Chemical Reactions

Ch. 8

Describing Chemical Change

8-1

Writing Chemical Equations

• Reactant = substance that undergoes a reaction

• Product = the new substance formed when reactants undergo a chemical change

• Ex: Fe+3 + O-2 Fe2O3 (rust)

reactants products

Equations

• Word Equations: – Ex: vinegar + baking soda sodium acetate

+ water + carbon dioxide• Chemical Equations:

– Ex: HC2H3O2 (aq) + NaHCO3 (s) NaC2H3O2 (aq) + H2O (l) + CO2 (g)

– Shows the state of substance: aq – aqueous, s – solid, l – liquid, g- gas

aqueous = solid dissolved in water.

Balancing Chemical Equations (REVIEW)

• Law of Conservation of Mass = matter is neither created nor destroyed…– Thus, mass of products = mass of reactants– Atoms in a chemical reaction don’t change,

they rearrange!

• The same number of each kind of atom must be on the left side as well as the right!

• Ex: H2CO3(aq) H2O(l) + CO2(g)

H = 2, C =1, O = 3 H = 2, C =1, O =3

A Balancing Act

• To indicate more than one unit of a compound, a coefficient is placed in front

– Ex: 2H2O

H = 4, O = 2• NEVER change the subscripts of a compound, that

would change its identity!!

– H2O is water, H2O2 is hydrogen peroxide

• Rule: start by balancing the elements that appear only once of each side of the equation.– Ex: Balance oxygen last!

H2CO3(aq) H2O(l) + CO2(g)

Writing Equations!

• Step 1: Write word equation– Hydrogen + oxygen water + energy

• Step 2: Replace words with formulas– H2(g) + O2(g) H2O(l) + energy

• Step 3: Balance the atoms on both sides!– 2H2(g) + O2(g) 2H2O(l) + energy

• Step 4: Check it!– H = 4, O = 2 H = 4, O =2

Example

• Write the balanced equation:

– Magnesium chloride + silver nitratemagnesium nitrate + silver chloride

– MgCl2 + AgNO3 Mg(NO3)2 + AgCl

– Now balance it!

– MgCl2 + 2AgNO3 Mg(NO3)2 + 2AgCl

You Try!1. Magnesium metal + water form solid magnesium

hydroxide and hydrogen gasMg(s) + H2O(l) Mg(OH)2(s) + H2(g)Mg(s) + 2H2O(l) Mg(OH)2(s) + H2(g)

• Potassium chlorate breaks down into potassium chloride and oxygen

KClO3 KCl + O2

2KClO3 2KCl + 3O2

3. Solid potassium reacts with liquid water to produce aqueous potassium hydroxide and hydrogen gas.K(s) + H2O(l) KOH(aq) + H2(g)2K(s) + 2H2O(l) 2KOH(aq) + H2(g)

Types of Reactions

8-2

Categories of Reactions

• There are 5 types of reactions:

1. Synthesis/Combination = 2 or more substances combine to form a single product• A + B AB

• Ex: 2Na + Cl2 2NaCl

2. Decomposition = a compound breaks down into 2 or more simpler substances

1. AB A + B

2. Ex: H2CO3 H2O + CO2

• Sodium + Chlorine --- Sodium chloride

3. Single-Replacement = an element that is not combined replaces an element that is part of a compound

-All compounds are aqueous!

A + BC AC + B

Ex: Mg(s) + Zn(NO3)2(aq) Mg(NO3)2(aq) + Zn(s)

• Double-Replacement = different atoms in 2 different compounds replace each other

-All reactants are aqueous

-Products will include a gas, a ppt (precipitate), or H2O

AC + BD AD + BC

Ex: AgNO3 + KCl AgCl + KNO3

5. Combustion = substance rapidly combines with oxygen producing heat and light.

-Reactants contain C,H,O

-Products are CO2 and H2O vapor

• CxHy + O2(g) CO2(g) + H2O(g)

• Ex: 5CH3OH + O2(g) 5CO2(g) + 8H2O(g)

You Try!

• H2SO4 H2 + SO4

– Decomposition

• Fe + O2 FeO

– 2Fe + O2 2FeO

– Synthesis

• Al + Cl2 AlCl3– 2Al + 3Cl2 2AlCl3– Synthesis

• BaCl2 + Na2SO4

BaSO4 + NaCl

– BaCl2 + Na2SO4

BaSO4 + 2NaCl

– Double Replacement

• Mg + CuSO4

Cu + MgSO4

– Single Replacement

First balance, then put into 1 of 5 categories: