Module 6 Flashcards
What are the strong acids?
HCl HBr HI HNO3 H2so4 HClO3 (chloric acid) HClO4 (perchloric acid)
What are the strong bases?
LiOH NaOH KOH RbOH CsOH Mg(OH)2 Ca(OH)2 Sr(OH)2 Ba(OH)2
Acid + Base
HX + MOH -> MX + H2O
Acid + Base -> salt + water
Acid + Carbonate
HX + MCO3 -> MX + H2O + CO2
Acid + Carbonate -> salt + water + carbon dioxide
Acid + Metal
HX + M -> H2 + MX
Acid + Metal -> hydrogen gas + salt
Compare the Arrhenius and Bronsted-Lowry definition of an acid and base, outlining its limitations.
Both definitions state that an acid contains H+. However, the Arrhenius definition states that a base must contain OH-. The Arrhenius definition has many limitations including that it only accounts for aqueous acids and bases, it does not explain why Ammonia is a base, and it does not explain why some neutralisation reactions are not neutral. The Bronsted-Lowry definition improves from this as it defines an acid as a proton donor and a base as a proton acceptor.
What does a large Ka value indicate?
A large Ka value indicates that the acid is strong as it completely dissociates. A large Ka value will give a low pKa value.
What does a large Kb value indicate?
A large Kb value indicates that the base is strong as it completely dissociates. A large Kb value will give a low pKb value.
What is Kw?
The Ionic product of water, the value is constant.
Kw = [H3O+][OH-] = 10↑-14
What is the relationship between Ka and pKa?
pKa = -log↓10(Ka)
Because of the nature of log scales, a large Ka means a small pKa and a small Ka means the pKa will be large.
What is the relationship between Kb and pKb?
pKb = -log↓10(Kb)
Because of the nature of log scales, a large Kb means a small pKb and a small Kb means the pKb will be large.
pKa + pKb =
= pKw = 14
Ka x Kb =
= Kw
pH + pOH =
= 14
What is a buffer and how does it work?
A buffer is an aqueous solution that resists rapid change in pH when small amounts of acid or base are added. It is made up of small and equal amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid. The equilibrium between these allows the buffer to work to resist changes in pH.
How to prepare a natural indicator
- Crush red cabbage using mortar and pestle
- Dissolve in ethanol, strain and filter to give you your indicator.
- Prepare solutions of known pH (0-14) to test indicator (this provides you with a colour change chart)
Red cabbage contains Anthocyanin pigment which allows it to be an indicator.
How does an indicator work?
Indicators are weak acid dyes that exist in equillibrium.
For example, Phenolphthalein works by the following equilibrium:
(Changes colour pH - 10)
X + OH- ⇌ XOH
Colourless Magenta/Pink
If you add OH- ⇌ shifts right (pink)
If you add H+ it reacts with OH- –> ↓[OH-] so ⇌ shifts left (colourless)
Why was the enthalpy of neutralisation less than expected?
This was due to the significant heat loss in the experiment. Since a lot of heat energy is being lost to the environment, it is not absorbed by the water, thus the end temperature is lower than it should be, and a lower energy released in the experiment is recorded.
*remember when calculating enthalpy of neutralisation remember it is exothermic.
What is an example of a neutralisation reaction in everyday life? Explain how it works.
A neutralisation reaction in everyday life includes the treatment of an insect bite. Most insect bites contain formic acid, and can therefore be treated by sodium hydrogen carbonate (or bicarb soda) which will neutralise the acid from the sting, and is not corrosive, so will not cause damage to the skin.
What is an example of a neutralisation reaction in industry? Explain how it works.
Limestone, a basic salt is used to neutralise acidic waste. It works by the following equation:
2H+ (aq) + CaCO2(s) –> Ca2+(aq) + H20(l) +CO2(g)
Describe a model you could use to distinguish between a strong and weak acid. What are its advantages and disadvantages?
Model of a Chocolate bar vs a lolly
HX + H2O –> H3O+ + X- (Chocolate doesn’t rewrap, therefor represents strong acid)
HX + H2O ⇌ H3O+ + X- (lolly rewraps, therefore represents weak acid)
Advantages of the model:
- Visualises concept/ makes it tangible
- Simplifies a complex idea for better understanding
- Represents key components/processes
- Allows for predictions
- The lolly and chocolate have equivalent parts, the sweet is the anion and the wrapper is the proton
- The parts are separate and can be separated (to represent ionisation in water)
Disadvantages:
- Can be oversimplified/lack detail
How do you prepare and test a buffer?
Control - Water
Dependent - Weak acid buffer and Weak base buffer
- Add 10 mL of a weak acid/base and 10 mL of its conjugate in equal amounts to a beaker. Use a measuring cylinder to measure. This will create an equilibrium as shown below:
CH3COOH + H2O ⇌ CH3COO- + H3O+
CH3COO- + H2O ⇌ CH3COOH + OH-
- Test the buffer by adding drop by drop a strong acid/base and recording the change in pH from the original pH.
Identify a buffer system that occurs in nature and explain its importance.
A buffer system that occurs in nature is human blood. The human body needs to maintain a certain pH range for optimal performance of its enzymes. Enzymes help catalyse all the biochemical reactions in the human body, so when pH varies extremely, the enzymes will cease to work. Without this catalyst, the reaction rate of the body’s metabolism will slow down eventually leading to death.
This buffer system works according to the following equations:
H2O + CO2 ⇌ H2CO3(aq) ⇌ H+(aq) + HCO3-(aq) ⇌ H+(aq) + CO3 2-(aq)
Methyl Orange
Use for titrations between a strong acid and a weak base (acidic salts) pH range: 3.1 - 4.4 Red > Yellow