Energy and Percent Yield

• enthalpy is the energy stored in chemical bonds
• symbol of enthalpy is H
• units of Joules (J)
• change in enthalpy is  ΔH
• in exothermic reactions enthalpy decreases
• in endothermic reactions enthalpy increases

BY: KRYSTA DEL ROSARIO :)

Percent Yield

• the theoretical yield of a reaction is the amount of products that should be formed
• the actual amount depends on the experiment
• the percent yield is like a measure of success. (How close is the amount to the predicted amount?)
• Percent Yield= actual/theoretical x 100

If 47.7g of Urea are produced, determine the theoretical (predicted) yield of CO2.

What is the percent yield of CO2 if 12.0g is produced?

47.7 x 1mol/60g x 1/1 x 44g/1mol = 34.98 g

12/34.9.98 = 34%

http://www.youtube.com/watch?v=TKNxdL7DN1I

BY: KRYSTA DEL ROSARIO :) 

Limiting Reactants

• in chemical reactions, usually one one chemical gets used up before the other
• the chemical used up first is called the limiting reactant
• one it's used up the reaction stops
• limiting reactants determines the quantity of products formed
• to fine the limiting reactant assume one reactant is used up. Determine how much of this reactant is required

2.5 mol of CuSo4 reacts with 2.5 mol of NaNO3. Determine the L.R and calculate how many moles Na2SO4 are formed.

CuSO4+ 2NaNO3= Na2SO4+Cu(NO3)2

2.5mol (CuSO4) x 2/1 = 5.0 mol of NaNO3                    NaNO3 is L.R

2.5mol (NaNO3) x 1/2 = 1.25 mol Na2SO4

http://www.youtube.com/watch?v=LifU5WQJi-4&feature=fvst

BY: KRYSTA DEL ROSARIO :)

Other Conversions: Volume and Heat

As you did before, the conversion factor for volume is 22.4 L/mol
Heat can be used as a separate term in the chemical reactions
    -This is called enthalpy
    - Rxns that release heat are exothermic
    - Rxns that absorb heat are endothermic
Both can be use in stoichiometry

Examples:

If 5.0g of methane (CH4) decomposed, what volume of hydrogen would you have?

1) Balance your equation

CH4= C + 2H2

2) Start writing the equation out to find the volume of hydrogen

5.0g  x  1 mole    x  2   x    2g       = 0.31 g of hydrogen
                16 g         1       1 mol


http://www.chemteam.info/Stoichiometry/vol-vol-stoichiometry.html
http://www.youtube.com/watch?v=84s2_zPQTUs

-Candace Chan

Stoichiometry Investigation

Today, we did the lab that needed to find the mass of the precipitate, which is stronium sulphate and copper (II0 nitrate. We separated the precipitate by filtering and drying it. Overall, we did well on the lab and it was a fun lab.

-Candace Chan :)

Mass to Mass Conversions

Mass to Mass Conversion only requires one more additional step from the stuff you've learned already.



Example:



???

http://www.youtube.com/watch?v=2Q8HtHItjxI

-Candace Chan

Mole to Mass & Mass to Mole

- questions may contain an amount of moles and ask you to determine the mass.
- Converting moles to mass only requires one additional step.


Steps:

1) Write the equations of the given formula
2) Balance the equations

Example:

Zn + HCl = ZnCl2 + H2

Then balance the equation

Zn + 2HCl = ZnCl2 + H2

then let's say the mole od zinc was 3.5, what is the mass of hydrogen?

3.5 mol x

multiply it over what you need over what you have!!

3.5 mol x  1   
                 1

Then you multiply it by the molar mass of the element you need, in this case, you need to find the molar mass of hydrogen. (remember the subscripts)

3.5mol x 1 x   2          =
               1   1 mol

 Multiply the whole equation and you get:

3.5 mol x 1  x   2    = 7 grams of hydrogen
                1    1 mol

Easy right?
Wait, we're not done yet...let's do the reverse.

How many moles are there in barium nitrate are consumed when 2.55 grams of silver chloride completely reacts?

First, remember to write the balanced equation:

Ba(NO3)2 + 2AgCl = BaCl2 + 2AgNO3

You do the reverse now, you use the molar mass of what you have. I repeat, what you have:

2.55 grams x 1 mol    x
                     143.3 g

Then multiply it by what you need over what you have!:

2.55 grams x 1 mol   x    1  =  0.00890 moles
                       143.3 g     2

http://www.youtube.com/watch?v=JIw_BmFZdhI
http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH4104

-Candace Chan