Acids, Bases and Salts Notes

Acids, Bases, and Salts

What are Acids and Bases?

• Substances are classified as acidic, basic, or neutral based on the pH scale, which ranges from 0 to 14.
• Acids have a pH below 7, bases have a pH above 7, and neutral substances have a pH of 7.
• Solutions exhibit specific properties that distinguish them as acidic or basic.
• Acids:
  • Taste sour.
  • Turn blue litmus paper red.
  • Feel wet.
  • Are proton donors (release H^+ ions into solution).
  • Examples: vinegar, citrus fruits, soda.
• Bases:
  • Taste bitter.
  • Turn red litmus paper blue.
  • Feel slippery.
  • Are proton acceptors (release OH^- ions into solution).
  • Examples: ammonia, antacids, detergents/soaps.

What Makes Something Acidic, Basic, or Neutral?

• Acids have a pH less than 7, turn litmus paper red, taste sour, feel wet, and are proton donors (releasing H^+ ions).
  • Example: vinegar, citrus fruits/juices (lemon juice), soda.
• Bases have a pH greater than 7, turn litmus paper blue, taste bitter, feel slippery, and are proton acceptors (releasing OH^- ions).
  • Example: ammonia, green leafy vegetables, detergents/soaps.
• Acids and bases react to form water and salt in a neutralization reaction.
  • The combination of H^+ and OH^- ions react to form water.
  • This results in a pH of 7 (pure water).
• pH indicators are used to determine whether a substance is an acid, base, or neutral using color changes but don't give an exact pH value.
  • Common indicators: litmus paper, phenolphthalein, and methyl orange.

Understanding the pH Scale: What Number is Neutral on the pH Scale?

• Neutral on the pH scale is 7, meaning the substance is neither acidic nor basic.
• Neutral substances have an equal amount of hydrogen (H^+) and hydroxide ions (OH^-).
• Water and human blood are common examples of substances with a neutral pH.

Is Water an Acid, Base, or Neutral?

• Water is a neutral substance with a pH of 7.
• It has an equal amount of hydrogen (H^+) and hydroxide ions (OH^-).
• Water exhibits properties of both acids and bases, but they cancel each other out.
• It serves as a reference point for both acids and bases.

pH of an Acidic Solution

• Acidic solutions have a pH below 7 and as low as 0.
• The closer the pH is to zero, the stronger the acid.
• A substance with a pH of 1 is an extremely strong acid.
• Common examples of acids include lemon juice, citrus fruits, tomatoes, and vinegar.

Uses for Acids, Bases, and Neutral Solutions

• Acids, bases, and neutral substances have many uses in medicine, industry, and at home.
• Sulfuric acid is used in chemotherapy drugs, to treat skin infections and canker sores, and as a component in detergents, fertilizers, and disinfectants.
• Lactic acid is found in milk, skin products (chemical peels), and textile dyeing.
• Bases are commonly found in soaps and cleaning agents.
  • Sodium hydroxide (NaOH) is used in the production of soaps and paper.
  • Calcium hydroxide is used as a food preservative.
  • Calcium oxide is used as a neutralizer in certain acidic soils.
• Water is the most important neutral substance.
  • It flushes out toxins, preventing buildup and helping to prevent diseases like colorectal cancer and urinary tract infections.
  • Consuming water-heavy foods and drinks promotes healthy growth, survival, and reproduction of cells.

The pH Scale

• The pH scale is a simplistic way to express measurements of the acidity of a solution.
• In science, we want to associate an actual number with acidity rather than arbitrarily placing it on a continuum.

Finding pH Values

• pH is defined as the negative log of the concentration of hydrogen ions.

• Equation: pH = -log[H^+] where [H^+] is the concentration of hydrogen ions.

• A hydrogen ion is a hydrogen atom that has lost one of its electrons.

• Logarithmic Scales

  • A log scale indicates a non-linear relationship between data points.
  • Each unit increase on the x-scale corresponds to a tenfold increase in the y-scale.
    * If I have an earthquake that is 4.0 on the Richter Scale, that has a certain power that's associated with it. If I talk about an earthquake that's 5.0, it's not that it was just one unit stronger than the 4.0 earthquake, it's actually 10 times more powerful than the 4.0 earthquake. And then if we talk about something that's 6.0, that's 10 times more than the 5.0 earthquake and 100 times more than the 4.0 earthquake.

• Example: The Richter Scale measures the intensity of earthquakes.

