Chem Rxns: quick revision, 15 bites

What is a Chemical Reaction?

Atoms rearrange, new substances form.

A **chemical reaction** is a process where **reactants** transform into **products** with new properties. This involves breaking and forming chemical bonds, rearranging atoms. Signs include color change, gas evolution, precipitate formation, or energy change.

Takeaway. Chemical reactions create new substances by rearranging atoms.

Chemical Equation

Shorthand for a reaction.

A **chemical equation** uses **symbols and formulas** to represent a reaction. Reactants are written on the left, products on the right, separated by an arrow ($\rightarrow$). For example: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$.

Takeaway. Chemical equations are symbolic representations of reactions.

Balanced Equations: Law of Conservation

Mass is neither created nor destroyed.

A **balanced chemical equation** has equal numbers of each atom on both sides, obeying the **Law of Conservation of Mass**. Coefficients are used to balance. Example: $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$.

Takeaway. Balancing ensures atoms are conserved in a reaction.

How to Balance an Equation

Step-by-step to balance atoms.

Takeaway. Balancing uses coefficients, not subscripts, to conserve atoms.

Balancing Example: Mg + O₂

From skeleton to balanced.

Skeleton: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$. Count: left has 1 Mg, 2 O; right has 1 Mg, 1 O. Balance oxygen by placing 2 before MgO: $\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. Now right has 2 Mg, so place 2 before Mg: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$.

Takeaway. Balance atoms one by one using coefficients.

Types of Chemical Reactions

Classifying reactions helps predict products.

Reactions are classified into types: **combination**, **decomposition**, **displacement**, **double displacement**, and **oxidation-reduction** (redox). Each type has a distinct pattern.

Takeaway. Reaction types follow recognizable patterns.

Combination Reaction

Two or more combine to one.

A **combination reaction** is when two or more reactants combine to form a single product. Example: $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$. Often exothermic (releases heat).

Takeaway. Combination reactions form one product from multiple reactants.

Decomposition Reaction

One breaks into many.

A **decomposition reaction** is when a single compound breaks down into two or more simpler substances. Often requires energy (heat, light, electricity). Example: $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$ (electrolysis).

Takeaway. Decomposition splits one compound into simpler products.

Displacement Reaction

One element replaces another.

A **displacement reaction** occurs when a more reactive element displaces a less reactive element from its compound. Example: $\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$ (iron displaces copper).

Takeaway. More reactive elements displace less reactive ones.

Double Displacement Reaction

Exchange of partners.

In a **double displacement reaction**, two compounds exchange ions to form two new compounds. Often produces a precipitate. Example: $\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3$.

Takeaway. Double displacement involves ion exchange, often forming a precipitate.

Oxidation and Reduction

Redox: electron transfer.

**Oxidation** is gain of oxygen or loss of hydrogen (or loss of electrons). **Reduction** is loss of oxygen or gain of hydrogen (or gain of electrons). They occur together in **redox reactions**.

Takeaway. Oxidation and reduction always happen together.

Oxidation vs. Reduction

Quick comparison.

Takeaway. Oxidation gains oxygen, reduction loses it.

Rancidity and Corrosion

Everyday effects of oxidation.

**Corrosion** is the gradual destruction of metals (e.g., rusting of iron). **Rancidity** is the oxidation of fats/oils in food, causing unpleasant smell and taste. Prevention: keep food in airtight containers, add antioxidants.

Takeaway. Oxidation causes rusting and food spoilage.

Never Change Subscripts

Subscripts define the compound.

Takeaway. Subscripts are fixed; coefficients balance.

Identify Reaction Type Quickly

Spot the pattern.

Look at the number of reactants and products: one reactant → many products is decomposition; many reactants → one product is combination; single element + compound → new element + compound is displacement; two compounds exchanging ions is double displacement.

Takeaway. Count reactants/products to classify.