Intro to Acids and Bases

300 Chemistry Acids and Bases

Definitions:

Acids… Bases… Have a low pH Have a high pH Taste sour Taste bitter React with metals to produce a salt and --- hydrogen gas React with bases to produce a salt and React with acids to produce a salt and water (neutralization reaction) water (neutralization reaction) Are electrolytes, conduct electricity Are electrolytes, conduct electricity Turn blue litmus paper red Turn red litmus paper blue --- Feel slippery

Different scientists have come up with a number of definitions regarding acids and bases due to the observation they made in their research. We will study the following: Arrhenius and Bronstead-Lowery Theories. ======Arrhenius Theory Around 1884, a Swedish scientist, Svante Arrhenius developed the theory that acids, when placed in water, acids form H+1ions and bases form OH-1 ions. This was the first modern definition of acids and bases and he won the Nobel Prize for it in 1903.

Arrhenius Acid = a substance that produces H+ ions in aqueous solution - Ex: HCl → H+1 + Cl 1 Note: Remember, a hydrogen atom only contains one electron and one proton and NO neutrons in its nucleus. As a result, when it forms and H+1 ion, it loses its only electron, and all that is left is the proton in the nucleus. As result, +1 H is called a “proton” as well as a “hydrogen ion.” A monoprotic (mono = 1) acid only has 1 acidic proton (H+); a diprotic (di = 2) has 2… etc.

Arrhenius Base = a substance that produces OH-1 ions in aqueous solution -1 Ex: NaOH → Na+1 + OH

Svante Arrhenius ======Bronsted-Lowry Theory

The Bronsted-Lowry definition is named for Johannes Bronsted and Thomas Lowry, who independently proposed it in 1923. A Bronsted-Lowry (BL) acid is defined as any substance that can donate a hydrogen ion (proton) and a Bronsted-Lowry base is any substance that can accept a hydrogen ion (proton). Thus, according to the BL definition, acids and bases must come in what is called conjugate pairs. (http://www.nyu.edu/classes/tuckerman/honors.chem/lectures/lecture_21/node3.html)

BL Acid = proton (H+) donor

BL Base = proton (H+) acceptor

Conjugate acid = what the base becomes after it accepts a proton Conjugate base = what the acid becomes after it donates a proton

+1 -1 Ex: NH3 + H2O → NH4 + OH

Ammonia is the Bronsted-Lowry base (it will accept a proton) Water is the Bronsted-Lowry acid (it will donate a proton) Ammonium ion is the conjugate acid of ammonia (it has accepted a proton) Hydroxide ion is the conjugate base of water (it has donated a proton)

Great link for further reading and many examples: http://www.mpcfaculty.net/mark_bishop/Bronsted_Lowry.pdf Fun website – easier to understand: http://www.blobs.org/science/article.php?article=3

======What are considered acids and bases?

Acids The six strong acids are: HCl (hydrochloric), HBr (hydrobromic), HI (hydroiodic), HNO3 (nitric), H2SO4 (sulfuric), and HClO4 (perchloric) Remember: Strong acids and bases are strong electrolytes, weak acids and bases are weak electrolytes (can break into ions and carry electric current. No ions = no current!) If a substance is not one of the strong acid listed above, it’s a weak acid! The weak acids are: HF (hydrofluoric), HC2H3O2 (acetic), H2CO3 (carbonic), and H3PO4 (phosphoric)

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Bases The strong bases are: any alkali metal hydroxides (LiOH, NaOH, KOH, RbOH, etc.), Ca(OH)2 (calcium hydroxide), Ba(OH)2 (barium hydroxide), and Sr(OH)2 (strontium hydroxide) If a substance is not a strong base, it’s a weak base! The most common weak one you’ll see if NH3 (ammonia) What is pH? pH (potential of hydrogen – or – how much hydrogen is in a substance) is very important biologically (pH of blood, pH of the ocean or lakes, pH of rain, etc). It is a measure of acidity (how much H+1) and basicity (how much OH-1) there is in a water solution.

If: pH = 7 the solution is neutral pH > 7 the solution is basic pH < 7 the solution is acidic

If we use Universal Paper to determine pH, the following colors will appear:

If litmus paper is used:

Red litmus: Base: turns paper blue Acid: paper stays red (looks wet)

Blue litmus: Base: paper stays blue (looks wet) Acid: paper turns red Determining pH For strong acids and bases, it is easy to determine their strength, using pH For weak acids and bases, it is a bit more complex to determine their strength and pH (we’re not going to worry about those)

Acid and Base Strength for Strong Acids and Bases Strong acids and bases dissociate 100% (equilibrium position is far to the right, reaction goes nearly completely to the products (ions))

For all aqueous solutions water also dissociates a little: H2O ↔ H+1 + OH-1 We can write Keq = Kw = [H+1] x [OH-1] = 1.0 x 10-14 [It sounds strange… but this means even in a basic solution, there are hydrogen ions, and even in an acidic solution, there are hydroxide ions… they come from the water] pH below 7 indicates an acidic solution ([H+1] > [OH-1]) pH of 7 indicates a neutral solution (not an acid or a base) ([H+1] = [OH-1] = 1.0x10-7) pH above 7 indicates a basic (alkaline) solution ([OH-1] > [H+1])

Neutralization In a neutralization reaction, an acid and base react to produce some salt and water… at what is called the “equivalence point”, we can say that: Moles of H+ = Moles of OH- For a monoprotic acid and a base with one hydroxide, use the following equation: Ma x Va = Mb x Vb

Titrations and Indicators Titration = a process used to determine the amount (concentration) of acid or base in a solution Indicator = a substance that marks the equivalence point of a titration (or identifies a pH range) by changing color Common indicators: litmus paper (red in acid, blue in base), phenolphthalein (clear in acid, pink in base), and universal indicator (a rainbow of colors depending on the pH) pH meter = device used to make rapid, accurate pH measurements Titration curve = a graph used to show how the pH of the unknown solution changes as various amounts of titrant are added How they are used: -A solution of known concentration (the titrant) is delivered from a buret into an unknown solution containing an indicator -At the endpoint of the titration, when the unknown substance is just consumed, the stoichiometric (equivalence) point is reached, and the indicator changes color -Then, the amount of titrant added can be used to determine the concentration of the unknown