UNIT IV: Tutorial 14 - Part II

HYDRONIUM IONS In Chemistry 11… HCl(g)  H+(aq) + Cl-(aq) Let’s talk about hydrogen atoms. Almost all hydrogen atoms consist of one proton, no neutrons and 1 electron. When a hydrogen atom (H) forms a hydrogen ion (H+), it loses an electron. When it does this, it also loses its electron cloud! So what’s left? For this reason, the H+ ion is often called a proton. Because, that’s exactly what it is!

FORMING HYDRONIUM IONS Reactions in which H+ ions are transferred from one species to another are called proton transfers. The 1+ charge on the H+ ion or proton, is concentrated in a very small volume, much smaller than in any other ion. Because this charge is concentrated in a very small volume, it acts like it is quite powerful and it is attracted strongly to anything even remotely negative:

FORMING HYDRONIUM IONS

FORMING HYDRONIUM IONS This thing (made up of a proton (H+) added to a water molecule) is an ion because it has a charge. Its formula is H3O+ and it’s called the hydronium ion. The hydronium ion always forms when an acid dissolves in water. The H+ from the acid always goes to the nearest water molecule and forms H3O+.

FORMING HYDRONIUM IONS All acid solutions contain hydronium (H3O+) ions. It is the hydronium ion which gives all acids their properties (like sour taste, indicator colours, reactivity with metals etc.) Now, recall that in Chemistry 11, when HCl gas dissolves in water, we wrote: HCl(g)  H+(aq) + Cl-(aq) dissociation reaction Now, in Chemistry 12, we write the following: HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq) ionization reaction

FORMING HYDRONIUM IONS The proton (H+) has been transferred from the HCl molecule to a water molecule, to form a hydronium (H3O+) ion and a Cl- ion.   This type of reaction is called ionization (because ions are being formed). Complete Question 1 on page 4 of your notes.

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES An acid is any substance which donates (gives) a proton (H+) to another substance. A base is any substance which accepts (takes) a proton from another substance. A Bronsted Acid is a proton donor A Bronsted Base is a proton acceptor

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES We see that the HCl is donating the proton and the water is accepting the proton. Therefore HCl is the Bronsted acid and H2O is the Bronsted base. HCl + H2O  H3O+ + Cl- acid base

BRONSTED-LOWRY DEFINITION OF ACIDS AND BASES Let’s look at another example: NH3 + H2O  NH4+ + OH- base acid acid base  

AMPHIPROTIC SUBSTANCES Some substances (ex: H2O) are capable of acting as an ACID (when surrounded by a stronger base) OR acting as a BASE (when surrounded by a stronger acid). Substances that act as acids or bases are called amphiprotic.

AMPHIPROTIC SUBSTANCES Other amphiprotic substances: H2PO4- HS- HCO3-   + H+ - H+ Example: H3PO4 H2PO4- HPO42-

AMPHIPROTIC SUBSTANCES Identify the acid and base in the reactants of the following reactions:   H2S + HCO3-  H2CO3 + HS- NH4+ + H2O  H3O+ + NH3 HCOOH + HSO3-  H2SO3 + HCOO- Complete Question 2 on page 6 of your notes.

ACID-BASE EQUILIBRIA In reality, most acid-base reactions go forward and in reverse. If a proton is transferred during the forward reaction, we can also assume there will be a proton transfer in the reverse reaction. HF + SO32- HSO3- + F- *hint: choose one species on the reactant side….look to the product side to see if it gains or loses an H+ Knowing that each side has an acid and a base, identify the others!

ACID-BASE EQULIBRIA Identify the ACIDS and BASES on BOTH SIDES: H3BO3 + NH3 ↔ H2BO3- + NH4+ NO2- + HIO3 ↔ HNO2 + IO3-   Complete Question 3 on page 7 of your notes.

CONJUGATE ACID-BASE PAIRS Look at the last example: NO2- + HIO3 ↔ HNO2 + IO3- HIO3 and IO3- are called a conjugate acid-base pair. Defn: a pair of chemical species that differ by one proton

CONJUGATE ACID-BASE PAIRS Conjugate acid – the species with one more proton (ex. HIO3) Conjugate base - the species with one less proton (ex. IO3-) *There will always be 2 conjugate pairs Complete Question 4 on page 8 of your notes.  

CONJUGATE ACID-BASE PAIRS To find the conjugate acid of something: Add one H and one + charge Example: HSO4- → H2SO4 Complete Question 5 on page 9 of your notes. To find the conjugate base of something: Subtract one H and one + charge Example: H2PO4 - → HPO42- Complete Question 6 on page 10 of your notes.

POLYPROTIC ACIDS The formula of an acid tells us how many protons (H+) the acid can donate.   An acid that can supply: ONE proton (ex: HCl) = monoprotic acid TWO protons (ex: H2SO4) = diprotic acid THREE protons (ex: H3PO4) = triprotic acid More than ONE proton = polyprotic acid Complete Questions 7 & 8 on pages 12 & 13 of your notes. Self-Test for Tutorial 14 Hebden Questions 10-19