chemsitry - energetics and electrolysis Flashcards
what is the symbol for the energy change that accompanies a reaction?
delta(change in) H
in exothermic reactions:
energy is given ? or ? when products form and the ? of the surroundings gets hotter/ increases
out, released, temperature
in exothermic reactions:
energy is taken ? when products form and the ? of the surroundings gets colder/ ?
in, temperature, decreases
Why are delta H values for Exothermic reaction negative?
As energy is released to the surroundings, so the products have less/lower energy than the reactants
Why are delta H values for endothermic reactions positive?
As energy is taken in from the surroundings, the products have more/higher energy than the reactants
what is the activation energy?
the minimum amount of energy that must be supplied in order to break the bonds in the reactants particles to initiate the reaction
old bonds between atoms in reactants are ?
broken
new bonds between atoms in products are ?
formed/made
what does bond breaking require and what type of process is this?
energy, so it is an endothermic reaction
what does bond breaking release and what type of process is this?
energy, it’s an exothermic reaction
Overall whether a reaction is endothermic or ? depends on the difference of ? needed to break old ? and the ? released in forming/? new bonds
exothermic, energy, bonds, energy, making
Exothermic reactions - when more ? is released in the formation of NEW bonds than is needed to ? old bonds. so delta H values for Exothermic ? are ?
energy, break, reactions, negative
Endothermic reactions - when less ? is released in the formation of NEW bonds than is needed to ? old bonds. so delta H values for Endothermic ? are ?
energy, break, reactions, positive
Exothermic reactions - greater than equation
energy released when new bonds are formed > energy required when old bonds are broken
Endothermic reactions greater than equation
Energy required to break the old bonds / when old bonds are broken> energy released when new bonds are formed
Energy profiles : what does an exothermic profile look like?
- have two axis - only y axis is labelled with energy
- start with a horizontal line at the top with the reactants
- draw a lower horizontal line at the bottom for the products
then draw a looping arrow up from the reactants down to the products -the peak of the arrow to the reactants should be the activation energy - the distance between reactants and products is delta H
energy profiles - what does an endothermic one look like?
- have two axis - only y axis is labelled with energy
- start with a horizontal line at the bottom with the reactants
- draw a higher horizontal line at the bottom for the products
then draw a looping arrow up from the reactants up to the products -the peak of the arrow to the reactants should be the activation energy - the distance between reactants and products is delta H
equation for delta H
sum of bonds broken - sum of bonds formed
4 steps to bond energy calculations
1 - write out symbol ? using displaced formulae showing all ?
2. work out energy needed to ? bonds
3. work out energy ? when bonds are ?
4. work out energy ? - delta ? using :
either :
• energy ? to break bonds - energy released when bonds are ?
or
• ? of bonds broken - sum of bonds ?
equation, bonds
break
released, formed
change, H
needed/required, made
sum, made
what is the alternative method for bond energy calculations?
- highlight the bonds that change between the reactants and the products
- only calculate the energy needed to break these bonds/ the energy released when new bonds are made
- then use the delta H equation to work out whether the equation is exothermic or endothermic (by the negative or positive delta H)!!!
what is the half equation for hydroxide being oxidised
4(OH-) –> 2(H20) +2(02) + 4e-
solid ionic ? do not conduct ? as there are no free ? to flow through the structure and all the charged ? are locked into a lattice ? in the solid ; the ions are not ? to move
compounds, electricity, electrons, particles/ions, structure, free
why is the cathode negative and therefore which ions go to it (and what is formed at it)?
because it has an excess of electrons, the cations go to it and so the metal element is formed at the cathode
why is the anode positive and therefore which ions go to it (and what is formed at it)?
because it has a lack/shortage of electrons (so is positive), the anions go to it and so the non-metal element is formed at the anode
given that electrons are taken from the ? and given to the cathode, there is an overall flow of ? and so an electric ? is said to flow
anode, electrons, current
when do ionic compounds conduct electricity and why
when they are dissolved in water or are in molten form as the ions are free to move
what are molten liquids and solutions (which both conduct electricity) also known as
electrolytes
how can a more reactive metal than carbon be extracted?
1. heat and ? metal to free ?
2. pass an electric ? through the molten ?
3. detail what happens at the cathode - half ?
