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1

 

How to Write and Balance a Chemical Reaction

1.) What element symbols do we need and what compounds do we have?

For example:

We want nitrogen trihydride from hydrogen and nitrogen.

Therefore, we have H (hydrogen), N (nitrogen), and we want NH3 (nitrogren trihydride).

2.) Put all reactants on the left, and all products on the right.

H, N, NH3

Reactants → Products

N + H → NH3

3.) Write the elements in how they exist in nature.

N2 + H2 → NH3

4.) Balance the equation. Remember

– Reactants must equal the same amount of products.

– All subscripts are fixed in (you cannot change them!).

– Adding a coefficient multiplies EVERY atom in that molecule.

– Coefficients must be in the smallest, whole number ratios.

– Since oxygen is all over the place, always balance oxygen last!

– Setting up your balancing process like this would help:

# of reactants – element – # of products

5.) Include the state (solid, gas, liquid) for each element. Solid = s, gas = g, liquid = aq.

N2 + 3H2 → 2NH6

N2 (g) + 3H2 (g) → 2NH6 (g)​

 

2

Ammonium carbonate decomposes into nitrogen trihydride, carbon dioxide, and dihydride monoxide.

Ammonium carbonate

  • Ammonium = NH4+
  • Carbonate = CO32-
  • NH4+ CO32-  =  (NH4)+ (CO3)2-  =  Switch charges to get (NH4)2- (CO3) =  (NH4)2 CO3

 

Nitrogen trihydride

  • Nitrogen = N
  • Trihydride = H3
  • NH3

Carbon dioxide

  • Carbon = C
  • Dioxide = O2
  • CO2

​Dihydride monoxide

  • Dihydride = H2
  • Monoxide = O
  • H2O

​(NH4)2 CO3 → NH3 + CO2​ + H2O

Now balance the reaction:

 

3

Balance the reaction by finding the missing coefficients:

 

__ Ti + __ F2 → __ TiF4

4

Balance the reaction by finding the missing coefficients:

 

__ Al + __ Cl2 → __ AlCl3

5

 

Types of Reactions

1.) Combustion Reaction

Reaction of an element or a compound with O2. Usually happens with compounds that contain carbon (C), hydrogen (H), and oxygen (O).

For example:

CH4 + O2 → CO2 + H2

 

2.) Combination Reaction

Formation of one product from two or more reactants.

A + B → C

For example:

Na (s) + Cl(g) → NaCl (s)

 

3.) Decomposition Reaction

One reactant producing two or more products.

A → B + C

For example:

H2CO3 → CO2 + H2O

4.) Single Displacement Reaction

An element substituting for another element that is already in a compound.

A + BC (aq) → B + AC (aq)

For example:

Zn (s) + CuCl2 (aq) → Cu (s) + ZuCl2 (aq)

5.) Double Displacement Reaction

Two aqueous compounds that swap anions.

AB (aq) + CD (aq) → BC + AD

There are two kinds of double displacement reaction:

  • Precipitation: Pb (C2H3O2)2 (aq) + KI (aq) → PBI2 (s) + KC2H3O2 (aq)
  • Acid-Base (or Neutralization): One of the products is H2O.

 

6

 

Combustion Reaction

 

Reaction of an element or a compound with O2. Usually happens with compounds that contain carbon (C), hydrogen (H), and oxygen (O).

For example:

CH4 + O2 → CO2 + H2

7

 

Combination Reaction

 

Formation of one product from two or more reactants.

A + B → C

For example:

Na (s) + Cl2 (g) → NaCl (s)

8

 

Decomposition Reaction

 

One reactant producing two or more products.

A → B + C

For example:

H2CO3 → CO2 + H2O

9

 

Counting by Weighing

Atomic Mass (weighted average of ALL isotopes of an element).

For example:

Oxygen (O) = 16 amu

- Turn 16 amu into grams (g).

- Use moles or 6.022 x 1023 (Avogadro's Number)

- 16g of oxygen = 1 mole. 

- Write as 16g/mole.

10

 

Double Displacement Reaction

 

Two aqueous compounds that swap anions.

