equilibrium chemistry 30

percent reaction compared with description of equilibrium

- <50% --> the reactants are favored - >50% --> products are favored - >99% --> quantitative reaction

Bronsted-Lowry neutralization

- A Bronsted-Lowry neutralization is a competition for protons that results in a proton transfer from the strongest acid to the strongest base present. - Therefore, strong acids have a weak attraction for protons and readily donate protons in a reaction. - Weak acids have a strong attraction for protons and do not readily give up the protons in a chemical reaction. - Strong bases, on the other hand, have a strong attraction for protons and accept protons readily. - Weak bases do not have a strong attraction for protons.

Volume changes for a system that has equal moles of gaseous reactants and products

- A change in volume of this system will have a direct effect on the concentration of ALL the gases in the above equilibrium but it will not result in an equilibrium shift as there are equal moles of gases on the reactant and product side

base strength

- According to Arrhenius, a base is a substance that increases the hydroxide ion concentration of a solution. - Strong bases are ionic hydroxides - Weak bases increase hydroxide ion concentration in solution --> Kb = [OH-(aq)]2 [WB] - NOTE: Kw = Ka x Kb

Arrhenius theory

- Arrhenius' Theory states the following: - Acids are substances that ionize in water to form hydrogen ions. The hydrogen ion is responsible for the acidic properties of the solution. - Bases are substances that dissociate in water to form hydroxide ions in solution. The hydroxide ion is responsible for the basic properties of the solution. - Neutral substances are those in which no hydrogen or hydroxide ions are formed, or are present.

as the reaction proceeds

- As the forward reaction proceeds, products are being made, initiating the start of the reverse reaction - It will be slow to begin with because the concentration of the products is low - As the reactants are consumed, the rate of the forward reaction will decrease and the rate of the reverse reaction will increase as more products are formed

the start of the chemical reaction

- At the start of a chemical reaction, the reactants are at their maximum concentration and the forward reaction will proceed at its maximum rate - Because there are no products at the start of the reaction, there is no reverse reaction

modified Arrhenius: formation of basic and acidic solutions

- Basic solutions are formed when substances react with water to form hydroxide ions. - Acidic solutions are formed when substances react with water to form hydronium ions.

weak acid and strong base titrations

- Because methanoic acid is a weak acid, it only partially ionizes in solution and as a result the initial pH of the solution will have to be determined using the Ka value. - As sodium hydroxide is added to the sample, the pH remains relatively constant - this again is an example of buffering action. - The solution that is formed in the buffer region is a true buffer solution consisting of methanoic acid and the conjugate base, the methanoate ion. - You will notice that the equivalence point for a weak acid-strong base titration occurs at a pH greater than 7 - why? The products of a neutralization reaction produce water and a weak base - therefore, the solution at equivalence point will be weakly basic and have a pH greater than 7. --> weak acid + strong base → water + weak base - The volume of titrant required to neutralize the weak acid sample still depends on the moles of acid and the molar ratio of the acid and base.

buffers

- Buffers are important in chemical and biological systems. - Blood consists of an important buffer (combination of H2CO3(aq) & HCO3-(aq)). - The normal pH range of blood is 7.38 to 7.42 - even a small change, either to 7.00 or to 8.00 would kill us. - Without the buffer, a glass of orange juice would prove to be fatal.

buffer solutions

- Buffers are solutions that resist a change in pH (buffer the system against a change in pH) when a small amount of strong acid or a strong base is added to them. - Buffers are mixtures of a weak acid and its conjugate base or a weak base and its conjugate acid. - They are prepared by mixing solutions of the weak acid (like acetic acid) and a salt of the weak acid (sodium acetate). - The salt of the weak acid provides the conjugate base (acetate ions). - Similarly, solutions of a weak base (like ammonia) and a salt of the weak base (ammonium chloride) may be used. - The salt of the weak base provides the conjugate acid (ammonium ions).

chemical equilibrium

- Chemical equilibrium can only be established for reactions that are reversible and are in a closed system where no matter can enter or leave.

two types of reactions

- Chemical reactions can be classified into two types: - those that are quantitative (go to completion) - those that proceed until an equilibrium is reached (products and reactants are both present)

pH and pOH calculations

- Concentration of H3O+/H+ ions present determines whether a solution is acidic, basic or neutral - Neutral --> [H3O] = 1.0x10-7 mol/L (pure water) - Acidic --> [H3O] > 1.0 x 10-7 mol/L - Basic --> [H3O] < 1.0 x 10-7 mol/L

amphiprotic/amphoteric

- Note: A substance may be a Bronsted-Lowry acid in one reaction and a base in another reaction. - The advantage of the Bronsted-Lowry definitions is that they enable us to define acids and bases in terms of chemical reactions, but they do not explain why a proton is donated or accepted.

percent reaction

- Percent reaction is defined as the yield of product measured at equilibrium compared to the maximum possible yield of the product if the reaction was quantitative (stoichiometric) --> % reaction = equilibrium [product] x 100 maximum [product] - to communicate that an equilibrium exists, equilibrium arrows are used - the extent of reaction is communicated by writing the percent reaction above the equilibrium arrow - It is also important to write down the temperature for which this percent reaction is valid; remember that % rxn is dependent on temperature! If no temperature is communicated, assume SATP

catalysts

- Provide an alternative reaction pathway that has a lower activation energy - The rate of forward and revers will increase

conductivity of water

- Pure water has a very slight conductivity that is only observable if measurements are made with a very sensitive conductivity meter. - This slight conductivity is due to the presence of ions produced by the ionization of some water molecules into the hydronium ion and hydroxide ion (fewer than 2/billion water molecules ionize at SATP)

modified Arrhenius

- Revised Arrhenius's theory, two main concepts must be considered: 1. The collision of substances with water molecules. 2. The nature of the hydrogen ion. - There is no evidence for unhydrated hydrogen ions in aqueous solution, but experiments have shown evidence of hydrated protons. - Because the hydrogen ion has a highly concentrated positive charge it will form a strong bond with a polar water molecule forming a hydronium ion, H3O+(aq). - This shows that water has a specific function in acid-base chemistry rather than only being a solvent.

rounding numbers to do with pH

- Sig Digs - The number of digits following the decimal point in the pH value is equal to the number of sig digs in the hydronium ion concentration

how to make an ICE table

- Step 1: Write the balanced chemical reaction and the equilibrium law expression. Notice that in order to solve for K, you must first find the equilibrium concentration of each molecule. - Step 2: Calculate the initial concentration of each reactant and product. - Step 3: Find the equilibrium concentration by comparing the initial concentrations to the amount each participant has changed. Set up your data in an ICE table - Step 4: Once all the equilibrium concentrations are calculated, plug in the values into the equilibrium law expression and solve for k.

temperature change

- The energy in a chemical equilibrium is treated as though it were a reactant (if the reaction is endothermic) or a product (if the reaction is exothermic)

predicting acid-base reactions

- Use the following Five Step Method for Predicting Acid Base Reactions: - List all entities (ions, atoms or molecules) initially present as they exist in a water environment. - Remember that weak acids are not broken apart. - Identify the strongest acid and the strongest base present, using the table of acids and bases in your data booklet - Transfer one proton from the acid to the base and predict the conjugate acid and conjugate base as the products. - Predict the position of equilibrium base on the position of the reactant acid and base.

volume/pressure changes

- Volume/pressure changes only affect systems where gases are present. This is because the concentration of gases are directly affected by volume changes: --> Because the number of moles of gas remains unchanged, increasing the volume of the container (decreasing the pressure of the system) will decrease the concentration of a gas (same numerator divided by a larger denominator) --> In the same way, decreasing the volume of the container (increasing the pressure of the system) will increase the concentration of a gas --> Therefore, we can see that pressure and concentration have a direct relationship (increasing one increases the other) and volume and concentration have an inverse relationship (increasing one decreases the other)

equivalence point

- When doing a titration there will be a point at which the reaction is complete; when chemically equivalent amounts of reactants have combined. - This is called the equivalence point - The point during a titration at which the exact theoretical chemical amount of titrant has been added to the sample (QUANTITATIVE) - To measure the equivalence point experimentally we look for a sudden change in an observable property such as, color, pH or conductivity. This is called the endpoint (QUALITATIVE)

factors that do not affect equilibrium constant

- initial concentration - pressure of gases - adding water - adding a catalyst

pH calculations

- pH = -log [H3O+(aq)] - [H3O+(aq) = 10^-pH

pOH formula

- pH is used more commonly, but sometimes it is more practical to describe hydroxide ion concentrations Anytime you are dealing with a basic solutions -YOU MUST use Hydroxide ion concentrations!! - The same format for pOH as pH - pOH = -log[OH-(aq)] - [OH-(aq)] = 10^-pOH

formula to convert between pH and pOH

- pOH + pH = 14

titration

--> Used to determine the amount concentration of substances in solution - The solution of the known concentration may be either the titrant or the sample, it makes no difference --> Breakdown - Carefully adding a solution (titrant) from a burette into a measured, fixed volume of another solution (sample) in an Erlenmeyer flask until the reaction is judged to be complete - In acid base titrations, indicators can help us determine when the reaction is complete

steps of a titration

- A known volume of solution is added to an Erlenmeyer flask(sample) + and indicator - An initial reading of the burette is made before any titrant is added to the sample - Then the titrant is added until the reaction is complete; when the final drop of titrant permanently changes the colour of the sample - The difference between the readings is the volume of titrant added - Near this endpoint, constant continuous swirling of the solution is important

the chatelier's principle

- According to Le Chatelier's Principle, when a chemical system at equilibrium is disturbed by a change in a property of the system, the system adjusts in a way that opposes that change - The application of Le Chatelier's principle involves a three-stage process that will be demonstrated by sketching a concentration-time graph: an initial equilibrium state, a shifting non-equilibrium state as a result of the stress applied, and a new equilibrium state - There are 3 stresses that can be applied to a system to cause a disturbance in equilibrium

la Chatelier's principle and hydronium concentration

- According to Le Chatelier's principle, an increase in the hydronium ion concentration by the addition of an acid will shift the equilibrium to the left changing the indicator color to the acid form, HIn(aq) (red for litmus). - When and indicator is placed in a basic solution, the hydroxide ions react with the hydronium ions, removing hydronium ions and therefore shifting the equilibrium to the right changing the indicator to the base form, In-(aq) (blue for litmus).

acid-base indicators

- Acid-base indicators are substances that change color when reacted with acids or bases. Most indicators exist in one of two conjugate forms(acid form and base form) that are reversible and distinctly different in color (see p. 10 of your data booklet). - An indicator is a weak conjugate acid-base pair formed when an indicator dye dissolves in water: --> HIn(aq) + H2O(l) ↔ In-(aq) + H3O+(aq) red blue - the above reaction is a generic indicator reaction

weak and strong acids

- Acidic solutions of different substances at the same concentration do not possess acidic properties to the same degree. - Weak Acids --> Weak electrolytes --> React at a slower rate than strong acids --> pH of weak acids are closer to 7 when compared to strong acids of equal concentration --> only partially ionizes in water [WA] ≠ [H3O+] - Strong acids --> Strong electrolyte --> Reacts at a faster rate than weak acids --> pH of strong acids is further away from 7 when compared to weak acids of equal strength --> completely ionizes in water [SA] = [H3O+]

Volume changes for a system with unequal moles of gaseous reactants and products

- An increase in volume of the container (a decrease in pressure) will cause equilibrium to favour the side with more moles of gaseous entities - A decrease in volume of the container (an increase in pressure) will cause equilibrium to favour the side with less moles of gaseous entities.

equilibrium constant, Kc formula

- Analysis of evidence from experiments reveal a mathematical relationship that provides a constant value for a chemical system over a range of initial concentrations --> This constant value is called the equilibrium constant, Kc - Evidence and analysis of many equilibrium systems have resulted in the equilibrium law: - For the reaction, aA + bB ↔ cC + dD (where the lower case letters represent coefficients and capital letters represent reactants and products) - The equilibrium law is: Kc = [C]c[D]d at a specific temp! [A]a[B]b

strong acid wand weak base titration

- Because ammonia is a weak base, it only partially dissociates in solution to produce hydroxide ions; as a result we must use the equilibrium law expression and Kb to solve for the initial pH of the ammonia solution: - As the hydrochloric acid is added to the ammonia sample for the first small additions, we don't see a large change in pH of the sample solution - that's buffering action again. - The solution formed in the buffer region is a true buffer consisting of ammonia and the conjugate acid, ammonium ions. - As the ammonia sample becomes neutralized, the sharp rise in pH indicates that the equivalence point has been reached - notice that the pH at equivalence point of a weak base-strong acid titration is below pH of 7 - Why? The products of the neutralization reaction between a weak base and a strong acid are water and a weak acid - therefore, the pH of the solution formed at equivalence point will have a pH of less than 7. --> strong acid + weak base → water + weak acid - Again the volume of titrant required to reach the equivalence point will depend on the moles of ammonia and the molar ratio of hydrochloric acid to ammonia.

strong acid and base buffers

- Consider the equilibrium in which there is are significant amounts of both HA and its conjugate base A-: --> HA(aq) + H2O(l) ↔ A-(aq) + H3O+(aq) - If a small amount of a strong acid is added to this equilibrium mixture, the additional hydronium ions would cause a shift in the equilibrium toward the left, causing the concentration of hydronium ions to eventually fall to near its original concentration. - Similarly, if a small amount of strong base is added to the equilibrium mixture, the hydroxide ions would react with the hydronium ions to form water. - The decrease in the concentration of the hydronium ion would cause equilibrium to shift to the right until the concentration of hydronium ions increases to near its original value. - In both cases, the effect of adding the small amount of strong acid or base is minimized and the change in the pH of the solution is minimal.

dynamic equilibrium

- Eventually the rates of the forward and reverse reaction will become equal - We can say that the system has reached a state of dynamic equilibrium where both the forward and reverse reactions continue, but at equal rates - Reactions are better written with a double headed equilibrium arrow indicating the reaction can go either way

equivalence point in pH curves

- Following the buffering region there is a very rapid change in pH for a very small addition of titrant. - The mid point of the sharp rise signifies the theoretical equivalence point at which chemically equivalent amounts of reactants are present (the reaction is complete; the sample is completely reacted). - If a proper indicator is used, the equivalence point should be the same as the indicator endpoint, where the indicator changes color. - For a strong acid-strong base titration, the products formed will always consist of water and a neutral salt - as a result the equivalence point will always occur at a pH of 7. - The indicator best used for a strong acid-strong base titration would be bromothymol blue.

when are ICE tables used?

- Ice tables must be used when: --> you are being asked to find the value of K --> when you are given the initial concentration of reactants and one equilibrium concentration --> you are given K and are asked to find the initial or equilibrium concentrations ---> Initial concentrations ---> Changed concentrations ---> Equilibrium concentrations

approximation/assumption rule

- If C/K is greater than 1000 we can ignore the -x on the bottom! - Reactants are favored, any change in the reactants is negligible. (This allows us to simplify the mathematical expression

perfect square rule

- If there is a square on the top and bottom square root both sides to simplify

proton transfer

- In a proton transfer reaction at equilibrium, both forward and reverse reactions involve Bronsted-Lowry acids and bases.

conjugate acid-base pair

- In any acid-base equilibrium, there will always be two acids and two bases. - The base on the right side of the equation is formed by the removal of a proton from the acid on the left side of the equation. - The acid on the right side of the equation is formed by the addition of a proton to the base on the left side of the equation. - A pair of substances that differ only by a proton is called a conjugate acid-base pair. - In the previous example, acetic acid and the acetate ions are a conjugate acid-base pair, and so is water and the hydronium ion. - Conjugate acid-base pairs appear opposite each other in a table of acids and bases in your data booklet.

