SCH 3U- Chemical Equations and Reactions
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Transcript of SCH 3U- Chemical Equations and Reactions
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.