Flashcards in Unit 3 AOS 2 Deck (125):
Electrolysis is used for (4)
extracting metals, making jewellery, protecting metal, recharging batteries
process in which a current is used to make a non-spontaneous redox reaction occur.
How does electrolysis work
electricity passes through a molten ionic compound or through an electrolyte, external power source forces electrons to move in the reverse direction so reactions are reversed
liquids that can conduct electricity
Where do positive ions gain electrons
Where do negative ions lose electrons
Define electrolytic cell
An electric cell in which a non-spontaneous redox reaction is made to occur by the application of an external potential difference across the electrodes.
Features of an electrolytic cell
electrolyte solution with free-moving ions which allow electrons to flow through external circuit, two electrodes, external source of electrons
What type of current is the electron flow
Polarity of cathode in electrolysis
Polarity of anode in electrolysis
What occurs at anode and cathode (reduction/oxidation)
reduction at cathode, oxidation at anode
Where are cations and anions attracted
cations to cathode, anions to anode
Why is cathode negative
the external DC source forces electrons onto it.
Why is the anode positive
The anode is positive because the DC source withdraws electrons from it.
How are polarities determined in galvanic cell
Factors affecting the electrolysis (3)
concentration of electrolyte, nature of the electrolyte, the nature of the electrodes
What happens to E naught when conditions aren’t standard
the standard the E naught values change
Why is electrolysis important
electrolysis provides the only practical way to prepare many metals
What reactive gases are prepared through electrolysis
fluorine, chlorine, sodium hydroxide
thin coating of metal is applied to more common base metal through electrolysis
Why is electroplating used
to make metal durable, protection from corrosion, attractive
Factors affecting quality of metal coating
concentration of cations to be reduced, unwanted side reactions must be avoided, type and concentration of electrolyte needs careful consideration, solution must contain compounds to control the acidity and increase the conductivity, it needs to be considered whether the metal coating makes colour brighter and smoother, in many electroplating cells the anode must be shaped like the object at the cathode to achieve an even metal coating.
What might happen in electroplating if conditions aren’t satisfactory
metal coating powdery and drops off
Why must cleaning acids and bases be replaced
Name for when using electrolysis to extract metal
What is Hall-Heroult method used for
aluminium production using molten aluminium
Why can’t the aluminium be aqueous
water would be reduced
electrolysis of brine (aqueous NaCl) - to produce Cl, H gases and NaOH. Semi-permeable plastic membrane separates anode half-cell from cathode half-cell, and traps chloride and water but not sodium and hydroxide.
Faraday’s 1st law of electrolysis
the amount of any substance deposited, evolved or dissolved at an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the cell.
What is an ammeter
measures rate at which charge flows in a circuit
How many electrons per Q
A constant that represents the amount of electric charge carried by 1 mole of electrons.
96500 C mol-1
Faraday’s 2nd law of electrolysis
to produce 1 mol of a substance by electrolysis, a certain number of moles of electrons (Faraday’s) must be consumed according to the relevant half equations
What is a lead-acid accumulator
A battery with lead electrodes using dilute sulfuric acid as the electrolyte; each cell generates about 2 volts.
Forcing electrons to travel in the reverse direction and, because the discharge products are still in contact with the electrodes, the original reactions are reversed.
What is a lithium ion cell
A battery where lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.
What happens to lithium ion cells at high temp
Most common secondary cell
is lead-acid battery
Silver and gold are… (poor or good conductors)
How to find Kc when reaction reversed
How to find Kc when reaction coefficient doubled
How to find Kc when reaction coefficient halved
square root Kc
Formula to find charge
What does Q stand for
charge in C
What does I stand for
current in amps
What does t stand for
time in seconds
Formula for moles and Q
Factors affecting battery life (4)
increase in temperature, overcharging (may not fully recharged/discharged, or reduce insulation - short circuits and danger of explosion), products of discharge fall from electrodes and when no contact reverse reaction cannot occur, corrosion due to acidic environment or constant use
What does molten mean
ions are separated
Can you electrolyse water
can’t unless salt dissolved in water
Positions of strongest oxidant/reductant
strongest oxidant - top right, strongest reductant - bottom left
How to determine electrolysis equation
pick strongest oxidant and reductant
If a solution is of high concentration (such as >6M)…
the ion will be more likely to react.
A reaction where a chemical equilibrium is reached. This is the state in which both reactants and products are present in concentrations that have no further tendency to change with time.
all species have same states
species have different states
The equilibrium law
The relationship between the concentrations of the products and the reactants, taking into account their stoichiometric values.
The value of the concentration fraction at equilibrium; also called the equilibrium constant, Kc. It allows us to describe the position of the chemical equilibrium quantitatively.
The reaction quotient Q
is the measure of the concentrations of the products and reactants at any particular time. Ratio of concentrations of the products divided by the concentrations of the reactants.
What is M
If the value is high
(greater than 10^4) large extent, favours forward
If the value is small
(less than 10^-4) small extent, favours backwards
Only factor that affects Kc
Why is temperature the only factor that affects Kc
changing energy available to system
What is extent (3)
the degree to which reactants are converted into products. How far the reaction goes. Some reactions will occur completely, some will never be complete, how much product is formed when the system reaches equilibrium.
What is rate of a reaction
an indication of how fast it occurs. The rate shows how long it takes to establish the position of equilibrium.
Why doesn’t catalysts affect Kc
alters the time taken to get to equilibrium, not the position of it.
Why we need reaction quotient
for slower equations. AKA concentration fraction.
If Q is bigger than Kc… (is the reaction net forwards or backwards)
net back reaction
Le chatelier’s principle
When a change is made to an equilibrium system, the system moves to counteract the imposed change and restore the system to equilibrium.
How equilibrium can be disturbed (3)
adding/removing substance involved, changing volume at constant temp, changing temp
Adding/removing reactants effect on equilibrium
concentrations will be different from original values, but value of equilibrium constant unchanged
Shift to right means
forward reaction favoured
Removal of reactants/products can be achieved through what types of means?
physical or chemical means
Concentration of solids and liquids for Q equation is...
How does volume that increases concentration affect equilibrium
to decrease this, the system reacts in the direction that produces fewer particles.
How does volume that decreases concentration affect equilibrium
to increase this concentration, the system must react in the direction that produces more particles if it is to re-establish equilibrium.
Increase in volume example
Increase temperature effect on Kc (exo+endo)
exothermic backwards favoured and Kc decreases, endothermic forwards favoured, and Kc increases
Decrease temperature effect on Kc
exothermic forwards favoured, and Kc increases, endothermic backwards favoured and Kc decreases,
What does change in temp look like on concentration-time graphs
effect on concentration without sudden change
What is yield
amount of product
How to calculate yield
actual mass obtained/theoretical maximum mass * 100
Other reasons for not 100% yield
practical how it is done, slow reaction
The oxygen-carrying pigment and predominant protein in red blood cells.
2 types of haemoglobin
Why does carbon monoxide replace O2
much more extent of reaction of carbon monoxide
What is a reversible reaction
reaction that can go both forwards and backwards - reactants form products and products can form the same reactants
What is an irreversible reactions
reactions that can only go forward.
What is an open system
exchange energy and matter with their surroundings
What is a closed system
only exchange energy with their surroundings.
Why concentration doesn’t affect equilibrium constant
By changing the concentration, we simply only redistribute the energy available.
Why is CO poisoning fatal
can’t transport oxygen to cells
More energy a collision has means
the more dramatic are the changes produced
What is activation energy
The minimum energy required by reactants in order to react
Where can the activation energy come from
can come from thermal energy, kinetic energy of colliding particles.
What is change in enthalpy + what is it denoted by
The amount of energy released or absorbed in a chemical reaction. Denoted by delta H.
Define exothermic reaction
Describes a chemical reaction in which energy is released to the surroundings. For exothermic reactions, the activation energy is less than the energy released when new bonds form. Consequently, there is a net release of energy (usually as heat released to the surroundings).
Define endothermic reaction
Describes a chemical reaction in which energy is absorbed from the surroundings. For endothermic reactions, the activation energy is greater than the energy released when new bonds form. Consequently, there is a net input of energy (in most cases, heat is absorbed from the surroundings).
What 3 things are in the collision theory
particles must collide, have the correct orientation for bond breaking to occur, have sufficient energy for the reaction to occur.
How chemical reactions rates are measured
volume of gas evolved, mass of solid formed, decrease in mass due to gas evolving, intensity of colour of solution, formation of a precipitate, pH, temperature.
A Maxwell–Boltzmann distribution curve shows…
the number of particles with a particular energy graphed against the value of that energy. During a reaction at a given temperature, only a small proportion of the reactant particles have kinetic energy that is equal to or greater than the activation energy and so are able to react. The particles in a sample have a wide range of kinetic energies. As kinetic energy is given by the formula 1/2mv^2, this means that there is also a range of velocities. This is due to the collisions that the particles are constantly undergoing.
Is a Maxwell–Boltzmann symmetrical
It is not symmetrical.
What does the highest point represent in a Maxwell–Boltzmann graph symbolise
The highest point represents the most probable velocity, however this is not the same as the average velocity.
What does the area under the Maxwell–Boltzmann graph represent
The area under the graph represents the total number of particles in the sample.
The effect of increasing the temperature on a Maxwell–Boltzmann graph
is to ‘stretch’ (not move) the graph to the right. As a result, there are more particles with higher kinetic energies and on average they all move faster and the average kinetic energy is higher.
Factors that affect the rate of a reaction
concentration, pressure, temperature, catalysts, catalytic converters, the effect off surface area,
Increasing concentration results in what for a reaction
an increased frequency of successful collisions during any given period of time and hence a higher rate of reaction.
Increasing pressure results in what for a reaction
more crowding together of the particles and hence more successful collisions within a certain time.
Increasing the temperature means what for a reaction
there are more particles with enough energy to overcome the activation energy barrier. Another effect of increasing the temperature is that there is an increased frequency of collisions due to the particles moving faster. However, a more sophisticated analysis of the situation reveals that this is secondary to the effect of the energy distributions mentioned above.
A substance that alters the rate of a reaction through providing an alternative reaction pathway with a lower activation energy which increases the proportion of collisions with energy greater than the activation energy; without a change in its own concentration
What is a catalyst called if it is used to slow down reaction
negative catalyst or inhibitor
Does catalyst alter delta H
Example of biological catalysts
How are catalytic converters used
How does surface area affect reaction
by increasing the surface area, more of a substance is brought into contact with other substances with which it might react. In terms of collision theory, an increase in surface area means that more reactant particles can collide with one another and, therefore, there are more successful collisions between them in a given period of time. This leads to an increased rate of reaction.
What are the two types of catalysts
homogenous catalysts, heterogenous catalysts
What are homogenous catalysts
a catalyst with the same state as the reactants
What are heterogenous catalysts
a catalyst with a different physical state to the reactants
How to increase collisions that occur in reaction
temp, concentration, SA
How to increase proportion of particles reacting
At absolute zero for Maxwell-Boltzman graph
graph at 0