3 main assumptions in dynamic equilibrium

- In systems at equilibrium forward and reverse reactions are taking place at the same rate - At equilibrium there will be no further change in the concentrations of reactants and products, although they are not equal - The system will have constant macroscopic properties --> Temperature --> Colour --> pH

the Chatelier's principle and products

- In the same way, the removal of a product from the equilibrium will cause a shift to the right to create more of the product removed, thereby increasing all product concentrations - In the same way, the addition of product to a system at equilibrium will cause equilibrium to shift left to use up the extra product added, thereby increasing the concentration of the reactants

ionization constant and equilibrium law expression

- In the study of acids and bases, the ionization constant and the equilibrium law expression for water can be used to calculate the concentration of either the hydronium ion or the hydroxide ion. - According to Arrhenius' theory, acids ionize in water increasing the hydronium ion concentration in water (greater than 1.0 x 10-7 mol/L). - Bases are substances that dissociate in water increasing the hydroxide ion concentration in water (greater than 1.0 x 10-7 mol/L)

the affects on other factors because of changes in pressure/volume

- It is important to note that an increase in pressure only increases the concentration of a gas IF the volume of the container is changed - If we were to add an inert gas to an equilibrium system with a fixed volume, the pressure of the system would increase, but the concentration of the gases in the container would not change

the Chatelier's principle and reactants

- Le Chatelier's Principle indicates that the addition of more reactant to a system at equilibrium will cause the equilibrium to shift right to use up the extra reactant added thereby increasing the yield of product - The removal of a reactant from a system at equilibrium will cause equilibrium to shift to the left to create more of the reactant removed, thereby increasing all reactant concentrations

Bronsted-Lowry

- The Bronsted-Lowry definition of acids and bases focused on the role of an acid and a base in a reaction rather than on the acidic and basic properties of their aqueous solutions. - Water does not have to be one of the reactants. - A Bronsted-Lowry acid is a proton donor and a Bronsted-Lowry base is a proton acceptor

predicting acid-base equilibria

- The bronsted lowry theory cannot predict acid-base strength, and therefore it cannot predict whether an acid-base system will favor reactants or products at equilibrium. - The table of acids and conjugate bases in your data booklet was empirically derived, based on measured strengths of acids in water. - The acids in the table are listed in order of decreasing strength. - Opposite each acid is its conjugate base, whose strength is related to the acid. - This relationship results in an ordered list of acids and bases in which the strongest acid is at the upper left and the strongest base is located at the lower right. - If the acid is above the base in the table, the forward reaction or products are favored. - If the acid is below the base in the table, the reverse reaction or reactants are favored

changed concentration of participants

- The changed concentrations of all participants are related by their mole ratios from the chemical equation - Also, for the changed concentrations, all the reactants decrease and all the products increase if the reaction or equilibrium is shifted to the right - If the shift is to the left, the opposite occurs

relative strengths of conjugate acid-base pairs

- The empirically derived table of acids and bases in your data booklet lists the position of equilibrium of acids reacting with water as in the following general reaction: --> HA + H2O H3O+ + A- - The extent of the proton transfer between HA and water determines the strength of HA. - According to Bronsted-Lowry, when a strong acid reacts with water, an almost complete transfer of protons results for the forward reaction and almost no transfer of protons for the reverse reaction. - Therefore, a strong acid has a very low attraction for its proton and easily donates it to a base. - This means that the conjugate base, A-, of a strong acid has a weak attraction for protons. - The stronger the acid, the weaker the conjugate base. - The weaker the acid, the stronger the conjugate base.

equilibrium law expression

- The equilibrium of the indicator is governed by the following equilibrium law expression: --> Ka = [H3O+(aq)][In-(aq)] [HIn(aq)]

trials and results of a titration

- The first trial is typically done very quickly, allowing you to determine what the endpoint looks like and the approximate volume of titrant needed - A titration should involve several trials to improve reliability of the answer - A typical requirement is to repeat titrations until three trials result in volumes that are within 0.2mL of each other - These three results are then averaged before carrying out the solution stoichiometry calculation; disregard any trial volumes that do not fall in the range

equilibrium constant, Kc compared with percent reaction

- The greater the equilibrium constant, Kc, the greater the percent reaction - Generally (that is not always), - If Kc is greater than 1, products are favored at equilibrium - If Kc is less than 1, reactants are favored at equilibrium - Note: Reactants and products that in solid or liquid state are NOT included in the equilibrium law expression as their concentrations are constant. Only gases and aqueous solutions are included in the equilibrium law expression