4. detail what happens at the ?, half equation
5. give reasons for why ? is expensive
melt, current, metal, equation, anode, electrolysis
in what form is aluminium found in the earth’s crust and what is this called
as a compound called bauxite (Al2O3)
before electrolysis, aluminium oxide has to be melted, however due to its high ? point this can be very ? and would require lots of heat ? . because of this, a substance called ? is added which dissolves the ? oxide and decreases the ? point - making the process simpler and ?
melting, expensive, energy, cryolite, aluminium, melting, cheaper
why is an electrolysis of a solution more complicated?
as it is dissolved in water there will be some dissociated ions of hydroxide and hydrogen
during an electrolysis of a solution what is usually made at the cathode plus half equation and what is the exception
hydrogen is usually made ( 2H+ + 2e- –> H2), unless an ion of a more unreactive metal than hydrogen eg copper is present, in which case we make the metal!! x
during an electrolysis of a solution what is usually made at the anode plus half equation and what is the exception
oxygen is usually made ( 4OH- –> O2 + 2H2O + 4e- ), unless an ion of a halide, eg bromide is present, in which case a halogen is made
why is aluminium used in planes?
it’s strong and malleable, low density and corrosion resistant
what are magnalium and duralumin both used for?
to make bikes and planes
what is duralumin
an alloy of copper and aluminium
how can solid sodium hydroxide be recovered from a solution of sodium hydroxide?
evaporating away the water
test for hydrogen?
burns in air with a squeaky pop/ put lit splint in presence of hydrogen and it produces a squeaky pop
test for chlorine
test for oxygen
- turns damp blue litmus paper red then bleached it white
- oxygen relights glowing splint
why is hydrogen normally made at the cathode?
as it is always the ion which is lower in the reactivity series that is discharged and gains electrons - and so hydrogen is normally below most metals in the reactivity series
if copper forms at the cathode what does it look like?
brown coating on the electrode
if sodium forms at the cathode what does it look like?
grey coating on the electrode
if hydrogen forms at the cathode what does it look like?
bubbles of gas
if oxygen forms at the anode what does it look like?
bubbles of gas form that do not bleach damp blue litmus paper
if chlorine forms at the anode what does it look like?
bubbles of gas that do bleach damp blue litmus paper, should also smell like swimming pool water!!!
what happens at the cathode and the anode in the electrolysis of copper sulphate using inert (graphite) electrodes
at the cathode, copper is formed (reduction)
at the anode, oxygen is formed (oxidation)
what happens in the electrolysis of copper sulphate using copper electrodes (non inert) and how is this different to other electrolysis (and what happens at the two electrodes)
as they are copper electrodes - the negative cathode attracts Cu2+ ions from the solution, while the positive anode replaces the Cu2+ ions - so the cathode increases in mass and then anode decreases. at the cathode, copper is made and at the anode copper ions are made - which is different as normally two different products are made
Purification of copper using electrolysis what happens?
1. Copper in the ? anode forms copper ? which dissolve into the electrolyte (?)
2. The ? ions move to the ? copper cathode and react to form a layer of pure ?
3. Any impurities from the ? copper anode sink to the bottom of the cell forming a ?
impure, ions, CuSO4
copper, pure, copper
impure, sludge
what does the anode and cathode start off as at the start of the purification of copper electrolysis and what is the electrolyte?
the anode is a big lump of impure copper, while the cathode is a thin piece of pure copper and the electrolyte is CuSO4 solution
electrolysis of sulphuric acid - what is made at the electrodes and what is the ratio of these substances?
at cathode - hydrogen is made and at anode oxygen is made. the ratio is 2:1 of hydrogen to oxygen - for every one molecule of oxygen there are two hydrogen molecules
what is a conductometric titration
a titration in which the electrical conductivity is measured as an acid is to tested against an alkali
Is silicon dioxide an electrolyte as a liquid or as dissolved in water?
no as it is just a melted covalent substance, and the same in water
what two substances are electrolysed when in liquid or aqueous form?
ionic substances or acids
how would electrical conductivity change as increasing amount of sulphuric acid is added to 25cm cubed of barium hydroxide?
the conductivity initially falls as the number of ions decreases because of the reactions occurring - making the mixture more and more neutral. however after the perfect ratio of barium hydroxide and sulphuric acid is surpassed, the conductivity keeps increasing as increasing amounts of sulphuric acid in excess are added providing more ions for higher conductivity
write the two ionic equations for the reaction between barium hydroxide and sulphuric acid
H+(aq) + OH-(aq) —> H2O(l)
Ba2+(aq) + SO4 2- (aq) —> BaSO4(s)
conductometric titration involving sulphuric acid and barium hydroxide - what are the pHs of both solutions
1/0 for the acid and 13/14 for the hydroxide
would aqueous sulphuric acid and barium hydroxide be electrolytes or not?
yes as they are either acids or ionic and are dissolved in water so the ions are free to move!!!
what happens at the electrolysis of dilute sulphuric acid
hydrogen is formed at the cathode and oxygen formed at the anode - as any acid is ALWAYS dissolved in water because it’s aqueous
