Introduction to Chemical Reactions

Posted on May 16, 2024 in Chemistry

Chemical Safety

Chemical reaction: a process in which atoms of one or more substances are rearranged to form different substances.

Chemical reactions break food down into nutrients that the body can use.
Chemical compounds are in our everyday life, from the plastics we use to the synthetic fibres that we wear and the healthcare products we put on ourselves.
Chemicals used at school and in the workplace can be potentially harmful.
WHMIS (Workplace Hazardous Materials Information System) labels provide information about the chemicals we use.
Safety Data Sheets (SDS) provide important information about hazards and first aid treatments for chemicals.

Safety in the Lab

It is important to know safety rules and how to handle chemicals in the lab.

Know the locations of the eyewash station, emergency shower, fire extinguisher, and the fire blanket in the classroom.
Handle hot objects with care.
Never leave an open flame unattended.
When heating a test tube, point it away from yourself and others.
During a lab, wear safety goggles (glasses) and a lab apron.
Tie back long hair.
Do not wear loose clothing like ties and scarves.
Do not wear open-toed shoes.
Hold beakers and test tubes away from your face.
When asked to smell some chemical, waft the fumes toward you.

Chemical Compounds

Atoms combine together in specific proportions to form compounds.
When atoms form compounds, they become more stable as they achieve full valence shells.
Depending on the type of bond that forms between the atoms, compounds are classified as ionic or covalent.
Lewis diagrams consist of an element’s chemical symbol surrounded by dots that represent its valence electrons.

Ionic Compounds

Ionic compounds are made up of oppositely charged ions that are strongly attracted to each other.
Atoms of a metal lose electron(s) and become positively charged ions (cations).
Atoms of a non-metal gain those electron(s) and become negatively charged ions (anions).
The electron transfer between atoms and the electrostatic attraction that is formed between the charged ions is called an ionic bond.
Ions of an ionic compound are arranged in a cubic crystal lattice.
The smallest repeating part of the crystal lattice is called a formula unit.
It is also the smallest whole number ratio of positive and negative ions to form a neutral ionic compound.

Covalent Compounds

Covalent compounds are formed when atoms of two or more non-metals share electrons.
The sharing of electrons causes the positive nucleus of each atom to be strongly attracted to the shared electrons.
This strong electrostatic attraction between atoms when atoms share valence electrons is called a covalent bond.
A molecule is the smallest independent unit of a covalent compound.
It is a particle made up of two or more atoms bonded together by covalent bonds.

Chemical Bonds and Energy

Chemical reactions take place when atoms are rearranged to form new substances.
This process involves the breaking of chemical bonds (electrostatic attractions) between atoms or ions and the forming of new chemical bonds.
To break bonds, energy is required; to form new and different bonds, energy is released.
The amount of energy required or released depends on the atoms or ions involved.

Conservation of Mass

Chemical reactions can occur in closed or open systems.
Within an open system, both matter and energy can enter or leave freely.
In a closed system, energy can enter, but matter cannot.
During chemical reactions, atoms are neither created nor destroyed.
They obey the law of conservation of mass: the total mass of reactants (substances used) is equal to the total mass of products (substances formed).

Chemical Equations

Chemical reactions can be represented with words as well as with chemical formulas.
Reactants are written on the left side of the equation; they are the starting material of chemical reaction.
Products are written on the right side of the equation; they are the new substances that form from a chemical reaction.
A word equation uses words to describe what happens to the reactants and products in a chemical reaction: methane + oxygen → water + carbon dioxide.
A skeleton equation provides the chemical formulas for the reactants and products, but it does not reflect the law of conservation of mass.
A balanced chemical equation is a complete description of a chemical reaction that provides the chemical formulas for the reactants and products and the coefficients.
A coefficient is a number placed in front of a chemical formula that shows the ratios of substances in a reaction.
It reflects the law of conservation of mass.

Energy Changes

Energy is needed for everyday processes.
Activation energy: minimum amount of energy needed for a reaction to happen.
Reactants must collide with enough energy to break bonds for a chemical reaction to occur.
Energy changes occur with all chemical reactions.
Photosynthesis is a process that captures sunlight to produce sugar molecules for plants.
Animals that eat the plants can use the energy stored in the sugar through cellular respiration.
Energy transfers between the system and the surroundings.
The system is all of the materials involved in the chemical reaction.
Everything else in the universe outside the system is called the surroundings.
Law of conservation of energy: the total energy of the universe is constant—energy cannot be created or destroyed.
Energy that leaves the system enters the surroundings.
Energy that enters the system came from the surroundings.

Exothermic and Endothermic Reactions

Exothermic reaction: a chemical reaction in which there is net release of energy to the surroundings reactants → products + energy.
Endothermic reaction: a chemical reaction in which there is net absorption of energy from the surroundings reactants + energy → products.
In an exothermic reaction, temperature increases as energy is released into the surroundings.
In an endothermic reaction, temperature decreases as energy is absorbed by the system.

Energy-Level Diagrams

Energy-level diagrams show energy changes in a reaction.
In an endothermic reaction, the energy of the products is greater than that of the reactants.
In an exothermic reaction, the energy of the products is less than that of the reactants.

Acids and Bases

Acid: compound that forms hydrogen (H+) ions when dissolved in water.
Two categories for acids:
- Binary acids: consist of hydrogen and non-metal (e.g., HCl). Change the suffix to -ic acid (e.g., HBr: hydrobromic acid).
- Oxyacids: consist of hydrogen, oxygen, and another element (e.g., H₂CO₃). Change the suffix -ate to -ic acid (e.g., H₂SO₄: sulfuric acid). Change the suffix -ite to -ous acid (e.g., HNO₂: nitrous acid).
The chemical formula for an acid has an “H” in it.
Acids taste sour, burn skin, corrode metals, and conduct electricity when dissolved in water.
Base: a compound that forms hydroxide (OH–) ions when dissolved in water.
The chemical formula for a base has an “OH” in it.
Bases taste bitter, burn skin, feel slippery, do not react with metals, and conduct electricity when dissolved in water.
A salt is an ionic compound that does not contain H+ or OH– ions (that is, neither an acid nor a base).

pH

Acid-base indicators can be used to identify acids and bases by changing colour (e.g., litmus paper).
pH is a measure of how acidic or basic (alkaline) a solution is.
The pH scale is used to measure pH.
- It ranges from 0 to 14.
- Acidic solutions have a pH less than 7.
- Basic solutions have a pH greater than 7.
- Neutral solutions have a pH equal to 7.
pH stands for “powers of 10 of concentration of H+ ions,” meaning each unit of change in pH represents a tenfold change in the degree of acidity.
- For instance, a decrease in two units in pH corresponds to a 10² (or 100) times increase in acidity.

Types of Chemical Reactions

Synthesis Reaction

Synthesis reaction: a chemical reaction in which two or more reactants combine to produce a single product.
- e.g., 2 K(s) + Cl₂(g) → 2 KCl(s)
Most synthesis reactions are exothermic.

Decomposition Reaction

Decomposition reaction: a chemical reaction in which a compound is broken down into elements or simpler compounds.
- e.g., 2 HgO(s) → 2 Hg(l) + O₂(g)
Most decomposition reactions are endothermic.

Single Replacement Reaction

Single replacement reaction: a chemical reaction in which an element and a compound react to produce another element and another compound.
- e.g., Cu(s) + 2 AgNO₃(aq) → Cu(NO₃)₂(aq) + 2 Ag(s)

Double Replacement Reaction

Double replacement reaction: a chemical reaction in which solutions of two ionic compounds react to produce two new compounds.
- e.g., BaCl₂(aq) + Na₂SO₄(aq) → 2 NaCl(aq) + BaSO₄(s)
A precipitate is usually produced in a double replacement reaction.

Combustion Reaction

Combustion reaction: a chemical reaction in which a compound or element reacts with oxygen to produce an oxide (compound that contains oxygen) of the element.
Many combustion reactions produce heat, light, carbon dioxide, and water.
A hydrocarbon is a compound that consists of carbon and hydrogen.
The general equation for the combustion of a hydrocarbon is: C_xH_y + O₂ → CO₂ + H₂O.
Incomplete combustion occurs when low amounts of oxygen are available.
It also produces carbon monoxide and soot (carbon).

Neutralization Reaction

Neutralization reaction: a chemical reaction in which an acid reacts with a base to form salt and water.
- e.g., HCl(aq) + NaOH(aq) → NaCl(s) + H₂O(l)
It is a type of double replacement reaction.
All neutralization reactions are exothermic.
The general equation for a neutralization reaction is: HA + BOH → BA + H₂O.