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
What is a Mole Ratio?A mole ration is a ration between the amounts in moles of any two compounds found in a chemical reaction. Mole ratios are used as conversion factors between products and reactants in many chemistry problems.
Key thingsto remember when doing Mole to Mole conversions: 1. Write the chemical equation 2. Balance the chemical equation 3. Understand what the question is asking 4. Now place what you need over what you have. It may not be easy to grasp so here's an example.. If 0.15 mol of methane are consumed in a combustion reaction. How many moles of CO2 are produced? CH4 + 02 ---> CO2 + H2O
(If you forgot..refer to the steps, for solving mole to mole conversions)
the answer is.... CH4 + 202---> CO2 + 2H2O
Want more problems??
How many moles of bauxite (Aluminum oxide) are required to produce 1.8 mol of pure Aluminum?
Al2O3 ----> Al + O2
Answer: 2AlO3------> 4Al+3O2
When 1.5 mol of Copper react with Fe (II) chloride. How many moles of Iron should be produced? 2Cu + FeCl2------> 2CuCl+Fe
What is a Mole Ratio? A mole ration is a ration between the amounts in moles of any two compounds found in a chemical reaction. Mole ratios are used as conversion factors betweenproducts and reactants in many chemistry problems.
Key things to remember when doing Mole to Mole conversions:
1. Write the chemical equation
2. Balance the chemical equation
3. Understand what the question is asking
4. Now place what you need over what you have. It may not be easy to grasp so here's an example..
If 0.15 mol of methane are consumed in a combustion reaction. How many moles of CO2 are produced?
CH4 + 02 ---> CO2 + H2O
(If you forgot..refer to the steps, for solving mole to mole conversions)
Stoiciometry is a branch of Chemistry that deals with the quantitative anaysis of chemical reactions
It is a generalization of mole conversions to chemical reactions
understanding the 6 types of chemical reactions is the foundation of stoichiometry
There are 6 types of reactions we use in stoichiometry, they are...
Synthesis (formation)
Decomposition
Single Replacement (SR)
Double Replacement (DR)
Neutralization
Combustion
SYNTHESIS
A synthesis reaction is when two or more simple compounds combine to form a more complicated one. The general form for this reaction is:
A + B ---> AB
DECOMPOSITON
A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones. These general form for this reaction is:
AB ---> A + B
Example: 2 H2O ---> 2 H2 + O
SINGLE REPLACEMENT
This is when one element trades places with another element in a compound. The general form for this reaction is:
A + BC ---> AC + B (A is a metal)
OR A +BC---> C+ BA (A is a non-metal)
Example: Mg + 2 H2O ---> Mg(OH)2 + H2
DOUBLE REPLACEMENT
This is when the anions and cations of two different molecules switch places, forming two entirely different compounds. These general form for this reaction is: AB + CD ---> AD + CB
Example: Pb(NO3)2 + 2 KI ---> PbI2 + 2 KNO3
NEUTRALIZATION
Also known as Acid-base neutralization: This is a kind of double displacement reaction that takes place when an acid and base react. The H+ ion in the acid reacts with the OH- ion in the base, causing water to form. The product of this reaction is some ionic salt and water: HA + BOH ---> H2O + BA
Example: HBr + NaOH ---> NaBr + H2O
COMBUSTION
Combustion reactions happen when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic.
* They show only the simplest rations, not the actual atoms.
* Molecular formulas give the actual # of atoms…
* To determine an empirical formula we need to know the ratio of each element. This is why we use the table below; to gather all the information we may need to solve each problem.
Atoms
Mass
Molar
Mass
Moles
Mol/ Smallest Mole
Ratio
C
8.4
12.0
0.7
2
H
2.1
1.0
2.1
6
O
5.6
16.0
0.35
1
Step # 1. Fill in the chart with what’s already given to you (the names and masses of the three atoms, plus their molar masses which you can find on the periodic table).
Step #2. Calculate the number of moles simply by following the conversion chart.
* 8.4g x 1mol / 12g = 0.7 mol
* 2.1g x 1mol / 1g = 2.1mol
* 5.6g x 1mol / 16g = 0.35mol
Step # 3. Find the smallest mole (0.35mol in this case) and divide it into itself and the rest of the mole #’s.
* 0.7 / 0.35 = 2
* 2.1 / 0.35 = 6
* 0.35 / 0.35 = 1
* If your answers are not “whole #’s you must multiply everything by a common #. For example:
* Say you end up with the numbers 1.0, 1.5, and 6.0… you would use the number 3 to change 1.5 into a whole # while still keeping the other two numbers whole as well. Thus you would end up with 2.0, 3.0, and 12.
= C2H6O
Atom
Mass
Molar
Mass
Moles
Mole/
Smallest mole
Ratio
Pd
42.56
106.4
0.3992
1
H
0.80
1.0
0.80
2
Step # 3. You must be given the compound’s molar mass (in this case it’s 216.8 g/mol) in order to figure out its molecular formula. Besides the molar mass, you also know from your previous calculations that the empirical formula is PdH2. You then calculate the empirical formula’s molar mass (by looking at the periodic table). In this case it is 108.4. You can then divide 216.8 by 108.4 to figure out what number to use for the molecular formula’s subscripts. It is 2… thus you change Pd into Pd2 and H2 into H4. And there you have your molecular formula!
