Enthalpy and Energy Calculations

remember the MOLE map?



Click to enlarge

 Well, we are going to add something to it.

=  KJ   

 1mol 

------------------------------->

MOLES                 ENERGY

<-------------------------------------

= 1mol

  KJ

So what is Enthalpy?

It is the total heat, or ENERGY contained in the system.

And what is ΔH?

It is the energy change of the reaction, expressed in kilojoules (KJ) per mole of one chemical.

For example:

4Na + 1O2  → 2Na2O + 520KJ

Calculate the energy released when 0.41mol of Na2O is produced.

*Think back to the mole map. Incorporate the MOLES ↔ ENERGY formula into the mole map.

To find the energy, we use the formula  KJ/1mol  because the mole is given.

                                                             

0.41mol Na2O X     -520KJ     =  -106.6KJ  =  110KJ energy released

                              2mol Na2O                             or  -110KJ

                                                            *DON'T FORGET SIG FIGS!

Now calculate how many grams of O2 would be needed to produce 1100KJ of energy.

We are now calculating  from energy to moles to grams.

formula:  Energy X 1mol   X  MMg

                                   KJ         1mol

-1100KJ  X  1mol O2   =  2.1mol O2 

                          -520KJ

2.1mol O2 X     16g     =  33.6g  =  34g O2

                     1mol O2 

34g O2 would be needed to produce 1100KJ of energy.

Endothermic and Exothermic Reactions!

-All chemical reactions involve changes in energy or heat

-Exothermic reactions release energy
-Endothermic reactions absorb energy


Using energy graphs, we will be able to see whether the reaction is endothermic or exothermic.

 -energy is represented in the form KJ

-The flat part of the graph before the bump  labelled "reactants" represents the energy of the reactants. This is how much energy the compounds START with.

-The flat part of the graph after the bump labeled "Product" represents the energy of the products. This is how much energy the compounds END with.

-The energy of activated complex,, is the potential energy of the transition state between the reactants and the products. 

-it's the highest possible amount of energy the compounds can reach in the reaction process

-The activation energy is the energy that is added to to the reactants to get them to progress. 

-Delta H or the change in enthalpy (heat contained in the system) during the reaction. This is found by subtracting the energy of the reactants from the energy of the products.
     -A negative H will signify an exothermic reaction - you end up with less energy than you started with
     -A positive H will signify an endothermic reaction - you end up with more energy than you started with

Once you find the H, you can use it in equations like this:

4Na + O2= 2Na2O + 520KJ

If the energy was written on the right side, it meant that the reaction is exothermic, therefore there is less energy in the products

If energy is written on left side, it meant that the reaction is endothermic.

This site might be very helpful!

http://www.saskschools.ca/curr_content/chem30_05/2_kinetics/practice/q3_2.htm

LAB 5B

In Lab 5B - Types of Chemical Reactions, we were able to observe synthesis, decomposition, single replacement, and double replacement reactions!



Reaction 1: 
-burning copper wire with a bunsen burner
- Using crucible tongs, hold the wire over the hottest part of the flame 
-copper will begin to change to silver

Reaction 2: 
-place a shiny nail in copper(II)sulphate--> SINGLE REPLACEMENT
- 15 minutes later, the part of the nail that was in the copper(II)sulphate will become copper  :)


the video uses cupric sulfate.

Reaction 3: 
Heating some copper(II)sulphate. 
What happens when a hydrate is heated?



Reaction 4: 
What happens when water is added back into the now anhydrous copper(II)sulphate?




Reaction 5: 
Pour calcium chloride solution into a sodium carbonate solution. 
- white precipitate is formed




Reaction 6: 
put zinc into HCL
-bubbling 

Reaction 7: 
Manganese(IV)oxide is added to hydrogen peroxide solution. Then, the gas is tested by placing a glowing splint into the mouth of the test tube - should form black precipitate on the sides of test tube. layer of gas forms on sides of test tube

MORE REACTION TYPES + HOW TO CALCULATE SOLUBILITY AND NET IONIC EQUATION

HOW TO DETERMINE THE STATE OF A COMPOUND (solid, gas, aqueous, liquid)

Here is the SOLUBILITY TABLE that was given during class:

