Name: __________________________________
CHEM 1212 Module 8 Reading Guide
Le Chateliers Principle
A system at ______________ has forward and reverse reactions taking place at ______________ rates and has reaction quotient, Q, ______________ to the equilibrium constant, K. If an equilibrium system experiences a change in conditions that affects the forward and reverse reaction rates differently, then the ______________ are no longer equal, Q will no longer equal ____, and the system is not at ______________. The system will subsequently experience a net reaction in the direction of greater rate (a shift) that will re-establish the equilibrium. Comparing the value of Q to K allows prediction of the ______________ the reaction will shift in order to return to equilibrium. These observations are summarized in Le Châteliers principle: if an equilibrium system is stressed, the system will shift in response to the stress to relieve the stress and reestablish equilibrium.
Either removing a product or adding more of a reactant result in a Q value ______________ K. The equilibrium will shift ______________ until concentrations are achieved that have Q equal to K again. ______________ a product or ______________ a reactant will result in a Q value greater than K and the reaction will shift ______________. In all of these cases, the ending concentrations of reactants and products will be different to the amounts present before the stress was applied, and the value of the equilibrium constant will be ______________.
KP is calculated using partial pressure, which is analogous to concentration. Changing the ______________ of the system or the total pressure will result in changes in concentration to both sides of the equation. This changes the position of equilibrium, but not the value of K, if ______________ are involved in the reaction, and then only when the chemical reaction produces a change in the total ______________ of gas in the system. Increasing the pressure or ______________ the volume result in a shift toward the side of the equation that has ______________ moles of gas. ______________ the pressure or increasing the volume results in a shift toward the side with the ______________ number of moles of gas. If the number of moles of gas is ______________ on both sides of the reaction, changing pressure or volume will not affect the reaction.
Changes to concentration or pressure affect the value of ______ but do not change the value of _____. The equilibrium constant for a reaction changes only if the temperature changes. When an equilibrium shifts in response to a temperature change it is re-established with a different relative composition that exhibits a new value for the equilibrium constant. This is explained by the dependence of the equilibrium constant on the rate constants of the forward and reverse reactions. Since the forward and reverse reactions typically experience different impacts from the change in temperature, the position of equilibrium will change. Considering the ______________ value can allow prediction of the effect caused by change in temperature. Raising the temperature of a system ______________ the energy of the system. The reaction will shift in the direct that uses up energy. For a reaction that is endothermic in the forward direction, increasing the temperature will ______________ the amount of products made. For an exothermic reaction, a temperature increase will result in a shift toward the ______________ so the extra energy can be pulled into bonds and the systemic stress removed. Cooling the system favors product formation in an ______________ reaction.
Addition of a ______________ does not affect the equilibrium position or constant for a reaction. Catalysts lower the ______________ of both the forward and reverse reactions, which causes the equilibrium to be established more ______________.
Practice Problems
Answer the questions about the following reaction that occurs at high temperature:
55. C(s) + H2O(g) ? CO(g) + H2(g)?K = 2.6 at 1000 K ?Hrxn = 125 kJ/mol
a. How will the reaction shift if carbon is removed from the reaction?
b. If the reaction is carried out in a sealed vessel, how will the reaction position shift if the volume of the vessel is reduced?
c. If the reaction is carried out at 800 K instead, will K change? Explain.
Acid Theory
Molecular substances that have hydrogen atoms that can ionize typically have the H written at the front of the formula where you would typically expect to put a metal cation. Any other hydrogen atoms are written after other elements such as carbon. Ex. HC2H3O2
According to the Arrhenius definitions of acid and base, an acid is a compound that ______________ the H+ (or hydronium, H3O+) concentration when dissolved in water, and a base is a compound that increases the _____ concentration.
In the BrønstedLowry definitions of acids and bases, an acid is a species that ______________ a ______________, and a base is a species that ______________ a ______________. This is the definition of acid/base we will use most commonly and it includes a larger number of compounds than the Arrhenius definition. In the BrønstedLowry theory, every reaction that includes an acid must also include a base because if there is a proton donor, there must also be a proton ______________. Some substances, like water, can function as either an acid or a base depending on what they are reacting with and are labelled as ______________. Once the proton is exchanged between an acid and base, the remaining parts of the molecules are labeled a conjugate base and conjugate acid, respectively. When a weak acid donates a proton, the remaining part of the acid is the ______________, which can now accept a proton. When a weak base accepts a proton, it becomes the ______________ of that base and can now donate a proton.
Practice Problems
10. Classify each species in each reaction as a Brønsted acid or Brønsted base, and identify the conjugate pairs.
HNO2(aq) + H2O(l) ? NO2?(aq) + H3O+(aq)
C5H5N(aq) + H2O(l) ? C5H5NH+(aq) + OH?(aq)
19. Write formulas for the conjugate acid and the conjugate base of each of the following.
a. HPO42?
b. H2PO4?
Acid/Base Strength
Acid strength determines to what extent an acid reacts with water to form ______________ ions. Strong acids and strong bases ionize ______________, so their ionization reactions are written with a normal, right-pointing arrow. The concentration of H3O+ in a strong acid solution is equal to the ______________ of the acid. The concentration of OH? in a strong base solution is either equal to the initial concentration of the base, or in the case of a dibasic strong base, equal to ______________ the initial concentration of the base. An acid, whether it is strong or weak, can be either concentrated or ______________.
Strong bases: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Ba(OH)2, Sr(OH)2
Strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO4
A strong acid exists entirely as ions in solution, whereas a ______________ exists mostly as intact molecules. Since weak acids and weak bases ionize only partially, their ionization reactions are written with ______________ arrows. The concentration of H3O+ or OH? in a weak acid or weak base solution is ______________ the initial concentration of the acid or base.
The stronger the weak acid or weak base, the weaker its conjugate base or conjugate acid, and the weaker the weak acid or weak base, the ______________ its conjugate base or conjugate acid.
Practice Problems
6. Write the ionization reaction for each of these bases.
a. KOH
b. CH3NH2
Calculation of pH
______________ of water results in there always being some hydronium ions and some hydroxide ions present in every aqueous solution, whether it is acidic, basic, or neutral.
In any aqueous solution, [H3O+][OH?] =______________ at 25°C. This constant is known as the water ionization constant, Kw. For pure water the concentration of hydronium and hydroxide are ______________ and have a value of 1×107 M. A solution is ______________ if it contains equal concentrations of hydronium and hydroxide ions; acidic if it contains a ______________ concentration of hydronium ions than hydroxide ions; and basic if it contains a lesser concentration of hydronium ions than ______________ ions.
For numbers that may span a very wide range of orders of magnitude (such as concentration of H+) it is helpful to compare them on a logarithmic scale. A popular scale is based on the p-function, where p means to take the base-10 log of the quantity of interest. pH is therefore an efficient way to express the concentration of H3O+ in solution, where pH = ______________. If the pH is known, [H3O+] can be calculated by taking the inverse log of the negative pH: [H3O+] = 10pH. A pH < 7 means the solution is ______________, a pH > 7 means the solution is ______________, and a pH = 7 means the solution is neutral. Since the autoionization constant Kw is ______________ dependent, these correlations between pH values and the acidic/neutral/basic adjectives will be different at temperatures other than 25 °C.
One can also apply the logarithm to both sides of the equation [H3O+][OH?] = 1.0×10?14, giving the expression: pH + pOH = ______. Similar to pH, pOH = -log[OH-]. These relationships can be used to go from a measured pH value to the amount of either H+ or OH- ions in a solution or to predict the pH from a concentration value. pH can be measured using a range of indicators or more precisely using a pH meter.
Practice Problems
30. Classify the following solutions as acidic, basic, or neutral at 25°C.
a. pH = 13.00
b. [H3O+] = 1.0 × 10?4 M
c. pOH = 8.00
d. [OH?] = 1.0 × 10?7 M
Text segments from OpenStax, Chemistry. OpenStax CNX. Sep 15, 2020 http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@13.1.;
White, J. et al. Interactive General Chemistry. MacMillan, NY. 2019.;
Chemistry LibreTexts, Libretexts, [online] Available fromhttps://chem.libretexts.org/Courses/University_of_Arkansas_Little_Rock/Chem_1403%3A_General_Chemistry_2/Text/16%3A_Acids_and_Bases/16.02%3A_Water_and_the_pH_Scale
Reading guide style Adapted from Ionic equations Study Guide by Montgomery College is licensed under CC BY 4.0
Document not to be reposted on the internet without express permission.
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