Unit 2: Chemical Reactions

Unit 2: Chemical Reactions

1) Chemical equations

2) Balancing Chemical Equations

3) Types of chemical reactions (predicting products)

Reactant (s) à Product (s)

You should review how to count # of atoms in compounds!

What is a Chemical Reaction?

Chemical reaction: Occurs when elements and/or compounds interact with one another to form NEW substances.

Element: A pure substance that cannot be separated chemically into any simpler substances. Example: O or H

Compound: A pure substance that results when two or more elements combine chemically to form a different substance. Example: Hydrogen and Oxygen react to form Water.

Different types of Chemical Reactions (see handout)

1) Synthesis Reactions

2) Decomposition reactions

3) Single Displacement reactions

4) Double displacement reactions (inc. neutralization rxn’s)

5) Combustion reactions (complete and incomplete)

What is the evidence of a chemical reaction having occurred?

Remember: A chemical reaction has occurred only when one or more NEW substances have formed! Change of state (liquid water à solid (ice) water) is not a chemical change!

1) Colour change: Two clear liquids react and form a green liquid.

2) Formation of a solid: After mixing two liquids a precipitate (little chunks of solid) appears.

3) Formation of a gas: Usually seen as smoke or bubbling.

4) Formation of a liquid: Sometimes two solids can react to form a liquid.

5) Heat (temperature) change: After reacting, the container feels hotter or colder.

6) Odour/Flavour Change: Spoiling of milk.

Representing Chemical Reactions

A) Word Equation: Identifies the reactants and products of a chemical reaction by name (does not use symbols).

B) Skeletal Equations: Uses formulas and symbols. Follows nomenclature rules.

C) Balanced Chemical Equations: Like B but incorporates balancing of compounds.

Symbol / Meaning
+ (on reactant side) / “Reacts with”
à (arrow) / Makes or produces
+ (on product side) / “And”
∆ (delta) / Change in Temp, heat added to reactants
(s) / Solid or precipitate
(l) / Liquid
(g) / Gas
(aq) / Aqueous (water) solution

Steps to Writing a Word Equation: a word equation does not include the states (gas, solid, liquid)

Sample Problem: Sodium metal and chlorine gas react to form sodium chloride.

Write a word equation for this reaction:

1) identify the reactant (s) in the description provided

Sodium and Chlorine

2) identify the product (s) in the description provided

Sodium Chloride

3) Draw an arrow (à)

4) On the left of the arrow, write the reactants down (separated by “+” if there is more than one reactant)

5) On the right side of the arrow, write the products down (separated by a “+” if there is more than one product formed)

Sodium + Chlorine à Sodium Chloride

Word Equations are too long and cumbersome to write (scientists are lazy) and do not provide any information about the elements or compounds themselves. For this reason, scientists will use skeleton equations.

B) Skeleton Equation: Represents a chemical reaction by using the chemical formula of reactants and products rather than using words. Na(s) + Cl2(g) à NaCl(s) (the compound is formed using the criss-cross rule)

Other Example: CO2(g) + H2O(l) à O2(g) + C6H12O6(aq)

Writing Chemical Equations

Steps to Writing a Word Equation: a word equation does not include the states (gas, solid, liquid)

1) Translate the word equation into the skeleton equation by replacing the words by the symbol for the element/compound of products and reactants (e.g. Sodium = Na, Chlorine = Cl, Sodium Chloride = NaCl).

2) Identify the type of reation

3) Predict the product(s) using the type of reaction as a model

4) If in indicated in the description, write the subscripts to indicate the state of both the products and the reactants: (e.g. Chlorine is gas, Sodium is metal (solid), Sodium Chloride is solid).

5) Balance it.

Don’t forget about the diatomic elements! (BrINClHOF) For example, Oxygen is O2 as an element.

In a compound, it can’t be a diatomic element because it’s not an element anymore, it’s a compound!

Sample Problems:

1) Write a balanced chemical equation for each of the following reactions. Include state symbols.

a) Hydrogen gas reacts explosively with chlorine gas to form hydrogen chloride gas.

b) Solid potassium chlorate breaks down into solid potassium chloride and oxygen when heated.

c) Ammonia gas, NH3, can be made by combining hydrogen gas and nitrogen gas.

d) Aluminum metal reacts with oxygen from the air to form a hard coating of aluminum oxide.

