Acids & Bases, Electrochemical Cells, Transition Metals, Aqueous ions Flashcards
How does the body ensure the concentration of the weak acid in blood doesn’t get too high?
- The carbonic acid is broken down, forming CO, which is exhaled via the lungs.
Why is a Burette preferred over a measuring cylinder when doing titrations
A burette is more precise
What is the difference between a Strong Base and a Weak Base?
The difference between a Strong Base and a Weak Base is their concentration of hydroxide ions (OH-) in water. Strong Bases have a high concentration of OH- ions and Weak Bases have a low concentration.
What is the difference between a Strong Base and a Weak Base?
The difference between a Strong Base and a Weak Base is their concentration of hydroxide ions (OH-) in water. Strong Bases have a high concentration of OH- ions and Weak Bases have a low concentration.
State why an indicator may not be required for a titration
the reactants are self-indicating
Would the pH of solution containing Mg(OH)2 have a differing pH to Ca(OH)2 and why?
Smaller/Lower pH because magnesium is less/sparingly soluble.
why is the pH probe washed with distilled water between each of the calibration measurements
to prevent contamination
Why is the volume added of a selected acid or base reduced between each pH measurement during and experiment?
To avoid missing the end point
What is a half-cell?
A half cell is one half of an electrochemical cell. They are constructed of a metal dipped in its ions, or a platinum electrode with two aqueous ions.
How do we make half cells with ions with no solid counterpart, or with two or more ions?
If there is a half cell with 2 aqueous ions we must use an inert but electrically conductive electrode e.g Platinum
What does an electrode potential show?
electrode potentials (V) show how easily the half cell gives up electrons (oxidation)
Define a Transition Metal
- A transition element is a d-block element that can form at *least$ one stable ion with a partially filled d-subshell
When given a single metal rod and solution of ions of the same chemical species, why may it be difficult to measure the potential difference and thus the reducing ability of the metal rod?
- You cannot measure the potential of a single half cell, we can only measure the potential difference between two different half cells
Redox Couple
A redox couple is the combination of two forms of the same chemic species separated by the loss or gain of electrons so that they have two different oxidation states
- the position of equilibrium will vary for different redox couples
Redox Equilbirum
- A dynamic redox equilibrium gets established when the rate of electron loss equals the rate of electron gain
Electrode Potential
- At the equilibrium, the electrons on the metal strip set up a potential between the metal and the ions in solution
- the potential is an indication of how easily the metal loses electrons. The greater the tendency for the metal to lose electrons, the greater the potential
- the potential of a single half cell cannot be measured but you can measure the potential difference of two half cells
Types of Half Cells
Metal/metal ion
metal rod dipped in a solution of one of its ions
Non-metal/non-metal ion
- a platinum or graphite (note the graphite gets weaker over time) is dipped into the non-metal ion solution
- the non-metal gaseous element can then be bubbled over the electrode
-the electrode has a dynamic equilibrium established on its surface (due to being in contact with the element and aqueous ions.)
Ion/Ion
- the half cell contains a solution of two different ions if the same element. A platinum or graphite rod is used as the electrode.
Electrochemical Cell Structure
- two half cells
- salt bridge, completed electrical circuit, typically filter paper soaked in a salt solution i.e. potassium nitrate, it allows ions to flow
- salt bridge mitigates the effects of an increasingly positive anode and increasingly negative cathode
- It does this by using it’s inert ions to move and balance the charges to keep the cell working
The voltmeter needs to have a very high resistance to prevent a current from flowing so that voltage can be measured
- The ions within the salt bridge have been exhausted
- The being oxidised has completely thinned out/run out
- a measure of the voltage generated by a half-cell in an electrochemical cell
- it is a measure of the energy that is generated by the movement of ions between the half-cell and the solution it is immersed in.
Explain how a Potential Difference is produced between to half cells
- The “left-side” electrode where oxidation of a metal occurs will (due to the position of equilibrium being to the right) release electrons as the metal is oxidised to it’s ions
- the influx of ions in the solution will create a potential difference with the other electrode, allowing the electrons to cross between them to the other electrode
- the metal ions of the “right-side” electrode will be reduced, forming more of its metal constituent
- this continues until the lefthand electrode is highly positive and right hand electrode highly negative which stops the flow of these electrons
- to reverse this effect the charged ions of the salt bridge will cancel out these charges.
- The more positive the value of the standard electrode potential, the greater the tendency for reduction, they are strong oxidising agents
-The more negative the value of the standard electrode potential, the greater the ability of the elements to be oxidised, that are strong reducing agents
e.g.
Factors Affecting Electrode Potentials
- changes in temperature, pressure or concentration will effectively change the position of the redox equilibrium and thus the electrode potential of the half-cell
- If the position of equilibrium is shifted in favour of the forwards reaction (the reduction),the electrode potential becomes more positive
- if instead the position of equilibrium is shifted in favour of the backwards reaction (the oxidation), the electrode potential becomes more negative
- therefore the cell that is most likely to be oxidised is the one with the most negative standard electrode potential.
Double vertical solid line - Salt Bridge
Vertical Solid Line - phase boundary, e.g between an aqueous solution and a solid
- Species with eg HIGHEST oxidation state is written closest to the salt bridge
- The half cell with the more negative potential goes to the left
- platinum electrode is used when no solid electrodes are in the half cell
Calculating Standard Electrode Potential
- electrons flow from the more negative electrode to the more positive electrode
- causing an EMF
EMF = E⦵ (reduced) – E⦵ (oxidised)
or
EMF = E⦵ (right electrode) – E⦵ (left electrode)
What directions to electrons flow in an electrochemical cell
- The electrons flow from the more reactive element (most negative electrode potential) to the less reactive element (metal with the most positive electrode potential)
How can Standard Electrode Potentials be used to predict Reaction Feasibility?
- A positive overall EMF indicates that a reaction is feasible
- however this prediction is only correct under standard conditions
- to determine whether a reaction is likely to occur reaction conditions must also be considered
- i.e a reaction may be feasible in terms of EMF but have a very high activation energy.
Advantages and Disadvantages of Non-Rechargeable Batteries
ADVANTAGES
Cheaper
Easy to use and has a wide range of Applications
Portable and easy to replace
DISADVANTAGES
once the reactants have been used up, the battery must be disposed of
toxic chemicals can leach from landfill sites into water sources
the casing of the battery can be corroded by the electrolyte, cussing leakages
Non-sustainable - the materials to make batteries are finite
ADVANTAGES AND DISADVANTAGES IF RECHARGEABLE BATTERIES
ADVANTAGES
- have a longer life-span than non-rechargeable batteries
- Can be recharged multiple times
- more energy efficient as chanting takes less energy than making new batteries
DISADVANTAGES
Cost more than non-rechargeable batteries
Need to be regularly recharged
Not all appliances are suitable to be used with rechargeable batteries
Advantages and Disadvantages of Hydrogen-Oxygen Fuel Cells
ADVANTAGES
only reaction product is water
all bond energy is converted in to electrical energy (more efficient than conventional engines)
no pollutants
can produce enough electricity to run vehicles which are much lighter and smaller than conventional engines and batteries
DISADVANTAGES
hard to store Hydrogen gas (needs to be under high pressure)
highly flammable/explosive
hydrogen is obtained via electrolysis (needs fossil fuels)
hydrogen is also obtained directly from fossil fuels
low energy density (for the same volume of other fuels hydrogen produced less)
Reaction occurring within Lithium Cells
- Rechargeable Batteries
How do Rechargeable Batteries work?
- when the battery is being used it operates like your standard non-rechargeable battery where a reactive electrode releases electrons which reduce ions at another electrode
- when being recharged, the reverse reactions occur and electrons flow in the opposite direction from the electricity supply to the e.g. lithium ions.
Fuel Cells
- a fuel cell produces electricity by using a fuel on the positive electrode and an oxidant on the negative electrode. They react in the presence of an electrolyte
- as long as there is a continuous supply of fuel, the cells can operate continuously
- most common in automotive industry
e.g Hydrogen-Oxygen fuel cell
pic
- Hydrogen enters at the negative electrode and releases electrons (oxidised by hydroxide ions to water)
- The electrons flow through the external circuit
(electrons flow to positive electrode) - The electrons are accepted and released hydroxide ions
(electrons are accepted by the oxygen entering at the positive electrode) - The hydroxide ions travel to the negative electrode
(hydroxide ions travel through the semi permeable membrane to negative electrode) - cycle repeats
What’s the Overall Equation for a Hydrogen-Oxygen Fuel Cell
What is an electrolyte
- an ionic compound that is melted or dissolved in water
Acid-Base Indicators
- acid-base indicators act as weak acids where the unionised acid and its conjugate base have different colours
- the end point of a titration is the point at which an indicator changes colour
- though due to indicators working of small ranges of pH, it may not be exactly at the equivalence point.
Factors Influencing Choice of Indicators
The colour change occurs at the equivalent point
- the colour change must occur at the equivalence point
The indicator has a narrow pH range
- the indicator chosen needs to cover a narrow pH range
The indicator shows a sudden distinct colour change
- only a few drops needed for a distinct colour change at neutralisation point
What are some Suitable Indicators for each Acid-Base Strength pairing?
(you do not need to know the indicators to detail)
Acid: STRONG Base: STRONG
pH range at equivalence point - 3-11
suitable indicators - ANY
Acid: WEAK Base: STRONG
pH range at equivalence point - 7-11
suitable indicators - phenol red(6-8), thymol blue(9-10), phenolphthalein (8-10)
Acid:STRONG Base:WEAK
pH range at equivalence point - 3-7
suitable indicators - methyl orange (3-4), methyl red (4-6)
Acid:WEAK Base:WEAK
- no clear equivalence point
- no suitable indicator
