6 min read•june 18, 2024
Dylan Black
Anika P
Dalia Savy
Dylan Black
Anika P
Dalia Savy
From electronic devices📱 to fizzy soda🥤, our world runs on chemical reactions. So what happens in chemical reactions? In a simplified definition, specific interactions with molecules result in the rearrangement of atoms to create new molecules. However, this magic has restrictions, as will be discussed in the next sections.
There are two types of changes in chemistry when discussing matter: physical changes and chemical changes.
A is one that changes an object but does not change its chemical structure. For example, you may boil water into steam, but H2O stays H2O💧 during that process. Similarly, shredding paper would be an example of a physical change. Paper may change from a piece to shreds, but it stays "paper"📄.
Changes in the state of matter of a substance or the formation/separation of substances are common physical changes. Nothing changes on a molecular level, rather, the properties of the substance change.
However, chemical changes, as the name implies, end with a brand-new product. There must be some sort of change on a molecular level with chemical changes! For example, if you leave iron out for too long, it rusts. is a between iron (Fe) and oxygen in the air (O2) to form rust (, Fe2O3).
Another example of a is mixing (NaHCO3) with (CH3COOH) to form and other . Chemical changes are the focus of AP Chemistry and are described using chemical reactions. The following are observations that are evidence of a chemical change, and therefore, a chemical reaction taking place:
A chemical reaction can be represented using a . A chemical equation is a written representation of the substances involved in a chemical reaction and the changes they undergo. Here is how to read a chemical equation:
There are five main types of reactions: synthesis, decomposition, combustion, single replacement, and double replacement. While we will go over these in later sections, let’s do a brief overview of each type:
Synthesis reactions are the easiest type to understand. Essentially, a synthesis reaction sees two reactants fuse to form products in the form A + B → AB, where A, B, and AB are arbitrary.
An example of a synthesis reaction is 2Na + Cl2 → 2NaCl, as it sees two reactants, Na and Cl2, become one product: NaCl. The synthesis of water from hydrogen and oxygen is another example of a synthesis reaction: 2H2 + O2 → 2H2O.
Decomposition reactions are the exact opposite: one reactant decomposes into two or more products. This can be represented by the general equation: AB → A + B.
For example, decomposes into hydrogen gas and water in the reaction 2H2O2 → 2H2O + H2. You can also reverse the synthesis of water in the last section to represent the decomposition of water: 2H2O → 2H2 + O2.
A combustion reaction is a special type of decomposition reaction involving , which are molecules that are carbon-based. A special class of organic molecules is called which are only made up of hydrogen and carbon molecules.
When in the presence of heat and oxygen, hydrocarbons combust or burn which is really just the energy being released from a chemical reaction! Therefore, a general combustion reaction sees a hydrocarbon being combusted into carbon dioxide and water in the presence of oxygen.
Combustion reactions always follow this format: Hydrocarbon + Oxygen → Carbon Dioxide + Water. For example, CH4 + 2O2 → CO2 + 2H2O is the combustion of (CH4). This definition of combustion is why oxygen is necessary for fires to start.
Single replacement reactions occur when you have a compound reacting with an element. The general form is AB + C → AC + B. The most common form of a single replacement reaction is one called a redox reaction, where electrons are transferred between atoms. An example is 3Mg + 2AlCl3 → 3MgCl2 + 2Al.
Single = one, replacement = a switch. These reactions involve the replacement of one element in a compound by another element. Only one switch takes place.
The most common type of reaction is a double replacement reaction, with the general form AB + CD → AD + CB.
For example, if you mix an acid and a base, you get a reaction that forms water and some type of salt. A formula example would be HCl + NaOH → NaCl + H2O. These reactions are the majority of the reactions you'll see in AP Chemistry, so get used to them!
Double = two, replacement = switch. These reactions involve the exchange of ions between two compounds to form two new compounds. Here, two switches take place.
Identify what type of reaction each of the following are: