Equilibrium, Acids and Bases Unit Summary:

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1 Equilibrium, Acids and Bases Unit Summary: Prerequisite Skills and Knowledge Understand concepts of concentration, solubility, saturation point, pressure, density, viscosity, flow rate, and temperature in terms of their effect on chemical reactions. Understand reactions between acids and bases. Describe the concept of ph and poh as well as calculate them. The Arrhenius theory states that acids are substances that react with water to produce hydronium ions. Bases are substances that react with water to produce hydroxide ions. Solve equations by either algebraic or graphical means. Equilibrium Systems A closed system is a chemical system that is separated from its surroundings by a definite boundary so that no matter can enter or leave, but energy can enter and leave. Up until this point in chemistry, the common assumption has been that all reactions proceed to completion and reactions start out as reactants and end as products. Equilibrium is a balance point. It occurs in a closed system at constant temperature. It is reached when: Rate of forward reaction = Rate of reverse reaction Concentration, volumes, temperature, pressure and color remain constant Static Equilibrium is when 2 opposing forces are balanced (fixed and unchanging) Dynamic Equilibrium is when 2 opposing forces proceed at the same rate. This type of equilibrium takes time to establish. All macroscopic properties remain constant; moles of reactants and products must be unchanged in any system at equilibrium Equilibrium Expression: aa + bb cc + dd where a, b, c, d are coefficients & A, B, C, D are entities present Kc > 10 Quantitative Percent Reaction is 100% Kc Favors the products Percent Reaction is >50% Kc < 1.0 Favors the reactants Percent Reaction is <50% Kc = 1.0 Neither reactants nor products are favored Percent Reaction is exactly 50% ICE Table For many gas reactions, a spectrophotometer can be used to find the concentrations of the gases involved. You can determine the concentrations at equilibrium of a reactant or product by using an ICE Table and the reaction equation. I Initial Concentration C Change in Concentration E Equilibrium Concentration Steps for using ICE Tables: 1) Write the balanced equation and equilibrium expression. 2) Calculate the initial concentration of all reactants and products.

2 3) Find the equilibrium concentration by comparing initial concentration to each participant that has changed using an ICE Table. Once all concentrations are found, plug values into equilibrium expression and solve for Kc. An equilibrium constant value depends on the system temperature, is independent of initial concentrations and any catalyst present, independent of time taken to reach equilibrium, and is always stated as a numerical value with no units. Le Chatelier s Principle When a system is disturbed by a stress, the system will oppose the changes that were made to it and reach a new equilibrium. Example used to show changes on graphs: N 2 (G) + 3H 2 (G) 2NH 3 (G) + ENERGY Concentration Changes Temperature Changes Volume/ Pressure Changes Concentration Changes: reactants or products = shift to products reactants or products = shift to reactants Temperature Changes: T = shift towards energy T = shift away from energy Volume / Pressure Changes: **Always look at moles of reactants to products** Equal moles (reactants = products) : no change V ( P) = shift towards more gas moles V ( P) = shift towards less gas moles Inert gas or a catalyst: Has no effect on equilibrium whatsoever Water Equilibrium (K W ) Distilled water has a very slight conductivity due to presence of little or no ions; so there must be some ionization of water into hydronium ions and hydroxide. H 2 O (L) H + (AQ) + OH (AQ) K W = [ H + (AQ)][ OH (AQ)] *Omit water as its concentration never changes.* K W is equal to 1.0 x therefore, [H 3 O + ] = [OH ] = 1.0 x 10-7 We can use these values to find unknown concentrations if others are known. Recall ph and poh calculations: Bronsted Lowry Theory Bronsted and Lowry thought of acid and base behaviour in terms of proton transfer.

3 An acid donates a proton (a hydrogen) and a base accepts that proton (a hydrogen). Amphiprotic (also called Amphoteric) substances can act as acids or bases. A polyprotic substance is an acid with more than one proton to donate. A polybasic substance is a base that can accept more than one proton. Conjugate Acid Base Pairs According to the Bronsted Lowry theory, acid-base reactions involve the transfer of 1 hydrogen. These reactions are universally reversible and always result in an acid-base equilibrium. Example: acetic acid solution Steps to Predicting Acid Base Reactions 1) List all entities as they exist in initial mixture. Ionic oxides exist as cation and hydroxide ion. 2) Identify all possible acids and bases using Bronsted Lowry theory. 3) Identify the strongest acid and strongest base. Ignore all other entities which cannot be identified as either acid or base. These are merely spectator ions and cannot be considered in the reaction. 4) Use SA and SB to show equilibrium and predict position. The Acid Ionization Constant (K A ) It communicates the strength of weak acids by using the equilibrium law expression for weak acids reacting in water. Remember that strong acids react quantitatively and so there is no equilibrium and Stoichiometry can be used to find [H 3 O + ] Assume for all equilibrium law expressions that the concentration of water will remain unchanged as long as the amount of water is much greater than acid reacting with it Example: The ph of mol/l sample of acetic acid is 2.65; so what is the K A? CH 3 COOH (AQ) + H 2 O (AQ) H 3 O + (AQ) + CH 3 COO - (AQ) I C [H 3 O + ] = = E K A = [H 3 O + ][CH 3 COO - ] = ( )( ) = 1.7 x 10-5 [CH 3 COOH] The Base Ionization Constant (K B ) A strong base is any soluble ionic compound that contains or produces hydroxide ions. Weak bases only react partially with water, and to communicate their strength we use an equilibrium constant K B Note that for any conjugate acid base pair, use K W = K A K B which, if we rearrange, we can find K B K B =

4 Example: Amphiprotic (also called Amphoteric) Substances For most substances, it is easy to see if they are a weak acid or a weak base. Amphiprotic substances must first be analyzed to determine if they predominantly act as an acid or a base in a solution. How to find if it is and acid or base: 1) Find the substance on the acid column on the Table of Relative Strengths of Acids & Bases at SATP and see what the K A value for that substance is. 2) Calculate K B using K B = 3) The higher value is its dominant nature in water. Interpreting ph Curves ph curves are graphs showing the continuous change of ph during an acid base titration until the titrant has been added in excess. ph curves have characteristic shapes. Endpoint: the point in a titration where the addition of titrant is stopped and is usually indicated by color change Equivalence Point: is the point in the titration where the titrant and sample have reacted in chemically equivalent amounts and is usually reported as the volume of titrant required to reach that endpoint. A sample ph Curve:

5 ph Curves for polyprotic substances Polyprotic acids and bases have the ability to accept or donate more than 1 proton. If more than one proton transfer occurs during a titration, they will occur as a series of single proton transfers; each transfer shown as an equilibrium point on a ph curve. Indicator is chosen based on last equivalence point!!!!! For every proton transfer, the strength of new acid or base is greatly decreased. Sulfuric acid is a unique polyprotic acid because it is a strong acid and the first proton transfer is done simply by placing it in water. Titration Generalizations: o STRONG ACID & STRONG BASE reactions are quantitative and equivalence point ph = 7 o STRONG ACID & WEAK BASE equivalence points have a ph < 7 o WEAK ACID & STRONG BASE equivalence points have a ph > 7 ph Curve Buffering Regions and Buffer Solutions Buffer: the combinations of any weak acid with its conjugate base. A buffer solution has a specific ph and when small amounts of acids and bases are added, the ph stays relatively constant. A small change in buffer ratio means a small ph change. END OF EQUILIBRIUM & ACID-BASE UNIT SUMMARY Find more Math and Science resources at rohansharma.weebly.com

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