Chapter 20 - Acid, Base and Redox 1 PS 101 Kim Cohn address

Chapter 20 - Acid, Base and Redox 2 Some 7th and 8th Grade Standards 1 Students know that compounds are formed by combining two or more elements. Students know structure of the atom. Students know how to use the periodic table Students know that each element has a specific number of protons in the nucleus. Students know that isotopes vary by the number of neutrons

Chapter 20 - Acid, Base and Redox 3 Definitions 1 Empirical Definition originating in or based on observation or experience relying on experience or observation alone often without due regard for system and theory capable of being verified or disproved by observation or experiment

Chapter 20 - Acid, Base and Redox 4 Definitions 2 Empirical Definitions Acids taste sour. Acids react with many metals to form hydrogen. Acids react with limestone to form carbon dioxide.

Chapter 20 - Acid, Base and Redox 5 Definitions 2 Empirical Definitions Bases taste bitter (e.g. unsweetened chocolate, many drugs). When wet bases make your skin slippery. Bases react acids to form a salt.

Chapter 20 - Acid, Base and Redox 6 Definitions 3 Arrenhius Definition Acids produce a H + ion when dissolved in water. Bases produce an OH - ion when dissolved in water. H + ion is called a hydrogen ion, or a proton or sometimes written as H 3 O +, a hydronium ion.

Chapter 20 - Acid, Base and Redox 7 Definitions 4 - Arrenhius Base Acid

Chapter 20 - Acid, Base and Redox 8 Definitions 5 Bronsted-Lowry Definition Acids donate a proton, H + Bases accept a proton.

Chapter 20 - Acid, Base and Redox 9 Definitions 6 – BL Acid HCl is a BL acid

Chapter 20 - Acid, Base and Redox 10 Definitions 7 – BL Base NH 3 is a BL Base

Chapter 20 - Acid, Base and Redox 11 Definitions 8 – BL All Species BaseAcid Base

Chapter 20 - Acid, Base and Redox 12 Salt Formation BaseAcid BaseAcidSaltWater

Chapter 20 - Acid, Base and Redox 13 Acid Strength 1 Remember equilibrium? If B were bigger and stronger, the equilibrium would lie to the left.

Chapter 20 - Acid, Base and Redox 14 Acid Strength 2 Strong acids have a strong tendency to donate their proton, H + ion. This equilibrium lies to the right with HCl

Chapter 20 - Acid, Base and Redox 15 Acid Strength 2 Weak acids do not have a strong tendency to donate their proton, H + ion. This equilibrium lies to the left with HF HF is a weak acid. Vinegar contain a weak acid, acetic acid.

Chapter 20 - Acid, Base and Redox 16 Acidic, Basic or Neutral Water both donates H + ions and OH - ions. H 2 0  H + + OH - We normally consider water as neutral. Not much water breaks up in this way. A liter of pure water will produce (1 x ) mole of H + ions and (1 x ) mole of OH - ions = 1 x 10 -7

Chapter 20 - Acid, Base and Redox 17 Acidic, Basic or Neutral 2 A liter of pure water will produce (1 x ) mole of H + ions and (1 x ) mole of OH - ions or: The molarity of the H + ions = the molarity of the OH - ions = (1 x ) M. Pure water is considered neither acid or basic. In water solutions of anything when the concentration of the H + ions = (1 x ) M, the solution is neutral.

Chapter 20 - Acid, Base and Redox 18 Acidic, Basic or Neutral 3 Because in pure water the concentration of H + ions = the concentration of the OH - ions = (1 x ) M you can say that Concentration H + x Concentration OH - = (1 x ) (1 x ) = 1 x If the concentration H + = 1 x what is the concentration of OH - ? 1 x 10 -9

Chapter 20 - Acid, Base and Redox 19 Acidic, Basic or Neutral 4 Instead of writing concentration or molarity, chemists often use brackets. In neutral water [H + ] = 1 x In neutral water [H + ] = [OH - ] = 1 x In acidic water [H + ] > [OH - ] In basic water [H + ] < [OH - ]

Chapter 20 - Acid, Base and Redox 20 pH scale Because it is difficult to explain to the average person what is meant by 1 x many people use a pH scale. The pH = - log [H + ] In neutral water [H + ] = 1 x pH = -log(1 x ) = 7 pH < 7 acidic, pH > 7 basic.

