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Unit 8 Lecture
Unit 8: Types of Reactions and Balancing Equations
When a chemical reaction occurs, it can be represented by a neat shorthand method called a balanced equation. The balanced equation tells us what we are starting out with (reactant) and what we end up with (product). It also makes sure the number of atoms for each element involved on the reactant side equals the number of atoms for each element involved on the product side (that’s the balancing part). That’s important because you have to end up with the same number of atoms you start with (atoms don’t just disappear and they don’t just appear – each one has to be accounted for).
There are three different types of reactions that we are going to work with in this course:
- Composition (or synthesis)
Whatever the type, the reactant(s) will be on the left side of the arrow (the yield sign) and the product(s) will be on the right side of the yield sign.
- Composition: A + B → AB (let’s use this for a generic form)
A + B are the reactants. There are 2 reactants in a composition reaction – they react together.
AB is the product. There is 1 product in a composition reaction – it’s what’s yielded or produced by the reaction.In a composition reaction there are 2 reactants and 1 product!
- Decomposition: AB → A + B
AB is the reactant.There is 1 reactant in a decomposition reaction – it is falling apart into the products.
A + B are the products. There are 2 products in a decomposition reaction. They are formed when the reactant falls apart (decomposes).In decomposition reaction there is 1 reactant and 2 products!
- Replacement: In a replacement reaction, there are 2 reactants and 2 productsThere are single replacement reactions and double replacement reactions.
SINGLE REPLACEMENT REACTIONS
(2 reactants, 2 products and only one – single – reactant is a compound, the other is an element)
In most single replacement reactions, metals will swap places with other metals or other components acting like metals (hydrogen or ammonium, for example). This is also called Cation Replacement because metals are always the first part of a compound since they always have a positive charge as ions and so do hydrogen and ammonium ions. There is a particular order of which metals can replace other metals and that information has been compiled in what is called the Activity Series of Metals. It is loosely arranged as to the location of the metal on the Periodic Table. In this course you will not be held responsible for memorizing the Activity Series – I just want you to know that there is a particular way metals can replace each other. In this course, the activities will be correct – I won’t throw you any curve balls! You won’t have to worry if the right metals are being replaced – we’ll just concentrate on identifying a reaction as single replacement and then we’ll balance it.
Activity Series of Metals
|Metal||Ion||React with oxygen||React with acid||React with water||Activity Notes|
This is a short activity list just to give you an idea of how it works:
Metals move from most reaction to least reactive moving from the top of the list downward.
A metal can replace metals listed below it in the activity series, but not above it
Metals that can replace hydrogen within acids
In the reactions we will work with, the reactivity will already by correct and you will not have to worry about that!
So in a single replacement reaction, most of the time it’s going to be metals replacing metals or things acting like metals. The generic form, then, would be:
AB + C → A + CB
(or the products could be written CB + A, the order isn’t important, the correct products are).
The A and the C are metals (the A on the reactant side is the first part of the compound formula, that’s now we know it’s a metal. You’ve just got to trust me that C is a metal as well in this generic equation.)
The metals swap places as products, since the C is a metal it’s placed first in the compound formula on the product side.
Every once in a while, you run into another kind of replacement reaction that involves not the metals swapping places but the halogens swapping places. That’s also known as Anion Replacement because the anions (halogens are the major group 7 elements, so they always have a -1 charge (remember the example of chlorine’s ion chloride back in unit 6?)) are the ones that are swapping places. Anions are always the second part to the compound formula.
There is also an Activity Series of Halogens. It looks like this:
The Activity Series of Halogens
Each halogen will replace any halogen below it in an anion replacement reaction.
Hey, wait a minute, that’s the order of these halogens on the Periodic Table!
Just as with the single replacement reactions involving metals, I just want you to be aware of this activity series. The halogens in the equations we will balance will all be able to swap places correctly.
All right, here we go:
AB + C → AC + B (or you could write the products B + AC)
The B and the C are halogens. They swap places. When they are in compounds they are the last part of the compound formula because they have negative charges as ions.
DOUBLE REPLACEMENT REACTIONS
(2 reactants, 2 products and both – double – reactants are compounds)
In Double Replacement Reactions, the two metals swap places:
AB + CD → CB + AD (or AD + CB would be fine too on the product side)
Something else to keep in mind with chemical equations – sometimes you might see little letters in parenthesis beside the symbols of elements or compounds. Letters like these:
That’s just to give you extra information about the reaction.
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous solution (something mixed with water)
On to the examples!
Ex: Zn + S → ZnS
First, what kind of reaction is this? 2 reactants, 1 product = composition
Next, is this equation balanced? Well, what does that mean?? Balancing an equation just means making sure that there are the same number of atoms for each element involved in the reaction on the reactant side as the product side.
Let’s count ’em up!
