Basics of Reactions
Reactants: The substances initially present in the chemical reaction that undergo a change to form new substances. They are consumed to form the products.
Products: The substances created as a result of the chemical reaction. The products are formed by the reaction.
Reactants → Products
As the reaction continues, there becomes less reactants, and more products.
Number Subscripts: Indicate the amount of that element in a molecule. For example, in H₂SO₄, there are two hydrogen, one sulfur, and four oxygen.
Coefficients: Indicate the amount of molecules. 2H₂SO₄ means that you have two entire molecules of H₂SO₄.
Phase Subscripts: There are 4 main subscripts you will see to indicate the phase of the reactants and products ↴
(s) means solid
(l) means liquid
(g) means gas
(aq) means aqueous, which indicates that something is dissolved in water
Chemical reactions must be balanced. You cannot create or destroy atoms, so there must be the same number of each type of atom on the reactants and products side.
You cannot change the subscripts (that would change the chemical formula), but you can change the coefficients (changing the number of molecules) to balance a reaction.
Synthesis
Two or more simple substances combine to form a more complex substance ↴
A + B → AB
Ex. Na + Cl → NaCl
Decomposition
A complex substance breaks down to form two or more simpler substances ↴
AB → A + B
Ex. CaCO₃ → CaO + CO₂
Single Replacement
A single element in one compound is substituted for another element ↴
AB + C → AC + B
Ex. Zn + CuCl₂ → ZnCl₂ + Cu
Double Replacement
Elements in two different compounds are swapped. It is essentially two single replacement reactions. Cations must swap with cations. Anions must swap with anions ↴
AB + CD → AD + CB
Ex. Na₂S + 2HCl → 2NaCl + H₂S
Combustion
A substance reacts with oxygen to form an oxide and release heat. Often, the substance being combusted is a hydro-carbon (made of hydrogen and carbon), which causes carbon dioxide and water to form. However, this is not always the case.
Ex. CH₄ + 2O₂ → CO₂ + 2H₂O
Endothermic vs. Exothermic
Endothermic: Reactions that absorb energy. The system gains heat, and the surroundings lose heat. The surrounding area may feel cooler because that’s where the system gains energy from.
Exothermic: Reactions that release energy. The system loses energy, and the surroundings gain energy. The surrounding area may feel warmer, because the energy is being sent out into the environment.
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