Unit 2b - Reaction Rates, Salts And Electrolysis Flashcards
What is one of the slowest reactions?
Rusting of iron
What is an example of a moderate speed of reaction?
Metal like magnesium reacting with acid to produce bubbles
What is an example of a really fast reaction?
An explosion where it’s all over in a fraction of a second
What four things does the rate of reaction depend on?
- Temperature
- Concentration (or pressure for gases)
- Catalyst
- Surface area of solids (or size of solid pieces)
On a graph what is the quickest reaction shown by?
The line with the steepest slope
On a graph how do you know a reaction has finished?
The faster the reaction the sooner it finishes shown by when the line becomes flat
What is the cause of a line on a graph showing more product being produced than the other lines showing the reaction?
This can only happen if more reactants are added at the start
What can the rate of reaction be observed by?
Either by measuring how quickly the reactants are used up or how quickly the products are formed (usually easier to do it this way)
How do you calculate the rate of reaction
Fine
How can you use precipitation to measure the rate of a reaction?
- this is when the product of the reaction is a precipitate which clouds the solution
- observe a mark through the solution and measure how long it takes to disappear
- the quicker it does the quicker the reaction
- only option when reaction doesn’t give off a gas
What are the downsides to using precipitation to measure the rate of reaction?
Only works for reactions where he initial solution is see through.
Results are subjective - different people might not agree to the exact point the mark disappears.
How does measuring the change in mass work as a way to measure the rate of reaction?
- measuring the speed of a reaction that produces gas can be done on a mass balance.
- as the gas is released the mass disappearing is easily measurable
- quicker the reading drops the faster
- rate of reaction graphs are easy to plot using the results from this
- accurate bc mass balance
Disadvantage of measuring change in mass as a way to measure the rate of reaction?
Releasing gas straight into the room.
If the flask is hot you lose mass through evaporation as well.
How does measuring the volume of mass given off work as a way to measure the rate of reaction?
- involves a gas syringe to measure volume given off
- the more gas given off given a time interval the faster the reaction
- a graph of gas volume against time elapsed could be plotted to give rate of reaction
- accurate because resolution on gas syringe is millilitre
Disadvantages of measuring the volume of gas given off to measure rate of reaction?
If the reaction is too vigorous you can easily blow the plunger out of the end f the syringe
How does the reaction of hydrochloric acid and marble chips work?
- Measure the volume of gas evolved with a gas syringe and take readings at regular intervals
- Make a table of readings and plot them as a graph. Regular time intervals with time on the x and volume on the y
- Repeat experiment with all variables the same but changing the factor you are testing e.g same mass of marbles but more crunched up
- Repeat with the same mass of powdered chalk instead of marble chips
What is the experiment of reacting hydrochloric acid and marble chips usually used for?
Experiment often used to demonstrate the effect of breaking the solid up into small bits
What does using finer particles mean?
Marple has a larger surface area
Why does a larger surface area increase the rate of reaction?
A larger surface area causes more frequent collisions so it’s faster.
What is reacting magnesium metal with dilute HCl good for?
Measuring the effects of increased concentration
How does the reaction of magnesium and dilute HCl work?
- Reaction gives off hydrogen gas which we can measure with a mass balance (or gas syringe)
- Time in x axis and volume on y
- Take readings at regular time intervals
- Put results in table and work out the loss in mass
- Repeat with more concentrated volumes but same other variables
What do sodium thiosulfate and hydrochloric acid produce?
A cloudy precipitate
What reacts to form a yellow precipitate of sulfur?
Sodium thiosulfate and HCl
What does the experiment of reaction sodium thiosulfate and HCl together involve?
- Watching a black mark dispappear through the cloudy sulfur and timing how long it takes
- Repeated for solutions at different temps
- Best way is to use a water bath to hear both solutions to the right temp before mixing because it’s hard to do accurately and safely
- Depth of liquid the same along nicely with all other variables
What do the results show in the reaction of sodium thiosulfate and HCl?
