SCH 3U- Chemical Equations and Reactions

What is a Chem. Rxn.?

Chemical Reaction: Process of one or more substances

converting to form new substances with different properties.

Some examples: Iron rusting Baking soda and vinegar mixed A campfire Plants converting CO2 and H2O to sugars

Indications of Chemical Rxns.

What are the five signs of a chemical reaction? Change in heat or light (energy) Production of a gas Production of a precipitate

Precipitate – an ionic solid formed as the product of a chemical rxn.

Change in odour Change in colour

Traits of Chem. Rxns.

Reactants: Substances that enter (starting materials) a chemical

reaction. Ex.: Vinegar (acetic acid) and baking soda (sodium

bicarbonate)

Products: Substances that are produced by a chemical reaction. Ex.: Carbon dioxide, sodium acetate, and water

Do all combinations result in a chemical reaction? Yes or No? Example?

Chemical Equations (Eqns.)

Chemical Equation: Expression using chemical symbols to represent a

chemical reaction A chemical equation represents the identities and

relative amounts of reactants and products in a chemical reaction

Word Equations: Ex. Acetic acid + Sodium Bicarbonate Carbon Dioxide

+ Sodium Acetate + Water

Formula Equations:)(

2)(

2)(

232)(

3)(

232lOHgCOaqOHNaCsNaHCOaqOHHC

Symbols for use in Chemical Equations

Symbol Meaning

(s) Solid

(l) Liquid

(g) Gas

(aq) In aqueous solution, dissolved in water

↑ Gas forms

↓ Solid precipitate forms

Symbol Meaning

→“produces” or “yields”, indicates result of reaction

↔Reaction in which products can reform into reactants, can go both ways

Heat is applied, may have word “heat” instead of triangle

Other conditions (temperature, pressure, etc.) may be indicated here, or below the arrow

other

Writing Equations

Example: Sodium metal is added to water. Hydrogen gas bubbles

off, and sodium hydroxide is left behind, in solution. Step 1:

Put text into word equation: Sodium + water hydrogen + sodium hydroxide

Step 2: Translate words in word equation to formulas Na + H2O H2 + NaOH

Step 3: Add symbols to tell states of reactants and products. Na(s) + H2O(l) H2(g) + NaOH(aq)

Do you remember the diatomic elements?

H I F N O Br Cl

BrO FINClH

Hockey Stick & Puck

Diatomic Molecules

H2, N2, O2, F2, Cl2, Br2, I2

Also-

P4, S8

Law of Conservation of Mass

Law of Conservation of Mass: Mass cannot be lost or gained in a chemical

reaction ALSO, the total mass of the reactants must

equal total mass of the products Therefore, the NUMBER of each type of atom

on each side of the equation MUST be the same!

Types of Chemical Rxns.

Synthesis (Direct Combination) Reaction:

Occurs when two or more simpler substances combine to form a more complex substance.

General Form: A + B AB

Eqn.: Fe(s) + O2(g) FeO2(s)

Synthesis Reactions

Signs of a synthesis reaction: Start with elements and end with a single compound OR start with simple compounds and end with a single

compound. What is always true of a synthesis reaction?

At the end, there is a SINGLE product, a compound.

Examples: Iron metal and oxygen gas combine to form rust

Sodium metal and chlorine gas combine to form salt

Decomposition Reaction

Occurs when a substance breaks down into two or more simpler substances.

Opposite of a synthesis reaction.

General Form: AB A + B

Eqn.: 2H2O(l) 2H2(g) +

O2(g)

Decomposition Reaction

Signs of a decomposition reaction A single, complex substance breaks into the elements that make it up OR A single, complex substance breaks into simpler substances

What is always true of a decomposition reaction? The reactant is ALWAYS a single, complex compound.

Example Liquid hydrogen peroxide, in the presence of light, breaks down to

form water and oxygen gas.

Atoms of one element replace atoms of another element in a compound.

General Form: A + BC AC + B

Eqn.: Cu(s) + AgNO2(aq) CuNO2(aq) + Ag(s)

Single Replacement (Displacement) Rxn.

Single Replacement Rxn.

Signs of a Single Replacement Rxn. One element reacts with a compound, and this produces another

free element and a new compound.

What is always true of a single replacement reaction? The reactants include an element and the products include a

different element.

Example Zinc is put into acid, and “eaten away”, producing hydrogen

bubbles.

Double Replacement (Displacement) Rxn.

Atoms in two different compounds trade places with each other.

General Form: AB + CD AD + CB

Eqn.: AgNO3(aq) + KCl(aq) AgCl(↓) + KNO3(aq)

Double Replacement Rxn. What are the signs of a Double Replacement Reaction?

Two compounds react and product two NEW compounds. There is USUALLY a gas or precipitate formed

What is always true of a DR Reaction? There are two compounds at the beginning, and two compounds at the

end of the reaction.

Example When vinegar and baking soda are combined, carbon dioxide gas

bubbles off, vigorously!

Combustion Reaction

Any hydrocarbon (compound made of only C, H, and occasionally O) reacts with oxygen to produce carbon dioxide and water.

A large amount of energy is released as light and heat.

General Form: CxHx + O2 CO2 + H2O

Eqn. CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Combustion Reaction

What are the signs of a combustion reaction? There is always light and heat, as well as the production of

gases (smoke). Something is BURNING!

What is always true of combustion reactions? They always take place in oxygen (in air), with a source of

heat (like a match), and start with a hydrocarbon (like oil, paper, or anything organic)

Example When paper burns, it produces light, heat, and gases. Anything else flammable: ethanol, gasoline, marshmallows

Complete Combustion

Incomplete Combustion

Exothermic and Endothermic

Exothermic reaction: A reaction that releases energy and gives off heat An exothermic reaction may also release energy in the form of light

or sound.

Endothermic reaction: A reaction that absorbs energy resulting in lowering of temperature. Endothermic reactions also include any reactions that need light to

proceed, such as photosynthesis.