In Chemical reactions, usually one chemical gets used up first before the other. The chemical used up first in a chemical reaction is called the limiting reactant.
Once it is used up, the reaction stops! Limiting reactants determines the quantity of products formed.
To find the Limiting Reactant, assume one reactant is used up. Determine how much of this reaction is required!
Here are the definitions of exothermic and endothermic reactions, in case you forget:
EXOTHERMIC:
The term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light, electricity or sound.
ENDOTHERMIC:
The word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat.
Calorimetry: To experimentally determine the heat released we need to know 3 things:
Temperature Change (∆T)
Mass (m)
Specific heat capacity (C)
These are related by the equation:
∆H = mC∆T
Example:
Calculate the heat required to warm a cup of 400 g of water (C= 41.8J/g°C) from 20.0°C to 50.0°C.
∆H = mC∆T
∆H =(400g)(4.18J/g°C)(50.0°C-20.0°C)
=50160 J
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?
********THIS EQUATION IS VERY IMPORTANT*********
Actual / Theoretical x 100% = % yield
or
Actual amount of product / Expected amount of product x 100% = % yield
Example: A student makes a single displacement reaction that produces 2.755 grams of copper. He determines that 3.150 grams of copper should have been produced. Find the student's percentage yield.
actual amount of product
percentage yield = ------------------------------------------- x 100
How many grams of O2 are produced from the decomposition of 3.0 g of Potassium Chlorate?
This question gives us grams, but we must change it to grams of another element. First we must write a balanced Chemical equation:
2KClO3 ----> 2K + Cl2 + 3O2
Than we take what we are given, which is 3.0 grams. Change 3.0 grams into moles, than find what you need over what you have, than with the moles you are given, find grams!
3.0 g x 1 mol/122.6 g = 0.024 mol x 3 O2 mol/ 2 KClO3 = 0.036 mol x 32.0 g/1mol
= 1.2 g
Let's try one more!
Example:
Determine the mass of lithium hydroxide produced when 0.38 g of lithium nitride reacts with water according to the following equation:
Li3N +3H2O ---> NH3+ 3LiOH
0.38 g x 1 mol/34.7 g = 0.011 mol x 23.95 g LiOH/1mol = 0.033 g x 23.9 g/ 1 mol
= 0.78 mol
