balanced chemical equations

Balanced Chemical EquationsBalanced Chemical EquationsYour Turn #1:

What happened in Lakehurst, NJ on May 6th, 1937? (click here to find out; requires Internet connection)

Answer:

The German airship Hindenburg exploded while landing at the naval air station in Lakehurst. On board were 61 crew and 36 passengers. 35 people died in the crash. There is still controversy as to what caused the explosion. More recent arguments contend that it was the coating on the surface of the ship that ignited. Others believe it was the gas which kept the Hindenburg afloat that exploded. To see an additional 5-6 minute video of Hindenburg footage click here.Your Turn #2:

Even if the gas was not the cause of the initial fire, it would have eventually ignited. What was the name of this highly flammable gas? (click for answer)Answer: hydrogen (today's blimps use the much safer helium)

Your Turn #5:

What would you predict is the product when hydrogen reacts with oxygen? (click for answer)

Your Turn #6:

What is the general term given to all substances that appear to the left of the arrow? (click for answer)

Your Turn #7:

What is the general term given to all substances that appear to the right of the arrow? (click for answer)

Balanced Chemical EquationsBalanced Chemical Equations

H2 + O2 H2O

reactantsproducts

Your Turn #3:

What is the formula of hydrogen gas? (click for answer)

Your Turn #4:

When something burns or explodes, what does it generally react with (name and formula)? (click for answer)

oxygenThe plus sign means “reacts with”.

The arrow means “to produce”.

To be balanced, a chemical equation has to have the same number of atoms of each element on the left as on the right. This is not the case here because there is one more oxygen on the left than the right.


H2 O2 H2O

What you see is an example of a chemical equation. However, it is not a balanced chemical equation.

Your Turn #8:How many atoms of hydrogen and how many atoms of oxygen appear on the reactants side of the equation? (click for answer)

2 hydrogen2 oxygen

+

Your Turn #9:How many atoms of hydrogen and how many atoms of oxygen appear on the products side of the equation? (click for answer)

2 hydrogen1 oxygen


H2 O2 H2O

2 hydrogen2 oxygen

+

2 hydrogen1 oxygen

So the question is, how can we balance an equation? First we will show you the WRONG thing to do.


H2 O2 H2O2

2 hydrogen2 oxygen

+

2 hydrogen2 oxygen


Wrong!


H2 O2 H2O2

2 hydrogen2 oxygen

+

2 hydrogen2 oxygen


NEVER change the subscripts when trying to balance a chemical equation.

I said NEVER! Were you listening???...The reason you can’t change the subscripts is because you are then changing what substance is represented. H2O2 is hydrogen peroxide and a completely different substance then what the product really was (H2O, water). Click to go back to what we had before.


H2 O2 H2O

2 hydrogen2 oxygen

+

2 hydrogen1 oxygen

We balance an equation by adding coefficients (numbers which appear in front of a formula). For example let’s add a 2 in front of H2O (click to add).


H2 O2 H2O

2 hydrogen2 oxygen

+

4 hydrogen2 oxygen

Notice that this does make two oxygen on the right (you multiply the coefficient, in this case 2, by the subscript for oxygen, in this case 1). Now oxygen is balanced! But….

2


H2 O2 H2O

2 hydrogen2 oxygen

+

4 hydrogen2 oxygen

…it also messes up the hydrogen!! Now there are 4 hydrogen on the right (coefficient of 2 multiplied by a subscript of 2) and 2 hydrogen on the left. So we need to fix the hydrogen…

2


H2 O2 H2O

2 hydrogen2 oxygen

+

4 hydrogen2 oxygen

Your Turn #10:

Predict how we can now balance the hydrogen. (click for answer)

2


H2 O2 H2O

4 hydrogen2 oxygen

+

4 hydrogen2 oxygen

Your Turn #10:

Predict how we can now balance the hydrogen. (click for answer)

22By adding the 2 in front of H2, we now make 4 hydrogen on the left hand side of the equation. This balances the 4 hydrogen on the right. Since the oxygen are also balanced, we now have a balanced equation!


