Lesson 1 Arrhenius Theory Bronsted-Lowry Theory ACIDS AND BASES

History of Acids and Bases In the early days of chemistry chemists were organizing physical and chemical properties of substances. They discovered that many substances could be placed in two different property categories: Substance A 1.Sour taste 2.Reacts with carbonates to make CO 2 3.Reacts with metals to produce H 2 4.Turns blue litmus pink 5.Reacts with B substances to make salt water Substance B 1.Bitter taste 2.Reacts with fats to make soaps 3.Do not react with metals 4.Turns red litmus blue 5.Reacts with A substances make salt and water Arrhenius was the first person to suggest a reason why substances are in A or B due to their ionization in water.

The Swedish chemist Svante Arrhenius proposed the first definition of acids and bases. (Substances A and B became known as acids and bases) According to the Arrhenius model: “acids are substances that dissociate in water to produce H + ions and bases are substances that dissociate in water to produce OH - ions” NaOH (aq)  Na + (aq) + OH - (aq) Base HCl (aq)  H + (aq) + Cl - (aq) Acid Arrhenius Theory

Acids and Bases Before… HCl(g)  H + (aq) + Cl - (aq) Acids dissociate in water H + and Cl - ions floating around in water Not really the case….

Acids and Bases Hydrogen 1 proton, 1 electron When it forms an ion it loses an electron H + Because it loses its electron we call H+ ions a proton Reactions where H+ ions are transferred from one thing to another are called proton transfers

Acids and Bases Charge on H+ ions is very small, and very concentrated Because of this it is attracted to anything remotely negative Recall Lewis Dot structures…. Oxygen 6 valence electrons Hydrogen 1 valence electron

Acids and Bases When they form water they shared electrons Oxygen has more electrons and because of the shared electrons are closer to oxygen then Hydrogen This makes water a polar covalent molecule more electrons close to “oxygen end” of the water molecule, that end has a partial negative charge. Less electrons around the “hydrogen end” that ends a has a partial positive charge:

Acids and Bases

Partial negative charge on oxygen very attractive to our H+ ion H+ will sit on one of electrons pairs of oxygen atoms Brings a positive charge H 3 O + hydronium ion This ion will always form when an acids is dissolved in water H + ion from acid will always go to nearest molecule to from H 3 O +

Acids and Bases For this reason, a hydronium ion a hydrated proton. Adding water to something is called hydration. So if you were a proton, you would have a water molecule “added to you”. 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. ) HCl (g)  H + (aq) + Cl - (aq) NO HCl (g) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) YES!!

Acids and Bases 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) NOT an H atom that is moving. The H atom leaves it’s electron behind with the Cl, so it is H+ (a proton) that moves to the water molecule. The Cl-, now having the electron that H left behind, gains a negative charge.

Acids and Bases All acids behave similarly in water; they donate (or give) a proton (H+) to the water, forming hydronium ion (H3O+) and the negative ion of the acid. Another example might be the ionization of nitric acid (HNO3): HNO3 (l) + H2O (l)  H3O+(aq) + NO3-(aq)

Acids and Bases NOT A WORKSHEET…but more practice problems! Complete equations for the following acids ionizing in water: HClO (g) H2SO4 (l) (assume only 1 H+ is removed.) CH3COOH (l) (assume the H on the right end comes off.) HSO4 - (aq)

Bronsted-Lowry Definition of Acids and Bases You might recall that the definition of an “acid” according to Arrhenius was a substance that released H+ ions (protons) in water. Bronsted and Lowry came up with a theory which is more useful when dealing with equilibrium and covers a wider range of substances. Bronsted-Lowry: 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 -OR- A Bronsted Acid is a proton donor A Bronsted Base is a proton acceptor

Bronsted-Lowry Definition of Acids and Bases HCl + H2O  H3O+ + Cl- 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 NH3 + H2O  NH4+ + OH- Now, the NH3 on the left has changed into NH4+ on the right, that means it must have accepted (taken) a proton. Since it has accepted a proton it’s called a base. The H2O, this time has donated (lost) a proton as it changed into OH-. Since it has donated a proton it’s called an acid. So now we can label these: NH3 + H2O  NH4+ + OH- base acid

Bronsted-Lowry Definition of Acids and Bases Water is an acid and base? Sometimes water acts like a base (takes a proton) and sometimes it acts like an acid (donates a proton). This is just like you. If you buy something (donate money) you are a buyer. If you sell something (accept money), you are a seller. I’m sure you have been both at various times. Animals that can live either in the water or on land are called amphibians. (Yes, this is still Chemistry just in case you’re wondering!) For things that can be “either / or ”, we can use the prefix “amphi” A substance that can act as either an acid or a base is called amphiprotic.

Bronsted-Lowry Definition of Acids and Bases Water (H2O) is an example of an amphiprotic substance. When it was with HCl, it acted like a base, but when it was with NH3, it acted like an acid. Not only molecules can lose or gain protons. Ions can too When something loses a proton (acts as an acid), it turns into something with one less H and one less (+) charge When something accepts a proton (acts as a base), it turns into something with one more H and one more (+) charge So what you have to do is look at the right side of the equation, and see whether the substance gained or lost a proton.

Bronsted-Lowry Definition of Acids and Bases HCO3- + HSO4-  H2CO3 + SO42- HCO3- must have accepted a proton to form H2CO3 So it is the base. HSO4- must have donated a proton to form SO42- so it must be the acid. so the answer is: HCO3- + HSO4-  H2CO3 + SO42- base acid