Higher temperature the quicker the reaction therefore the less time it takes for the mark go disappear but it doesn’t give a set of graphs.
What is the decomposition of hydrogen peroxide a good reaction for showing?
The effect of different catalysts.
Also can be used for measuring effects of temp or concentration
What is the word equation for the decomposition of hydrogen peroxide?
2H2O2 –> 2H2O + O2
What is the disadvantage of using the decomposition of hydrogen peroxide to measure the rate of reaction?
It is quite slow but a sprinkle of manganese oxide catalyst speeds it up
What are three catalysts that work in the decomposition of hydride peroxide?
Manganese oxide, potato peel and blood
How does the decomposition of hydrogen peroxide work?
- Oxygen gas is given off so is a good way to measure rate of reaction using a gas syringe
- Time on x vol. of gas on y
- Better catalysts give a quicker reaction which is shown by a steeper graph which levels off quickly
What does the collision theory state?
The rate of a reaction simply depends on how often and how hard the reacting particles collide with each other. The particles have to collide in order to react and they have to collide hard enough with enough energy.
How does increasing the temperature increase the rate of reaction?
When the temperature is increased the particles move quicker and collide more often
How does increasing the concentration or pressure increase the rate of reaction?
If a solution is more concentrated it means there are more particles of reactant knocking about between the water molecules which makes collisions more likely.
Increasing the pressure in a gas means the particles are more squashed up together so more frequent collisions.
How does increasing the surface area of a solid increase the rate of reaction?
The particles around it in the solution will have more area to work on so there’ll be more frequent collisions
What is the minimum amount of energy needed by the particles to react called?
The activation energy
What is a catalyst?
A substance that speeds up a reaction without being changed or used up in the reaction
How does a solid catalyst work?
By giving the reacting particles a surface to stick to this increases the number of successful collisions and speeds up the reaction
How do catalysts help save money?
They increase the rate of reaction so the plant doesn’t need to operate as long to produce the same amount of stuff
How does a catalyst being able to work at a lower temperature help?
It reduces the energy used up in the reaction (the energy cost) which is good for sustainable development and can save money
What are a few disadvantages of catalysts?
- very expensive to buy and often need to be removed and cleaned (but once you have it it can be reused)
- different reactions use different catalysts so you need to buy multiple ones
- catalysts can be poisoned by impurities so they stop working
Example of an impurity in a catalyst stopping it working?
Sulfur impurities can poison the iron catalyst in the haven’t process so you have to keep mixture clean
What happens to energy in chemical reactions?
It is transferred to or from the surroundings
What is an exothermic reaction?
One which transfers energy to the surrounding usually in the form of heat shown by a rise in temperature
Examples of exothermic reactions
- burning fuels in computation gives out a lot of heat
- neutralisation reactions
- oxidation reactions e.g adding sodium to water produces heat. The sodium emits heat and moves about on the surface of the water as it is oxidised
What are some everyday uses of exothermic reactions?
Hand warmers use exothermic oxidation of iron in air with a salt solution catalyst to generate heat.
Self heating cans of hot chocolate and coffee rely on exothermic reactions between chemicals in their bases.
What is an endothermic reaction?
One which takes in energy from the surroundings usually in the form of heat and is shown by a fall in temperature
What is an example of an endothermic reaction?
Thermal decomposition.
E.g heat must be supplied to make Calcium carbonate decompose into quicklime.
Everyday uses of endothermic reactions
Sports injury packs use endothermic reactions to take in heat and become more cold. More convenient than carrying around ice.
What are the reactions in reversible reactions?
The reaction is endothermic in one direction and exothermic in the other.
Why are reversible reactions that are endothermic in one direction be exothermic in the other?
The energy absorbed by the endothermic reaction is equal to the energy released during the exothermic reaction e.g thermal decomposition of hydrated copper sulfate -> is endothermic and
What does anhydrous mean?
Without water
What happens if you heat blue hydrated copper(II) sulfate crystals and then add water again?
It drives the water off and leaves white anhydrous copper sulfate powder (endothermic). If you add a couple drops of water to the White powder you get the blue crystals back again (exothermic)
How do you test the pH of a solution?