  • An earthquake of 5.0 is 10 times more powerful than an earthquake of 4.0.
  • An earthquake of 6.0 is 100 times more powerful than an earthquake of 4.0.

• Log scales allow us to measure a wider range of values.

• Water has a hydrogen ion concentration of 10^{-7}.

• The negative sign is used to make pH values positive because concentration values are usually less than one.

Defining an Acid

• Water translates to a pH of 7.
• Anything more than 7 is a base; anything less than 7 is an acid.
• Acids are defined as molecules that donate hydrogen ions.
• Example: Hydrogen chloride (HCl) is a strong acid that completely dissociates into one hydrogen ion and one chlorine ion.
• Because it donates a hydrogen ion, it increases the hydrogen ion concentration of the solution.
• If the concentration of hydrogen ions in water is 10^{-7}, adding more hydrogen ions increases the concentration (e.g., 10^{-5}).
• Any value less than 7 is defined as an acid because it has a higher concentration of hydrogen ions than water.

Defining a Base

• A base is any molecule that can accept a hydrogen ion.
• Example: Sodium hydroxide (NaOH), also known as lye, is a strong base.
• A strong base completely dissociates into a sodium ion and a hydroxide ion (OH^-).
• H^+ and OH^- can react to produce water (H_2O).
• A base essentially removes hydrogen ions from the solution, lowering the concentration of H^+ ions.
• Adding concentration values into the pH formula results in a pH of 7 for water and a pH of 9 in this case.
• Anything with a value greater than 7 is defined as a base.

Bronsted-Lowry Acid and Base

• Arrhenius Acid Definition
  • Proposed by Svante Arrhenius in 1887.
  • An acid is a substance that can give away a hydrogen atom in the form of an ion (H^+) when dissolved in water, producing an aqueous acid.
  • Drawbacks of Arrhenius Theory- It limits acids to being only aqueous solutions and does not account for an acid, like hydrochloric acid, in gaseous form, also it does not account for substances that can sometimes act as an acid and sometimes as a base.
• In 1923, Johannes Bronsted and Thomas Lowry independently proposed that an acid is any substance that can donate a proton (a hydrogen atom that has lost its electron to become a positively charged ion of H^+).
• This is now called a Bronsted-Lowry acid, and an example is when hydrogen chloride dissolves in water.
• HCl + H2O \rightarrow H3O^+ + Cl^-
• In contrast, a Bronsted-Lowry base is a substance that accepts protons, for example ammonia, (NH3).
• NH3 + H2O \rightarrow NH_4^+ + OH^-

Conjugate Acids and Bases

• In the example of ammonia dissolving in water (NH3 + H2O \rightarrow NH_4^+ + OH^-), water donates a proton and is acidic, while ammonia accepts the proton and is the base.
• This reaction is reversible.
• The ammonium ion (NH4^+) can donate a proton back to the hydroxide ion (OH^-) to form NH3 and H_2O again.
• Because the ammonium ion is donating a proton, it is called a conjugate acid.
• The hydroxide ion is accepting the proton and is called a conjugate base.
• Every Bronsted-Lowry acid has a conjugate base, and every Bronsted-Lowry base has a conjugate acid.
  • Conjugate acid: an acid that forms when a base gains a proton.
  • Conjugate base: a base that forms when an acid loses a proton.

Lewis Acid and Base

• G.N. Lewis created the electron dot structure illustrating valence electrons.
• All Arrhenius acids and bases are also Bronsted-Lowry acids and bases, but the opposite isn't true.
  • An Arrhenius acid/base must be a substance dissolved in water.
  • A Bronsted-Lowry acid/base can be dissolved in water, like an Arrhenius acid/base, but it does not have to be.
• The Arrhenius and Bronsted-Lowry theories assume that an acid must have a hydrogen ion.
• G.N. Lewis proposed that an acid is a substance that accepts an electron pair to form a covalent bond.
• This is called a Lewis acid.
• In the Lewis theory, any compound with a central atom that can form a covalent bond is an acid, even without containing hydrogen.
• A silver ion can accept electron pairs from ammonia to form a covalent bond, so it is a Lewis acid. Ag+ + 2NH3 → (Ag(NH3 )2 )+ .
• A Lewis base is a substance that donates an electron pair to form a covalent bond.
• An anion is a Lewis base when it reacts to form a covalent bond by donating a pair of electrons.
• A Lewis acid-base reaction is a reaction in which a covalent bond is formed between an electron pair donor and an electron pair acceptor.
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