(TG) Limiting Reactants! FUNFUNFUN.

LIMITING REACTANTS.

Before we dive into the wonderful world of Chemistry I believe it is important to first understand exactly what a limiting reactant is. With limiting reactants it is best, and most important to understand exactly what are you doing, as opposed to just memorizing a process. 

A limiting reactant, is one of the reactants in a chemical reaction that gets used up in the rxn first, and "limits" the rest of the reaction. A great comparison to this process in chemistry appears in our cafeteria: On pancake day one meal consists of:

1. 2 pancakes 
2. 1 sausage

Every single breakfast has to have these components. Now if we have 30 sausages and 2 pancakes how many meals can we make? Only one! Then pancakes are the limiting reactants because they limit/prevent the making of any further breakfasts. The same is true in chemical reactions.

Now on to some examples:


Identify the limiting reactant when 1.22 g of 0(2) reacts with 1.05 t of H(2) to produce H(2)0

2H(2) + 0(2) = 2H(2)O

1.22g x mol/ 32g x 2/1 x 2 = .15g

Now because we only need .15 g of H and we have 1.05 g of it. We know that Hydrogen is NOT the limiting reactant leaving Oxygen as our only option :)!

(TG) Heat and the Stoichiometric Method

* I know that these two topics were taught several weeks apart, but I want to keep the blog updated, and 1 post is simpler than 2.

ENERGY (in kJ's)

• Often times in chemistry you will get a chemical reaction that is either: EXOTHERMIC, or ENDOTHERMIC. These words may seem complicated but really they just mean, releasing heat (exo) and absorbing heat (endo)
• Whenever you see these in a formula (ie. NaBr + I = NaI + Br + 234kJ) all you have to do is treat the kJ's like another element.
• Since energy will only appear on one side of the equation you dont even have to balance it
• Whenever using energy pretend as if there is the coefficient of "1" infront of it

Now lets look at an example:

How much energy is released when 2.0 mol of I is used up?

2NaBr + I(2) = 2NaI + Br(2) + 345kJ

2.0 x 1/2 = 1 x 345 = 345kJ

(NR) Feb 8, 2011 : 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 delta H
• In Exothermic reactions enthalpy decreases
• In Endothermic reactions enthalpy increases
  
       CALORIMETRY
  • To experimentally determine the heat released we need to know THREE things:
    
    1. TEMPERATURE CHANGE (delta T)
    2. Mass (m)
    3. Specific heat capacity (C)
       Cw= 4.19 J/g degrees C
     These are related bythe equation:
          delta H = mCdeltaT            
                     = mC(Tf-Ti)
    
    
    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 actual amount to the predicted amount?
      
      
      %YIELD = [ACTUAL/THEORETICAL] x 100