Chemical Reactions

• Reactions involve chemical changes in matter resulting in new substances being formed.

• Reactions involve rearrangement and exchange of atoms to produce new substances.

Reactants Products

Evidence of Chemical Reactions

• Look for evidence of a new substance• Visual clues (permanent) that indicate a reaction has

occurred:

– color change

– precipitate formation • solid that forms when liquid solutions are mixed

– gas bubbles

– large energy changes• emission of light• Production of flame

• Other indications:– new odor – whooshing sound from a tube – permanent new state

– temperature change

Describing chemical reactions by formulas

• We want to put enough information into the formula so that we can understand exactly what happens.

• Sodium reacts with water to give off hydrogen and produces sodium hydroxide solution.

• Na(s) + H2O(l) H2(g) + NaOH(aq)

• We show that solid sodium reacts with liquid water to give off gaseous H2 and aqueous sodium hydroxide.

Chemical Equation

• Shorthand way to describe a reaction

• Provides information about the reaction

– Formulas of the reactants and products

– Gives the “states” of the reactants and products

– Relative numbers of reactant and product molecules that are required

– Can be used to determine amounts of reactants used and amounts of products produced in the reaction

• Matter cannot be created or destroyed

– Therefore the total mass cannot change

– and the total mass of the reactants will be the same as the total mass of the products

• In a chemical reaction, all the atoms present at the beginning must still be present at the end of the reaction

– if all the atoms are still there, then the mass cannot change

Atoms are conserved in a chemical reaction!

Law of Conservation of Mass

Na(s) + H2O(l) H2(g) + NaOH(aq)

The number of each type of element in the reactants is the same as the number of each type of element in the products. To achieve this we need to balance the chemical equation.

In balancing a chemical equation we can change the number of reactants or products, but never their chemical composition (identities or formulas)

Balancing a Chemical Equation

Allowed: Balance the equation by adding coefficients to the left of the chemical formulas:

Na(s) + H2O(l) H2(g) + 2 NaOH(aq)

Not allowed: Change a chemical formula to make an element balance (balancing H)

Na(s) + H3O(l) H2(g) + NaOH(aq)

Not allowed: Add more chemical species for convenience (balancing all elements)

Na(s) + H2O(l) + H H2(g) + NaOH(aq)

Balanced Chemical Equation

2 Na(s) + 2 H2O(l) H2(g) + 2 NaOH(aq)

Element

Na

H

O

# Atoms in reactants

2

4

2

# Atoms in products

2

4

2

Balancing Chemical Equations: “Formal Approach”

To balance a chemical equation, do not change any molecules or atoms, but change the number of molecules

KClO4 KCl + O2

1. Place a ‘1’ in front of the molecule with the greatest number of atoms. If two molecules have the same number of atoms, then choose the molecule with the greatest number of different elements (this number may change)

1 KClO4 KCl + O2

Balancing Equations

2. Insert coefficients that balance elements that are in compounds with more than one different element

1 KClO4 1 KCl + O2

3. Place coefficients in front of all else to balance remaining elements

1 KClO4 1 KCl + 2 O2

4. Clear fractions, if any, and multiply/divide by a common denominator. If, for instance, we get the following balanced equation...

4 KClO4 4 KCl + 8 O2

...then divide by 4 to get...

1 KClO4 1 KCl + 2 O2

5. Check to be sure the final equation is balanced

Reactants Products

1 K, 1 Cl, 4 O 1 K, 1 Cl, 4 O

Balanced Chemical Equation:

KClO4 KCl + 2 O2

Simple, Systematic Approach

Balance the following equation:

MgCO3 + HCl MgCl2 + CO2 + H2O

1. In how many compounds does each element appear? Choose the element with the lowest number.

Element Number of compounds in equation Mg 2 C 2 O 3 H 2 Cl 2

1 MgCO3 + HCl 1 MgCl2 + CO2 + H2O

1 MgCO3 + HCl 1 MgCl2 + 1 CO2 + H2O

1 MgCO3 + 2 HCl 1 MgCl2 + 1 CO2 + H2O

Note that you can ‘coincidentally’ balance more than one element at a time (such as Cl)

2. After choosing the element that appears in the fewest number of compounds, balance the number of atoms of that element (i.e., balance the number of Mg, C, H, Cl atoms)

Mg

C

H

MgCO3 + 2 HCl MgCl2 + CO2 + H2O

3. Always check to confirm that the equation is balanced

Reactants Products

1 Mg, 1 C, 3 O, 2 H, 2 Cl 1 Mg, 2 Cl, 1 C, 3 O, 2 H

Balanced Chemical Equation:

MgCO3 + 2 HCl MgCl2 + CO2 + H2O

Balance by Inspection

Cu + S Cu2S

1. Determine the required number of compounds by visually inspecting the chemical equation. This is often the fastest method for simple equations.

2 Cu + S Cu2S

2. Always check to confirm that the equation is balanced

Reactants Products

2 Cu, 1 S 2 Cu, 1 S

Now Your Turn

Balance the following chemical equations:

CO + O2 CO2

P4 + Cl2 PCl3

P + O2 P2O3

C3H8 + O2 CO2 + H2O

Notice that on at least one reaction, you may be able to balance the reaction just by inspecting it