> See also: > - [[Relative Strengths of Acids and Bases]] > - [[Buffer Solutions]] # Acids and Bases There are three primary theories surrounding the concept of acids and bases: The **Bronsted-Lowry** and **Lewis** definitions are the most relevant in modern chemistry, however the *Arrhenius* definition was the first definitions which paved the way for future classifications. | Name | Acids | Bases | | :---: | :---: | :---: | | Arrhenius | Generates $H^+$ in a solution | Produces $OH^-$ in a solution | | Bronsted-Lowry | A proton donor | A proton acceptor | | Lewis | An electron lone-pair acceptor | An electron lone-pair donor | --- Generally, the following definitions are used to define acids and bases: - **Bases:** Anything that *accepts* a proton - All bases must have a lone pair of electrons that can bond to $H+$ - **Acids:** Anything that *donates* a proton - All acids must have a hydrogen atom The "proton" here refers to a hydrogen [[Ionization|cation]] ($H^+$). Hydrogen atoms only have one proton and no neutrons so removing it's electron would leave a singular proton. > [!info] Lewis Acids and Bases > - All Bronsted-Lowry acids are Lewis acids > - Lewis Bases can also be referred to as **nucleophiles** --- **Amphiprotic:** Any chemical species capable of either donating or accepting protons **Amphoteric:** Any chemical species capable of acting as either an acid or a base (as per the Bronsted-Lowry definition). A *neutral substance* is any substance that is not acidic or basic. - Ex: Salts ### Properties of Acids & Bases > [!info]- Properties of Acids > - Sour > - When reacted with metals, these substances produce hydrogen gas. > [!info]- Properties of Bases > - Soapy > - Bitter > - Good conductors of electricity when immersed in water TODO: - common examples - To determine whether a substance is an acid or base: 1. Count the number of hydrogen in each substance before and after the reaction. 2. If the number has *decreased* that substance is the **acid** (which donates hydrogen ions) . 3. If the number of hydrogen has *increased* that substance is the **base** (accepts hydrogen ions). ## Acid-Base Reactions > [!abstract] Acid-Base Reactions > A chemical reaction where a *hydrogen ion*, $H^+$ , is transferred from one chemical species to another. > > The following equation depicts the transfer of the hydrogen ion in terms of the acid and base reactants and their conjugate products as an equation: > $HA + B \rightleftharpoons A^- + BH^+$ > - $HA$ = Proton Donator (Acid) > - $B$ = Proton Acceptor (Base) > - $A^-$ = [[Conjugate Pairs|Conjugate Base]] > - $BH^+$ = [[Conjugate Pairs|Conjugate Acid]] The [[Chemical Equilibria|equilibrium constants]] of reversible acid-base reactions are known as acid/base [[Ionization Constants]]. ### Acid-Base Conjugate Pairs The concept of **conjugate pairs** is used when analyzing [[Chemical Equilibria#Reversible Reactions|reversible reactions]] to describe the role reactants/products play in the forward and reverse reactions. ![[Conjugate Pairs.png|500]] When an [[Acids and Bases|acid]] *donates* a proton ($H^+$), what remains is called the **conjugate base** and serves as the proton *acceptor* in the reverse reaction. When a [[Acids and Bases|base]] *accepts* a proton ($H^+$), what remains is called the **conjugate acid** and serves as the proton *donator* in the reverse reaction. ## Water's Role in Acidity/Basicity > See also: > - [[Chemical Equilibria#Ionization Constants|Water Ionization Constant]] Acid-base reactions frequently occur in [[Solutions & Concentrations|aqueous solutions]] (where water is the solvent). *Water* is the *most common acid* **AND** the *most common base* (meaning it is amphoteric/amphiprotic). In fact, can simultaneously be the acid and the base in a reaction with other water molecules: $H_2O (l) + H_2O (l) \rightleftharpoons H_3O^+ (aq) + OH^- (aq)$ The reaction above is between two molecules of water, where one acts as the base (acceptor) and the other as the acid (donor). This process where similar molecules react to yield [[Ionization|ions]] is known as **autoionization**. > [!warning] Free Hydrogen Ions Don't Exist in Water > It's important to remember that **hydronium ions** ($H_3O^+$) and **hydroxide ions** ($OH^-$) are the two key pieces to acid-base reactions that occur in [[Solutions & Concentrations|aqueous solutions]]. > > Free hydrogen ions that do not react with other molecules cannot exist This reaction is not occurring with every single molecule of water within a solution. At room temperature, approximately two out of every billion water molecules are ionized. > [!abstract] Autoionization (Ion-Product) Constant for Water > The extent of this reversible reaction can be represented as an [[Chemical Equilibria|equilibrium constant]] known as the **autoionization (ion-product) constant for water**: > > $K_w = [H_3O^+][OH^-]$ > >$K_w = 1.0 \times 10^{-14}$ at room temperature (25 $\degree C$)