Developing metals Flashcards
What is oxidation?
A loss of electrons
What is reduction?
A gain of electrons
What is a reducing agent?
Donates electrons
What is an oxidising agent?
Takes/accepts electrons
Does an oxidising agent lose or gain electrons during a redox reaction?
Gain
Does a reducing agent lose or gain electrons during a redox reaction?
lose
Apart from multiplying up the reactants and products, name three things you can add to balance a half equation.
H+ ions
Electrons
Water
What are acid base titrations used for?
To find how much acid is needed to neutralise a base
In an acid base titration what goes into the burette?
The acid
What is the purpose of a redox titration?
How much oxidising agent is needed to react with a quantity of reducing agent
What is a common oxidising agent used in redox titrations?
manganate(VII) ions
How do you do a redox titration?
1) Measure quantity of reducing agent using pipette and put in conical flask
2) Add dilute sulfuric acid in excess to conical flask
3) Add oxidising agent to the burette
4) Swirl the conical flask as you add it
5) Stop when you reach end point
6) repeat until concordant titrations
In a redox reaction what goes in the burette?
The oxidising agent
Why do you add dilute sulfuric acid to the conical flask in a redox titration?
The acid is added to make sure there are plenty of H+ to allow the oxidising agent to be reduced
What is the colour change in a redox reaction using potassium manganate(VII)?
colourless –> pink end point
What are the reactions in an electrochemical cell?
oxidation and reduction
What reaction happens at the anode in an electrochemical cell?
oxidation
What reaction happens at the cathode in an electrochemical cell?
reduction
What does an electrochemical cell consist of?
two different metals dipped in salt solutions of their own ion and connected by a wire and a salt bridge
What direction do electrons flow through a wire in an electrochemical cell?
from the most reactive metal to the least reactive metal
What is cell potential?
The voltage between the two half-cells
EMF Ecell
What electrode do you need when the half cell consists of two aqueous solutions of the same element?
Needs to conduct electricity but be very inert
eg.platinum or graphite
How do you set up an electrochemical cell?
1) Clean surface of strip of metals you’re using as electrodes with sand paper
2) Clean electrodes using propanone
3) Place electrode into beaker filled with solution containing ions of that metal
4) Create a salt bridge by dipping a piece of filter paper in salt solution and then drape it between the two beakers
5) connect electrodes with voltmeter crocodile clips and wires
Does the reaction go forwards at the half cell if the electrode potential is more or less positive?
more positive
What happens at the half cell with a more negative electrode potential?
it is oxidised so goes backwards
What are standard conditions for electrode potentials?
298K
100kPa
1.00 moldm^-3
What are electrode potential measured against?
Stand hydrogen electrodes
What is standard electrode potential?
Voltage of half cell measured under standard conditions when half cell is connected to a standard hydrogen potential
What is the equation for Ecell?
Ecell = (E more positive) - (E more negative)
Do more reactive metals have more negative or positive electrode potentials?
more negative
Why do more reactive metals have more negative electrode potentials?
more likely to lose electrons to form a positive ion
Do less reactive metals have more negative or positive electrode potentials?
more positive
Why do less reactive metals have more positive electrode potentials?
more likely to gain electrons to form a negative ion
How can you tell if a reaction is feasible, at standard conditions, using electrode potentials?
Use them to calculate Ecell
if Ecell is positive it is feasible if it is negative it is not
What is the voltage of the standard hydrogen electrode half-cell?
0.00V
Why is rust less likely under alkaline conditions?
OH- ions added means
2(H2O) + O2 +4e- –> 4OH-
is shifted to left to so more electrons produced so
Fe2+ + 2e- –> Fe
Shifts to the right to balance this so less Fe2+ to form Fe(OH)2
What are the half equations in the start of rusting of Fe?
Fe2+ + 2e- –> Fe
2(H2O) + O2 +4e- –> 4OH-
What equations show how Fe2+ react to make Fe(OH)3 in the process of rusting?
Fe2+ +2OH- –> Fe(OH)2
2(H2O) + O2 + 4Fe(OH)2 –> Fe(OH)3
What is rust?
Fe2O3 xH2O
What was rust before it became hydrated iron(III) oxide?
iron(III) hydroxide
Fe(OH)3
How do you prevent rusting?
Barrier
Sacrificial metal
What are two examples of the barrier method of rust prevention?
- Coating with a polymer (Painting)
- Oiling or greasing
When would you use oiling or greasing to prevent rust?
When moving parts are involved
What is the sacrificial method?
Placing a more reactive metal with the iron so the water and oxygen react with that metal instead
What sacrificial metal is most often used? Why?
Zinc
It has a more negative electrode potential than Fe so it will be oxidised instead
What is galvanising?
Coating something zinc
What two substances react with iron to produce rust?
Water and Oxygen
What is a complex ion?
A central metal ion surrounded by datively bonded ligands
What is a dative covalent bond?
Both shared electrons come from the same atom
In a complex where do the shared pair of electrons in the dative bond come from?
The ligands
What is a ligand?
Something that donates a pair of electrons to a central transition metal ion to form a dative bond
What is a monodentate ligand?
Has one lone pair of electrons
What is a bidentate ligand?
Has two lone pairs of electrons
What is a polydentate ligand?
Has multiple lone pairs of electrons
What is are four examples of monodentate ligands?
H2O
NH3
CN-
halide ions
What is an example of a bidentate ligand?
ethanedioate
What is the coordination number?
The number of dative bonds formed with ligand
What shape would a molecule with a coordination number of 6?
Octahedral
What two shapes would a molecule with coordination number of 4 have?
Tetrahedral
Square planar
What size bond angles does an octahedral complex have?
90 degrees
What size bond angle does a tetrahedral shape have?
109.5 degrees
What size bond angle does a square planar shape have?
90 degrees
How do ligands affect the 3d sub-shell of transition metal ions?
Split it into two energy levels because some orbitals gain energy
When will the compound look colourless or white? why?
If the 3d sub shell is full or empty
because the electrons won’t jump up to higher orbitals
What do you mix with aqueous transition metal ions for them to become coloured?
NaOH or NH3 to make a coloured hydroxide precipitate
What is ligand substitution? what does it cause?
One ligand being swapped for another
a colour change
Why do transition metal compounds make good catalysts
They can change oxidation states by gaining or losing electrons in their D orbitals so they can transfer electrons to speed up reactions
Why can transition metal compounds cause health risks?
Their compounds can be toxic
Stages of heterogeneous catalysis
- Adsorption
- Bonds in molecule break
- New bonds form
- desorption
What makes a good heterogeneous catalyst?
Attract the reactant strong enough so it is held to the surface long enough to bond but not too strong it won’t desorb
What is a heterogeneous catalyst?
A catalyst in a different state to the reactants
What is a homogeneous catalyst?
A catalyst in the same state as the reactant
How does a homogeneous catalyst work?
Combine with reactants to form intermediate species which then reacts to form products and reform the catalyst
What happens to white light that makes a transition metal ion colour?
One frequency of light is absorbed
the rest are transmitted (the colour you see)
How do you set up a colorimeter?
- Pick a filter that is a colour that’s absorbed
- Set zero using a blank sample
- Place sample in a cuvette and measure
What does a high absorbance on the colorimeter mean?
Very concentrated
What is monochromatic light?
light of a single colour