There is 1 zinc on the reactant side and 1 zinc on the product side
There is 1 sulfur on the reactant side and 1 sulfur on the product side
This is equation is balanced! We are done with it!
Ex: NaCl + H2SO4 → Na2SO4 + HCl
What kind of reaction is this? 2 reactants, 2 products make it replacement. 2 compounds on the reactant sides make it double replacement.
Is this equation balanced? Count ’em up!
There is 1 Na on the reactant side but . . . wait a minute . . . there are 2 Na atoms on the product side!
There are 2 H atoms on the reactant side but 1 on the product side.
This equation is not balanced! We are not done with it!
The Cl and the SO4 are OK for now. (Here’s a tip: if you’ve got the same polyatomic ion on both the reactant and the product side, don’t break it down to it’s parts – consider it a unit, so call SO4 sulfate , don’t break it down to sulfur and oxygen. It will make things easier if you do.)
We need to get the same number of sodium atoms on both sides!
Here’s what you CAN’T do to make that happen:
You can’t put numbers just anywhere – for example, you can’t put a 2 after the Na in NaCl to balance the sodium. NaCl is the correct formula for sodium chloride, if you stuck a 2 in the middle of that formula, it would not have a net charge of zero.
Here’s all you CAN do to make that happen:
You can only put numbers in front of the entire compound formula or element.
Ditto for the hydrogen atoms!
So, if we need 2 sodium atoms on the reactant side to balance the 2 sodium atoms on the product side:
And if we need 2 hydrogen atoms on the reactant side to balance out the 2 hydrogen atoms on the product side:
Here’s the way the balanced equation will look:
2NaCl + H2SO4 → Na2SO4 + 2HCl
Count ’em up:
2 sodiums and 2 sodiums
2 chlorines and 2 chlorines
2 hydrogens and 2 hydrogens
1 sulfate and 1 sulfate
Now it’s balanced and we are done with this rascal!
Say, this balancing equations stuff is not too bad at all!
Here are a few suggestions on balancing equations to keep in mind:
- The number of each element’s atoms on the left must be the same as its atoms on the right
- If you have the same polyatomic ion on one side as the other, think of it as a unit – don’t break it down to its parts
- You can’t just stick numbers any old place to balance an equation, they can only go in front of a compound or element – not in the middle or the end
- If an element has an even number of atoms on one side and an odd number on the other, first try multiplying the odd number by 2 to make it even – see if that helps; it’s usually a good place to start.
How about a couple more examples?
Ex: MgBr2 + Cl2 → MgCl2 + Br2
There are two things I want to show you about this reaction!
First, this is one of those special single replacement reactions in which the halogens swap places. Take a look, the chlorine replaces the bromine.
Second, what is up with that chlorine on the reactant side being written Cl2 rather than Cl and that bromine on the product side being written Br2 rather than Br?
Chlorine and bromine are two of seven elements that you have to be able to identify because they are the:
There are only seven of these guys that you have to know and here’s a memory tool to help you remember them:
NO Fun Having Invisible Brown Clothes
I know you think I’ve gone off the deep end putting a weird statement like that here, but it’s a mnemonic device and here’s what it stands for:
N = nitrogen
O = oxygen
F = fluorine
H = hydrogen
I = iodine
Br = bromine
Cl = chlorine
When these seven elements are BY THEMSELVES they always pair up in nature and in a chemical equation we symbolize this by putting a 2 subscript after their symbol:
Please note: The automatic 2 subscript only applies when these seven elements are by themselves in a chemical reaction – it does not apply when these elements are in compounds!
In this equation example, the magnesium chloride is written MgCl2 because magnesium has a +2 charge as an ion and chloride has a -1 charge as an ion so you need 2 chlorides to balance the charge of the one magnesium – same deal with the magnesium bromide – but it has nothing to do with chlorine and bromine being diatomic.
So, to balance the equation:
MgBr2 + Cl2 → MgCl2 + Br2
what do we need to do? Well, there is:
1 magnesium and 1 magnesium
2 bromines and 2 bromines
2 chlorines and 2 chlorines
We don’t have to do anything – this bad boy is balanced! We are done here!
Let’s try one more example!
Ex: Ni(ClO3)2 → NiCl2 + O2 (1 reactant and 2 products = decomposition)
There is 1 nickel and 1 nickel
There are 2 chlorines and 2 chlorines (that reactant chlorine is inside parenthesis and there’s a 2 subscript on the outside of the parenthesis so it’s 1 x 2 = 2)
But hold the phone, there are 6 oxygens (3 x 2 = 6) and 2 oxygens
This equation is not balanced! The oxygens are out of whack and we’ve got to balance them!
What can we do? To get our oxygens balanced, we need the same number of oxygen atoms on each side. So let’s put a big ‘ol 3 in front of that O2 on the product side! 3 x 2 = 6! 6 = 6!
So here is how the balanced equation looks:
Ni(ClO3)2 → NiCl2 + 3O2