3.Chemical Reactions

Reactants

Substances present at the start of a chemical reaction that participate in the reaction.

Coefficient

The number placed before a molecule that indicates how many molecules are involved (e.g., 2H₂ means two molecules of hydrogen).

Subscript

The small number written after an element that shows how many atoms of that element are in the molecule (e.g., H₂ means two atoms of hydrogen).

Irreversible Reaction

A chemical reaction that cannot easily go back to the original substances.

How a reversible reaction looks like?

Note: The arrows in the middle pointed both ways.

If we add more reactants, it will push the reactants towards the product side and vice versa.

5 Types of Chemical Reactions - Overview

Reaction Type	Description	General Formula	Example (not in image)
Combination / Synthesis	Two or more substances combine to form one product.	A + B → AB	2Na + Cl₂ → 2NaCl
Decomposition	A single compound breaks down into two or more simpler substances.	AB → A + B	2H₂O → 2H₂ + O₂
Single Displacement	One element in a compound is replaced by another element.	A + BC → AC + B	Zn + HCl → ZnCl₂ + H₂
Double Displacement	Elements in two different compounds swap places with each other to form two new compounds. Note: The inner elements combine and the outer elements combin to form new compounds.	AB + CD → AD + CB	AgNO₃ + NaCl → AgCl + NaNO₃
Combustion: Hallmark sign of combustion: The second reactant is oxygen, and the products are CO2 and H20	A substance (usually a hydrocarbon)reacts with oxygen to produce CO₂, H₂O, and energy (heat/light).	CₓHᵧ + O₂ → CO₂ + H₂O + energy	CH₄ + 2O₂ → CO₂ + 2H₂O + 🔥

5 Types of Chemical Reactions - Quiz

Flashcard Question (Front)	Flashcard Answer (Back)
What happens in a combination/synthesis reaction?	Two or more substances combine to form a single product (A + B → AB).
What happens in a decomposition reaction?	A compound breaks into simpler substances (AB → A + B).
What happens in a single displacement reaction?	One element replaces another in a compound (A + BC → AC + B).
What happens in a double displacement reaction?	Two compounds exchange elements (AB + CD → AD + CB).
What is a combustion reaction?	A hydrocarbon reacts with O₂ to produce CO₂, H₂O, and heat/light.
What are the products of a combustion reaction?	Carbon dioxide (CO₂), water (H₂O), and heat or light energy.
In which reaction does energy in the form of heat and light appear?	Combustion.
Give an example of a synthesis reaction.	2Na + Cl₂ → 2NaCl.
Give an example of a double displacement reaction.	AgNO₃ + NaCl → AgCl + NaNO₃.

📝 Additional TEAS Exam Notes (Not in Images)

Concept	Explanation
Recognizing Reaction Types by Formula	On the TEAS, you may be asked to identify reaction types just from the formula format (e.g., A + B → AB = Synthesis).
Balancing Reaction Equations	TEAS will test your ability to balance chemical equations so that atoms on both sides are equal.
Physical vs Chemical Changes	Chemical changes involve new substances forming. Reaction types like decomposition, combustion, and synthesis are all chemical changes.
Energy Transfer in Reactions	Endothermic (absorbs energy) vs Exothermic (releases energy—like combustion). TEAS may ask which reaction types release or absorb heat.
State Symbols	Know these: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water). E.g., NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq).

What is Diatomic Molecules?

🔹 Most commonly, on the TEAS, this refers to diatomic elements:

Elements that naturally exist as pairs of the same atom when in their pure form.

🧪 The 7 Diatomic Elements (memorize for TEAS):

Element	Formula	Mnemonic
Hydrogen	H₂
Nitrogen	N₂
Oxygen	O₂
Fluorine	F₂
Chlorine	Cl₂
Bromine	Br₂
Iodine	I₂

Balancing Chemical Reactions

Do practice!!

The goal: Reactants are the same exact number as your products.

Tips:

1. Save Oxygen and Hydrogen balancing for last.

   1. -Save Oxygen to the last last!!!!!

2. If Polyatomic atoms are on the same side. Do not separate them.

Balancing Chemical Reactions Practice

The goal: Reactants are the same exact number as your products.

Tips:

1. Save Oxygen and Hydrogen balancing for last.

   1. -Save Oxygen to the last last!!!!!

2. If Polyatomic atoms are on the same side. Do not separate them.

What is Polyatomic atoms?

🧪 What Are Polyatomic Ions?

Definition:
A polyatomic ion is a charged group of two or more atoms that are covalently bonded and act as a single ion in chemical reactions.

• "Polyatomic" = many atoms

• "Ion" = has a charge (positive or negative)

🔋 Charge:

• The group has an overall charge even though the atoms are bonded together.

• Example: NO₃⁻ (nitrate) is made of nitrogen and oxygen but behaves like a single unit with a -1 charge.

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🧠 Common Polyatomic Ions to Know for TEAS

Name	Formula	Charge
Ammonium	NH₄⁺	+1
Hydroxide	OH⁻	-1
Nitrate	NO₃⁻	-1
Sulfate	SO₄²⁻	-2
Carbonate	CO₃²⁻	-2
Phosphate	PO₄³⁻	-3
Bicarbonate	HCO₃⁻	-1
Acetate	C₂H₃O₂⁻ or CH₃COO⁻	-1

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🔍 Key TEAS Points

• They appear in ionic compounds (e.g., NaNO₃ is made of Na⁺ and NO₃⁻).

• You must keep them together as a unit when writing formulas or balancing equations.

• Use parentheses when you need more than one in a compound (e.g., Ca(NO₃)₂).

Moles

Unit of measurement that is the amount of a pure substance containing the same number of chemical units.

Practice Example of Moles calculations:

Collision Theory Table – Factors That Affect Chemical

Factor	Description	TEAS Key Term
Collision Theory	Particles must collide with sufficient energy to cause a chemical reaction.	Collision Theory
Activation Energy	Minimum amount of energy required for a reaction to occur.	Activation Energy
1. Energy of Particles	The more kinetic energy particles have, the more likely they will overcome activation energy.	Temperature
2. Frequency of Collisions	The more often particles collide, the higher the chance for successful collisions (reactions).	Concentration / Surface Area

Factors that affect Chemical Reactions - Temperature

Factors that affect Chemical Reactions - Concentration and Pressure

Factors that affect Chemical Reactions - Surface Area

Factors that affect Chemical Reactions - Catalyst

Higher rate of reaction with less energy!

Exothermic vs Endothermic Reactions

Exothermic Reactions - Release energy to surroundings - Release HEAT !!

• Tip: Exothermic = Think Exit

• Heat is exiting during this reaction.

Examples:

1. Combustion Reaction

2. Oxidation

3. Nutralizing

Exothermic vs Endothermic Reactions

Endothermic Reactions - Absorb energy from surroundings - Absorb HEAT !!

• Tip: Endothermic = Think Enter

• Heat is absorbed during this reaction.

Examples:

• Cooking an egg

• Photosynthesis

• Liquid Evaporation

Chemical Equilibrium

When the rate of the forward reaction reaction equals the rate of the reverse reaction in a close system.

Two Type of Chemical Equilibrium

• Static

• Dynamic