to make Cu and Mg(s) lost electrons to make Mg²⁺  ions.

-         These reactions we observed were single replacement reactions.

o       Sn(s) + Pb(NO₃)_{2 (aq)} à Pb(s) + Sn(NO₃)_{2 (aq)}   

-Candace