H2 O2 H2O

4 hydrogen2 oxygen

+

4 hydrogen2 oxygen

22

Remember, we balance equations by adding coefficients, NOT by changing the subscripts. Below, the coefficient on O2 is assumed to be 1.

coefficients

Balanced Chemical EquationsBalanced Chemical EquationsBefore you go on to practice balancing more equations, it is worth noting that there really is no advantage to starting on the left or right hand side of the equation. It pretty much is just a trial and error process until you get it balanced. Having said that, there are a few hints that will help you generally and we’ll cover two of them in this lesson. The first is, in general, it is best to balance the more complicated formula(s) first (for example, you would generally balance C6H12O6 before you balance O2). The second hint relates to polyatomic ions and we’ll get to that after you practice a few others first.Your Turn #11:

Balance the following equations. (click for answer one equation at a time) Cl2 + H2 HCl2

Mg + AgCl MgCl2 + Ag (hint: start with AgCl or MgCl2 because they

are more complicated than Mg or Ag)

2 2

BiCl3 + H2S Bi2S3 + HCl (hint: HCl looks the least complicated so start

with one of the others)

63

2


Balance the following equations. (click for answer one equation at a time) MgCO3 CO2 + MgO

C3H8 + O2 CO2 + H2O (hint: which one of these formulas do you think is least complicated,

thus suggesting you’ll want to balance that last, or at least not first)

Al + HCl AlCl3 + H2

3 4

already balanced!

5

22 6 3

Now for your second hint. It relates to equations which contain polyatomic ions. In particular for equations which have the same polyatomic ion on the left as the right. In these cases, it is easiest to balance the polyatomic ion as a complete unit, not element by element. To see what I mean, try Assisted Problem #1 on the next slide.

Balanced Chemical EquationsBalanced Chemical EquationsAssisted Problem #1:

Balance the following equation. (click for answer one step at a time)

Al2(SO4)3 + KOH Al(OH)3 + K2SO426 3

By adding 2 as a coefficient, this makes 2 Al on the right to go with the 2 on the left. But now think of the oxygen and hydrogen in Al(OH)3 not as individual elements but as part of a hydroxide unit. This would make 6 hydroxides (OH-). 6 because the coefficient of 2 is multiplied by the subscript of 3 for the hydroxide. Now predict how we can adjust the left side of the equation in order to balance the hydroxides. Remember, you are trying to balance the OH- as a complete group. For now, don’t think of it as O and H separate from one another (click for answer).

2 aluminum6 hydroxide (OH-)

6 hydroxide (OH-)6 potassiumThis makes 6 hydroxide on the left to balance the 6 on the right. However, it also makes 6 K. Predict how we can adjust the right side of the equation to balance the K (click for answer).

6 potassium3 sulfate (SO4

2-)

Now there are 6 K on both sides of the equation (because the coefficient of 3 times the subscript of 2 equals 6). But this also makes 3 sulfates (SO4

2-). This is good though because there are already three sulfates on the left hand side of the equation! This equation is now balanced!


Balance the following equations (remember to balance the polyatomic ions as complete units). (click for answer one equation at a time) Ca + H3PO4 Ca3(PO4)2 + H2

NH4Cl + Pb(NO3)2 NH4NO3 + PbCl2

3 2 3

22

3 Ca

6 H2 PO4

3-

3 Ca2 PO4

3-6 H think of as

2 NH4+

2 Cl-1 Pb2 NO3

-

2 NH4+

2 NO3-

1 Pb2 Cl-

think of as

Balanced Chemical EquationsBalanced Chemical EquationsReview Questions:

Go to this link to practice balancing equations. Start with #’s 2 and 20 (remember to save the least complicated formula(s) for last) and #’s 7 and 17 (remember to balance polyatomic ions as complete units). You can check your answers by clicking the “Check” button on the bottom of the page.

The End

balanced chemical equations

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