Chapter 20 - Acid, Base and Redox 21 Question What is the pH if [OH - ] = 1 x There are two steps to this problem, first find the [H 3 O + ] – on board. Next, take the negative log of the number you found – on board. pH=9, basic. Answer makes sense because the [OH - ] = 1 x > 1 x 10 -7, therefore the solution has to be basic.

Chapter 20 - Acid, Base and Redox 22 Acid Rain Sulfur burns in air to form SO 2 SO 2 dissolves in water to form an acid, H 2 SO 3. SO 2 also reacts with more O 2 to form SO 3 and This dissolves in water to form H 2 SO 4, sulfuric acid. SO 3 + H 2 O  H 2 SO 4

Chapter 20 - Acid, Base and Redox 23 Acid Rain 2 Carbon from coal or gasoline also burns in air to form CO 2 CO 2 dissolves in water to form an acid, H 2 CO 3, carbonic acid.

Chapter 20 - Acid, Base and Redox 24 Acid Rain 3 You may notice that you have trouble when there is fog in Bakersfield. This is because the build-up of SO 2.

Chapter 20 - Acid, Base and Redox 25 Acid Rain 4 Much of the acid, both H 2 SO 4 and H 2 CO 3, in the air is removed by Ca 2+ ions in the ocean. Ca 2+ + CO 3 2-  CaCO 3 (s) Ca 2+ + SO 4 2-  CaSO 4 (s)

Chapter 20 - Acid, Base and Redox 26 Acid Rain 5 The buildup of CO 2 may also be causing the earth to warm up because of the greenhouse effect. The layer of CO 2 allows ultraviolet radiation to come through. The ultraviolet hits the earth and excites electrons in anything it hits. The excited electrons in the atom fall to the ground state and emit infrared, or heat, which can not pass trough the CO 2 layer.

Chapter 20 - Acid, Base and Redox 27 Buffers A buffer contains a weak acid and the salt of a weak acid. The weak acid absorbs OH - ions. H + + OH -  H 2 O The salt of the weak acid absorbs H + ions. CO H +  H 2 CO 3 The pH is controlled by these two reactions. Discuss pH in blood.

Chapter 20 - Acid, Base and Redox 28 Electrochemistry In the same way that some atoms or ions can accept or donate protons, H + some atoms or ions can accept or donate electrons, e -

Chapter 20 - Acid, Base and Redox 29 Electrochemistry 2 Atoms that lose or donate electrons are oxidized. Zn  Zn e - Loss of electrons is oxidation. Atoms that gain electrons are reduced. Cl 2 + 2e -  2Cl 1- Gain of electrons is reduction.

Chapter 20 - Acid, Base and Redox 30 Electrochemistry 4 Demo – Copper wire in AgNO 3 solution. Cu  Cu e - and Ag + + e -  Ag Notice that one species is losing and electron and the other is gaining an electron. You can also make a battery from this reaction by isolating the reactants.

Chapter 20 - Acid, Base and Redox 31 Electrochemistry 5

Chapter 20 - Acid, Base and Redox 32 Electrochemistry 6 All batteries work the same way, but are put together in different ways with different materials. Alkaline batteries use the following reactions - Zn  Zn e - and Mn 4+ +e -  Mn +3 The reactions above have been simplified.

Chapter 20 - Acid, Base and Redox 33 Oxygen Oxygen is easily reduced to the oxide ion. O 2 + 2e -  2O 2- It is a good agent. It will oxidize iron to form rust. 4Fe + 3O 2 + 6H 2 O  2Fe 2 O 3. 3H 2 O And burn fuels.