NEGATIVE IONS (ANIONS)	POSITIVE IONS (CATIONS)	SOLUBILITY OF COMPOUNDS (solubility: >0.1mol/L)
ALL	All Alkali ions	Soluble
ALL	Hydrogen ion, H+	Soluble
ALL	Ammonium ion, NH4+	Soluble
Nitrate, NO3¯	ALL cations	Soluble
Acetate, CH3COO¯	ALL cations, except Ag+	Soluble
Chloride, Cl¯ Bromide, Br¯ Iodide, I¯	Ag²+, Pb²+, Hg2²+, Cu+ ALL other cations	Not (low) Soluble Soluble
Sulfate, SO4²¯	Ca²+, Sr²+, Ba²+, Ra²+, Ag+, Pb²+ All other cations	Not (low) Soluble Soluble
Sulfide, S²¯	Alkali ions, H+, NH4+ Be²+, Mg²+, Ca²+, Sr²+, Ba²+, Ra²+ Any other cations	Soluble Soluble Not (low) Soluble
Hydroxide, OH¯	Alkali ions, H+, NH4+, Sr²+ Any other cations	Soluble Not (low) Soluble
Phosphate, PO4³¯ Carbonate, CO3²¯ Sulfite, SO3²¯	Alkali ions, H+, NH4+ ALL other cations	Soluble Not (low) Soluble

Example:

2BeI2(   ) + 1Sn(NO3)4(   ) → 2Be(NO3)2(   ) + 1SnI4(   )

Instructions:

1. Is  2BeI2(   )  a solid or an aqueous substance? First take a look at the Anion, in this case, Iodide. Find Iodide in the left column of the Solubility Table. This is what you should see:

Chloride, Cl¯ Bromide, Br¯ Iodide, I¯

2. When you have found Iodide in the left column, look at the Cation, in this case, Beryllium. On the Solubility Table, look at the box with Iodide, then look at the box to its immediate right, in the middle column. This is what you should see:

Chloride, Cl¯ Bromide, Br¯ Iodide, I¯

Ag²+, Pb²+, Hg2²+, Cu+ ALL other cations

3. Look for Beryllium in the middle column. It if is not stated specifically, the it is considered in the “ALL other cations” category.

Chloride, Cl¯ Bromide, Br¯ Iodide, I¯

Ag²+, Pb²+, Hg2²+, Cu+ ALL other cations

4. Now look to the immediate right of the middle box. This is what you should see:

Chloride, Cl¯ Bromide, Br¯ Iodide, I¯

Ag²+, Pb²+, Hg2²+, Cu+ ALL other cations

Not (low) Soluble Soluble

Look at the one that is directly to the left of the highlighted part, not the other one. It should be like this:

Chloride, Cl¯ Bromide, Br¯ Iodide, I¯

Ag²+, Pb²+, Hg2²+, Cu+ ALL other cations  →

Not (low) Soluble Soluble

When it is SOLUBLE, it means it is an AQUEOUS. When it is NOT (low) SOLUBLE, it means it is a SOLID.

Therefore, 2BeI2( aq )

When you do the whole equation, it should look like this:

2BeI2( aq ) + 1Sn(NO3)4( aq ) → 2Be(NO3)2( aq ) + 1SnI4( s )

DOUBLE REPLACEMENT

• The reaction between 2 ionic compounds
• The positive ions switch places with one another

General formula:

AB + CD → CB + AD

For example:

When a solution of silver nitrate is combined with a solution of sodium chloride, silver chloride and Sodium Nitrate are formed.

AgNO3 + NaCl → AgCl + NaNO3

BUT, how can we determine whether or not a reaction actually occurs?

• Look back at the Solubility Table mentioned earlier.

• There is a reaction if the reactants change states during reaction.
• The change usually forms a precipitate (aqueous).
• There is no reaction if there is no change in states.

When you’ve figured out the states, it should look like this:

AgNO3( aq ) + NaCl( aq ) → AgCl( s ) + NaNO3( aq )

The reaction changed states, therefore, this reaction can occur.

NEUTRALIZATION

• A special type of Double Replacement
• Happens when an ACID and a BASE reacts to produce WATER and an ionic SALT
• The formula for an ACID always contain H⁺ as the cation
• The formula for a BASE always contain OH¯ as the anion
• An exception to this is H2O, aka. HOH, because it is neutral.

General formula:

HA + BOH → H2O + BA

For example:

Hydrochloric acid combined with Sodium Hydroxide is neutralized to form Sodium Chloride (salt) and water.

HCl + NaOH → NaCl + H2O

COMBUSTION

• a COMBUSTION reaction happens when a compound is burned with oxygen

General formula:

AB + O2 → AO + BO

For example:

When methane burns with oxygen, it produces carbon dioxide and water.

1CH4 + 2O2 → 1CO2 + 2H2O  *don’t forget to balance!!

How to Calculate the Net Ionic Equation 

~JZ