hydronium ion concentration

- The greater the hydronium ion concentration the lower the pH is (inverse/indirect relationship) - [H3O+(aq)] x [OH-(aq)] = 1.00 x 10-14

pH of strong acids and strong bases

- The initial pH of the sample depends on the nature of the acid or base sample. - In this case the sample is a strong acid which means that the concentration of hydronium ions will be equal to the concentration of the acid; [H3O+(aq)] = [acid] - The initial addition of a titrant (sodium hydroxide) to the hydrochloric acid sample did not produce a large change in the pH of the solution. Why? - This relatively flat region of a pH curve is where buffering action occurs. - The pH remains relatively constant even though small amounts of strong base are being added. - Remember that buffers are solutions consisting of a weak acid and its conjugate base - even though we see buffering action, the solution in the beaker is not a true buffer solution.

pH curves

- The pH curve is a method of monitoring the progress of an acid-base reaction and allows us to study the nature of the acid and base participating in the reaction. - A pH curve is a graph showing the continuous change in pH as titrant is added to the sample. - It is a graph of pH of a reaction solution vs. volume of titrant added. - The pH curves for acid-base reactions have characteristic shapes based on the nature of the acid and base participating in the reaction. - The initial pH, the equivalence point(s), and the final pH of the solution will depend on the type of acid/base reacting.

acid strength

- The reason why weak acids have weaker acidic properties s than strong acids of equal concentration is because there are fewer hydrogen ions present in the weak acid solution. - The hydrogen ion concentration of any acid solution can be calculated by multiplying the percent ionization by the molar concentration of the acid solute.

equilibrium constants for acids

- The strength of an acid can also be communicated using an equilibrium constant that expresses the extent of ion formation by the acid. - Weak acids placed in water reach a state of equilibrium and the equilibrium law can be used to calculate the equilibrium constant, known as the acid ionization constant, Ka. - Ionization constants can be found on page 8 & 9 in your data book. --> Ka = [H3O+(aq)]2 [WA]

volume of titrant and final pH in titration

- The volume of titrant needed to neutralize the sample (to reach the equivalence point) is determined from the moles of the sample reacted and on the molar ratio (stoich!). - The final pH reached will depend on the concentration of the base added and the total volume of the final solution (no need to worry about how to find this).

position equilibrium

- There are two ways of describing the position of equilibrium - Both methods describe the position of equilibrium as a numerical value that holds true for the reaction in question for any given initial concentrations as long as the temperature remains CONSTANT - If the temperature at which the reaction is studied is changed, both the percent reaction and the equilibrium constant are no longer valid

partial ionization of water and ionization constant

- This partial ionization indicates that the equilibrium at SATP greatly favors the reactant side of the equation which means that the equilibrium constant for water, called the ionization constant for water, Kw, is very small: - Because hydronium ions and hydroxide ions are formed in a 1:1 ratio, the concentration of these ions in pure water and neutral solutions is equal

chemical reactions

- Up until now we have assumed that all chemical reactions go to completion - The reactants get used up and turned into products until one or both run out - We write these reactions as --> A + B C + D - We assume that there is no back reaction

communicating acid/base concentrations

- pH and pOH - Because of the tremendous range of hydronium and hydroxide ion concentrations, scientists rely on the pH scale (power of hydrogen) and the pOH scale (power of hydroxide) for communicating concentrations. - Expressed as a numerical value without units, the pH of a solution is the negative of the logarithm to the base ten of the hydronium ion concentration: --> pH = -log [H3O+(aq)] and pOH = -log [OH-(aq)] - Note: Only the digits following the decimal point in the pH/pOH value are significant!!

pH scale

- pH's of substances are arranged on a scale from 1-14 where 7 is neutral, >7 is basic and < 7 is acidic - Changes in pH can be deceptive. Adding vinegar to pure water might change the pH from 7 to 4. While the change of 3 pH units may not appear significant, the change in hydronium ion concentration is 1000 time larger or 103

factors that do affect equilibrium constant

- temperature