Unit 7: Chemical Reactions
Key Terms
Describing Reactions
Types of Reactions
Energy Changes in
Reactions
Reaction Rates
Equilibrium
reactants
products
chemical equation
coefficients
mole
molar mass
synthesis reaction
decomposition
reaction
singlereplacement
reaction
doublereplacement
reaction
combustion
reaction
oxidationreduction reaction
chemical energy
exothermic
reaction
endothermic
reaction
reaction rate
equilibrium
catalyst
reversible reaction
Essential Questions
I.
Describing Reactions
Chemical reaction – when a substance undergoes a chemical change
reactant: the substance that undergoes the change
product: the new substance
reactant  product
Conservation of Mass (matter): mass is neither created nor destroyed in a chemical
reaction.
charcoal burns: Carbon + Oxygen  cabon dioxide
- if you weighed the amount of carbon and oxygen before and after, they
would match.
DEMO: light a candle:
- wick: chemical change string (organic: containing C)  CO2
- wax: physical change Solid  Liquid  Solid
- cover it with glass to starve it of oxygen to put it out.
Q: What’s going on?
II.

Types of Reaction
What are the main types of chemical reaction?
1. synthesis reaction: two or more substances react to form a single substance.
A + B  AB
EX:
2Na + Cl2  2NaCl
2H2 + O2  2H2O
2. decomposition reaction: the opposite of synthesis
AB  A + B
EX:
Electricity passes through water it decomposes into oxygen and hydrogen gases.
2H2O  2H2 + O2
Calcium carbonate decompose into lime and carbon dioxide
CaCO3  CaO + CO2
3. single-replacement reaction: This is when one element trades places with another
element in a compound. These reactions come in the general form of:
A + BC  AC + B
EX:
One example of a single displacement reaction is when magnesium replaces
hydrogen in water to make magnesium hydroxide and hydrogen gas:
Mg + 2H2O  Mg(OH)2 + H2
4. double-replacement reaction: This is when the anions and cations of two different molecules
switch places, forming two entirely different compounds. These reactions are in the general
form:
AB + CD  AD + CB
EX:
One example of a double displacement reaction is the reaction of lead (II)
nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:
Pb(NO3)2 + 2KI PbI2 + 2KNO3
5. combustion reaction: A combustion reaction is when oxygen combines with another
compound to form water and carbon dioxide. These reactions are exothermic, meaning they
produce heat.
EX:
An example of this kind of reaction is the burning of napthalene:
C10H8 + 12O2  10CO2 + 4H2O
6. oxidation-reduction reaction (redox): a reaction in which electrons are transferred from one
reactant to another.
oxidation : any process in which an element loses electrons during a chemical reaction
EX: iron + oxygen  iron(III) oxide (rust)
EX: calcium + oxygen  calcium oxide
2Ca + O2  2CaO (lime)
- each neutral calcium atom loses two electrons and becomes a calcium
ion with a charge of 2+.
Ca  Ca2+ + 2ereduction: the process in which an element gains electrons during a chemical reaction
EX: as calcium atoms lose electrons during the synthesis of calcium
oxide, the oxygen atoms gain electrons.
O + 2e- O2* oxidation and reduction always happen together.
7. acid-base: This is a special kind of double displacement reaction that takes place when an
acid and base react with each other. The H+ ion in the acid reacts with the OH- ion in the
base, causing the formation of water. Generally, the product of this reaction is some ionic
salt and water:
H(A) + (B)OH ⇌ H2O + BA
EX:
One example of an acid-base reaction is the reaction of hydrobromic acid (HBr)
with sodium hydroxide:
HBr + NaOH ⇌ NaBr + H2O

How can you identify in the laboratory that a chemical change has occurred?
Observations: - production of light / energy
- color change
- odor produced
- precipitate forms
- gas produced
III. Energy Changes in Reactions

Why and how is a chemical equation balanced?
Since the “Law of Conservation of Mass” says nothing is lost or gained, you must have the same
amounts on both sides. Atoms are arranged in a chemical reaction and the same number of
each type of atom is present before and after the reaction.
Energy Changes in Reactions
Breaking bonds – requires energy
Forming bonds – releases energy
Does breaking chemical bonds require energy or release energy?

What is an endothermic process? (breaking bonds)
During a chemical reaction, energy is either released or absorbed.
endothermic reaction: a chemical reaction that absorbs energy from its surroundings

What is an exothermic process? (forming bonds)
exothermic reaction: a chemical reaction that releases energy to its surroundings
III.

Reaction Rates
What factors impact the rate of a chemical reaction?
reaction rates are influenced by:
- temperature : decrease temp = slow down (spoiling milk)
increase temp = speed up (fry egg)
- surface area : the smaller the particle size, the more surface area it has.
EX: grain dust suspended in the air (max surface area) and oxygen
Q: why are silo’s tall and not wide?
- concentration : refers to the number of particles in a given volume. More particles, the
faster the reaction rate.
EX: wash your white socks with a red shirt = pink socks
pour in red pigment = red socks
- catalysts : a substance that affects the rate without being used up in the reaction
In the chemical equation it is written above the reaction
EX: What do you do when you are running a fever? (Tylenol )
EX: enzymes function as catalysts for biological functions:
Pepsin = in stomach
trypsin = small intestine
What happens if the stomach is too basic? (drink a bunch of milk)
Enzyme activity decreases
What happens if acid from the stomach gets into the small intestine? (eat VERY spicy food)
- pH drops, enzyme activity drops
What does our body sometimes do when this happens?
- it says, “Evacuate!” Diariah!
IV. Equilibrium
Equilibrium – a state which the forward and reveres paths of a change take place at the same
rate.
Physical equilibrium – pour water into a contain (some evaporates, then condenses)
EX: (water) H2O(l) ⇌ H2O(g)
Chemical equilibrium – when a chemical reaction does not go to completion
reversible reaction: a reaction in which the conversion of reactant into product and the
conversion of the product into reactant can happen simultaneously.
EX: synthesis of sulfur trioxide (to then make sulfuric acid) from sulfur dioxide (toxic gas)
and oxygen:
2SO2(g) + O2(g) ⇌ 2SO3(g)
Factors to affect chemical equilibrium: temperature, pressure, and concentration.