Chap. 9: Chemical Reactions

Question 1

How to Write and Balance a Chemical Reaction

Answer 1

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)​

Question 2

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

Answer 2

Ammonium carbonate

• Ammonium = NH₄⁺
• Carbonate = CO₃^2-
• NH₄⁺ CO₃^2- = (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:

Question 3

Balance the reaction by finding the missing coefficients:

__ Ti + __ F₂ → __ TiF₄

Answer 3

Question 4

Balance the reaction by finding the missing coefficients:

__ Al + __ Cl₂ → __ AlCl₃

Answer 4

Question 5

Types of Reactions

Answer 5

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 + H₂O

2.) Combination Reaction

Formation of one product from two or more reactants.

A + B → C

For example:

Na (s) + Cl₂(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.

Question 6

Combustion Reaction

Answer 6

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 + H₂O

Question 7

Combination Reaction

Answer 7

Formation of one product from two or more reactants.

A + B → C

For example:

Na (s) + Cl₂ (g) → NaCl (s)

Question 8

Decomposition Reaction

Answer 8

One reactant producing two or more products.

A → B + C

For example:

H₂CO₃ → CO₂ + H₂O

Question 9

Counting by Weighing

Answer 9

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.

Question 10

Double Displacement Reaction

Answer 10

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

Question 11

Single Displacement Reaction

Answer 11

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)

Question 12

How many moles are in 45 g of carbon?

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

Answer 12

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.

Question 13

The Sum of Molecules and Compounds

Answer 13

For example:

How many moles of FeCl₃ are 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

Question 14

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

Answer 14

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

Question 15

Stoichiometry

Answer 15

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.

Question 16

N₂H₄ → NH₃ + N₂

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

Answer 16

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?

Question 17

N₂H₄ → NH₃ + N₂

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

Answer 17

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! *

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

Question 18

Mass of a Given

Answer 18

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.

Question 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)?

Answer 19

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.

Question 20

Percent Yield

Answer 20

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.