Chemical Reactions - Balancing and Writing Chemical Equations

Chemical Reactions - Balancing and Writing Chemical Equations

​Learning Target

​I know...

​

how to count the number of atoms in a chemical formula

Success Criteria

​I can...

​

use coefficients to balance a chemical equation

Numbers in ​Chemical Equations

• ​Subscripts are whole numbers on the right-hand side of an element symbol

  • ​they tell us how many atoms of the element are in the chemical formula

Multiple Choice

How many Fe (iron) atoms are in Fe₂(SO₄)₃?

• 2

• 4

• 6

• 8

Numbers in ​Chemical Equations

• ​Coefficients are whole numbers in front of a chemical formula

  • ​they tell ushow many molecules of a formula are present in the equation

  • ​they are also multiplied by the subscript to help us count atoms

Multiple Choice

How many H (hydrogen) atoms are in 5H₂O?

• 2

• 4

• 8

• 10

​How do we describe coefficients?

an ​individual atom = "atom"

​2Mg means that there are 2 atoms of magnesium

​

covalent substance = "molecule"

​ 3CO₂ means that there are 3 molecules of carbon dioxide

​REMEMBER: covalent compounds only contain nonmetals

ionic substance = "unit"

​4MgO means that there are 4 units of magnesium oxide

​REMEMBER: ionic compounds contain metals and nonmetals

Multiple Choice

How would we write 2NaCl? (hint: what types of elements are present?)

• 2 molecules of NaCl

• 2 atoms of NaCl

• 2 units of NaCl

Multiple Choice

How would we write 3CO₂?

• 3 units of carbon dioxide

• 3 molecules of carbon dioxide

• 3 atoms of carbon dioxide

​Why do we use chemical equations?

Answer:

Chemical equations help us describe chemical reactions without using a ton of words.

Example:

​

• ​Instead of writing...

  • Solid aluminum reacts with solid iron (III) oxide to produce solid aluminum oxide and liquid iron.

• ​We write...

​2Al(s) + Fe₂O₃(s) --> Al₂O₃(s) + 2Fe(l)

​What do the symbols in a chemical equation mean?

What are the rules for writing equations?

• ​ The equation has to show known facts

• ​All reactants and products must be identified

• ​the equation must have the correct chemical formulas for the reactants and products

• ​the Law of conservation of mass must be satisfied

Multiple Choice

What does the symbol "(s)" mean in the chemical equation?

CaO(s) + H₂O(l) --> Ca(OH)₂(s)

• CaO is a solid

• CaO is a liquid

• CaO is a gas

• gas is produced

Multiple Choice

What does the symbol "+" mean in a chemical equation?

• solid

• heat is added

• aqueous solution

• plus, and

Multiple Choice

What does the Law of Conservation of Matter tell us?

• atoms can be created and destroyed

• you can have less atoms on the product side than on the reactant side

• there will be more atoms on the left side of the arrow than on the right side

• atoms cannot be created or destroyed, only rearranged

​How do we satisfy the Law of Conservation of Mass?

Answer: by using coefficients to balance the equation

​Using Coefficients to Balance Equations

• ​You can add coefficients in front of the compounds to balance the equation, but you CANNOT change the subscripts

  • ​Why can't we change the subscript?

    • ​because changing the subscript will change the compound

Explanation

Look at the subscripts in the formulas to the right.

1. They both have the same element symbols in their formulas, but the subscripts are different.

​

• ​Which one is safe for us to drink?

​What are the steps for balancing equations?

1. ​Write out the unbalanced equation

2. ​Count how many atoms of each element are on each side of the arrow

3. ​Add coefficients to make the number of atoms equal on the left and right.

4. ​Reduce the coefficients to the lowest possible ratio (only if necessary; if the numbers can be divided by 2 and stay whole numbers then reduce)

5. ​DOUBLE CHECK YOURSELF!!!

​A helpful tip.

​make a chart like the one below to help you balance the atoms

Example 1

Example 2

​Example 3