AB (aq) + CD (aq) → BC + AD

There are two kinds of double displacement reaction:

  • Precipitation: Pb (C2H3O2)(aq) + KI (aq) → PBI2 (s) + KC2H3O2 (aq)
  • Acid-Base (or Neutralization): One of the products is H2O

11

 

Single Displacement Reaction

 

An element substituting for another element that is already in a compound.

A + BC (aq) → B + AC (aq)

For example:

Zn (s) + CuCl2 (aq) → Cu (s) + ZuCl2 (aq)

12

 

How many moles are in 45 g of carbon?

How much mass of He would give us the same number of atoms?

From the periodic table, we see that carbon has an atomic mass of 16, or 16 amu. Therefore:

16g Carbon = 1 mole Carbon

(See picture below)

To find the mass of He, helium, that would give us the same number of atoms as carbon, we will also need 3.75 moles of He. From the periodic table, He has an atomic mass of 4, or 4 amu. 

4g Helium = 1 mole Helium

We use the above information as a conversion factor and solve.

The answer: We will need 15g of He to give us the same number of atoms as carbon.

 

13

 

The Sum of Molecules and Compounds

For example:

How many moles of FeClare in 269g?

From the periodic table:

Fe = 55.845 amu

Cl3 = 35.4527 x 3 = 106.3581 amu

55.845 g/mole + 106.3581 g/mole = 162.2 g/mole

14

 

I need 0.627 moles of C12H22O11. How many grams is this?

C = 12 amu x 12 = 144 amu

H = 1 amu x 22 = 22 amu

O = 16 amu x 11 = 176 amu

24 amu C + 22 amu H + 176 amu O = 342 g/mole  

15

 

Stoichiometry

The calculation of relative quantities of reactants and products in chemical reactions.

When presented with a stoichiometry problem, you must:

  1. Make sure the reaction is balanced.
  2. Find the atomic weight of each element. Remember that if an element has a subscript, multiply that subscript by the atomic weight.
  3. Solve the question using certain conversion factors depending on what is being asked.

16

N2H4 → NH3 + N2

How many moles of NH3 will form from 5.3 moles of N2H4?

First, balance the reaction. You should get:  

3N2H4 → 4NH3 + N2

Or, in other words:

3 moles of N2H4 decomposes into 4 moles of NH3 and 1 mole of N2.

* This is your conversion factor! *

How many moles of NH3 will form from 5.3 moles of N2H4?

 

17

N2H4 → NH3 + N2

If N2H2 completely decomposes and 0.25 moles of N2 is produced, how much NH3 is produced?

First, balance the reaction. You should get:  

3N2H→ 4NH3 + N2

Or, in other words:

3 moles of N2H4 decomposes into 4 moles of NH3 and 1 mole of N2.

* This is your conversion factor! *

 

If N2H2 completely decomposes and 0.25 moles of N2 is produced, how much NH3 is produced?

18

 

Mass of a Given

Follow these steps in order.

1.) MASS OF GIVEN. Add all of elements' atomic weights together to get the molar mass in grams (g). 

1 mole

Molar mass in grams (g)

2.) MOLES OF GIVEN. These involve the coefficients in the chemical equation. 

# of moles requested

# of moles given

3.) MOLES OF REQUESTED. 

Molar mass

1 mole

4.) MASS OF REQUESTED.

19

2 Sr (s) + O2 (g) → 2 SrO (s)

How many grams of SrO (s) can be produced from 21.0 g of Sr (s)? 

Follow the steps listed in Mass of a Given.

1.) MASS OF GIVEN. 

1 mole

Molar mass in grams (g)

2.) MOLES OF GIVEN. These involve the coefficients in the chemical equation. 

# of moles requested

# of moles given

3.) MOLES OF REQUESTED. 

Molar mass

1 mole

4.) MASS OF REQUESTED.

20

 

Percent Yield

A way to compare what we produce in an experiment to what we calculated. To compare an experimental value to a calculated value.

First, find the Actual Yield - Experimental Value (given in the problem)

Then, find the Theoretical Yield - Calculated Value (stoichiometry)

 

Percent Yield = Actual / Theoretical (100%) 

 

The percent yield should be a little lower than 100%. If the percent yield is over 100%, you did something wrong.