2) Write a word equation and a balanced equation for the following reaction.

Solid Sodium metal reacts with liquid water to produce Hydrogen gas and aqueous sodium hydroxide.

a) Word Equation:

b) Balanced (skeleton) Equation:

Synthesis Reactions : (combination or formation): 2 or more compounds or elements combine to form a new substance. General Equation: A + B à AB

General Types of Synthesis Reactions

1) Metal + O2 à Metal Oxide MxOy (X and Y are subscripts determined by the criss-cross method, M = metal).
Ex: Mg(s) + O2(g) à MgO
Other examples:
Na + O2 à ______
Ca + O2 à______/ 2) Non-Metal + O2 à Non-Metal Oxide NxOy (X/Y are subscripts determined by criss-cross, N = non- metal).
Ex: H2(g) + O2(g) à H2O(l)
Other examples:
C + O2 à ______/ 3) Metal + Non-Metal (other than Oxygen) à Binary Ionic Compound (compound made of two elements)
Ex: Na(s) + Cl2(g) à NaCl(s)
Subscripts of the product are determined by criss-cross.
Other examples:
K + I2 à ______
Al + S8 à ______
4) Non-metal + Non-Metal à Binary Covalent Compound (compound made of two elements)
Ex: P4 + Cl2 à PCl3 Subscripts of the product are determined by criss-cross.
Other examples:
O2 + Cl2 à ______/ 5) Non-metal Oxide + H2O à Acid H x(polyatomic ion) (x = subscript determined by criss-cross)
Ex: SO3(g) + H2O(l) à H2SO4(aq)
Other examples:
Cl2O + H2O à ______
CO2 + H2O à______/ 6) Metal-Oxide + H2O à Base M(OH)x (x = subscript determined by criss-cross)
Ex: CaO(S) + H2O(l) à Ca(OH)2 Subscripts of the product are determined by criss-cross.
Other examples:
Al2O3 + H2O à______
K2O + H2O à______
7) Metal-Oxide + CO2(g) à Metal Carbonate, Mx(CO3)y (M= metal, X/Y= subscripts determined by criss-cross)
Ex: ZnO(s) + CO2(g) à ZnCO3(s) Subscripts of the product are determined by criss-cross.
Other examples:
CaO + CO2 à______
CuO + CO2 à______
MgO + CO2 à______

1) Decomposition reactions: “Exact” Opposite of Synthesis Reactions: Decomposition Reaction: A compound breaks down into elements or other simpler compounds. General Equation: AB à A + B

General Types of Decomposition Reactions

1) Metal Oxide à Metal + O2
Ex: MgO à Mg(s) + O2(g)
Other examples:
Li2O à ______
CaO à ______/ 2) Non-Metal Oxide à Non-Metal + O2
Ex: H2O(l) à H2(g) + O2(g)
Other examples:
CO2 à ______
SO3 à ______
Cl2Oà ______/ 3) Binary Ionic Compound (compound made of two elements) à Metal + Non-Metal (other than Oxygen)
Ex: NaCl(s) à Na(s) + Cl2(g)
Other examples:
ZnCl à ______
MgCl2à ______
4) Binary Covalent Compound (compound made of two elements) à Non-metal + Non-Metal
Ex: PCl3 àP4 + Cl2
Other examples:
HI à ______
HCl à ______/ 5) Acid (carbonate or sulphate) à Non-metal Oxide (gas) + H2O
Ex: H2SO4(aq) à SO3(g) + H2O(l) (Trick: 1st subtract water from the formula of the acid: H2SO4 – H2O = SO3 (take out 2 H and one O).
Other examples:
H2CO3 à ______/ 6) Base à Metal-Oxide + H2O
Ex: Ca(OH)2 à CaO(S) + H2O(l)
Other examples:
KOH à ______
Ca(OH)2 à ______
7) Metal Carbonate à Metal-Oxide + CO2(g)
Ex: ZnCO3(s) à ZnO(s) + CO2(g)
Other examples:
CaCO3 à ______
CuCO3 à ______