Using an indicator
Explain the pH scale
- the scale is a measure of how acidic or alkaline a solution is
- the stringed acid had a pH 0 and alkaline has it 14
- a neutral substance has pH 7
What is an indicator?
A dye that changes colour. It changes colour depending on whether it is above or below a certain pH.
What is a universal indicator and what is it useful for?
A combination of dyes which give the colours of red - green - purple on the scale and it’s useful for estimating the pH of a solution
What is an acid and what does it form in water?
A substance with a pH of less than 7.
H+ ions.
What is a base?
A substance with a pH of greater than 7
What is an alkali and what does it form in water?
A base that dissolves in water and forms OH- ions.
What is the reaction between acids and bases called?
Neutralisation (the products are neutral with a pH 7)
What is the word equation for neutralisation?
Acid + base –> salt + water
What is the equation for neutralisation in terms of H+ ions and OH- ions?
H+ + OH- –> H2O
What colour will the universal indicator go when the neutralisation reaction is over?
Green because it has been neutralised
What do (s) (l) (g) (aq) mean?
Solid
Liquid
Gas
Dissolved in water
Word equation for metals reacting with acids
Acid + metal –> salt + hydrogen
what would you find when reacting a metal and an acid?
- the more reactive the metal the faster the reaction will go (very reactive metals like sodium react explosively)
- copper does not react with dilute acids at all because it’s less reactive than hydrogen
- speed of reaction is indicated by the rate at which bubbles of hydrogen are given off
- hydrogen is confirmed by the burning splint test giving the notorious squeaky pop
what is the typical experiment to test acids and metals?
Put a bunch of metals in dilute acid and the speed of reaction is indicated by the hydrogen bubbles. Hydrogen confirmed by the burning splint test giving the squeaky pop
What kind of salts will hydrochloric acid always produce?
Chloride salts
E.g hydrochloric acid and magnesium make magnesium chloride
What kind of salts does sulfuric acid always produce?
Sulfate salts
E.g magnesium reacting with sulfuric acid gives magnesium sulfate
When do nitric acids produce nitrate salts?
When reacted with alkalis (neutralisation)but with metals it produces nitrogen oxides instead
Why can’t you react potassium or sodium with an acid in a school lab?
they are too react and would explode
Why don’t some metals react with acids?
Because they are less reactive than hydrogen
What are metal oxides and metal hydroxides?
Bases
What is a soluble compound?
One that dissolves in water e.g metal oxides and metal hydroxides
What is the word equation of reacting a metal oxide or hydroxide with an acid?
Acid + metal (hydr)oxide –> salt + water
Neutralisation reactions
What is the symbol equation for reacting nitric acid and potassium hydroxide?
HN03 + KOH –> KN03 + H2O
You always know water will be formed so it’s just a trial and error
What can ammonia be neutralised with to make fertiliser?
HNO3
What does ammonia dissolving in water make?
An alkaline solution
When ammonia reacts with nitric acid what do you get and what are the equations?
A neutral salt called ammonia nitrate
NH3 + HNO3 –> NH4NO3
Ammonia + nitric acid –> ammonia nitrate
Why is ammonia neutralising different from most neutralisation reactions?
Because no water is produced just the ammonia salt
Why is ammonia nitrate a good fertiliser?
Because it has nitrogen from two sources - ammonia and nitric acid - like a double dose and plants need nitrogen to make protein.
What are soluble mostly in water?
Chlorides, sulfates, nitrates (beside lead chloride, lead sulfate and silver chloride)
What are mainly insoluble in water?
Oxides and hydroxides
How do you make soluble salts using a metal or an insoluble base?
- you need to pick the right acid and metal or insoluble base (if you wanna make copper chloride mix hydrochloric acid and copper oxide)
- add metal or base to the acid and the solid will dissolve in the acid as it reacts
- you will know all acid has been neutralised when the excess solid sinks to the bottom of the flask
- filter it using filter paper to get salt solution
- evaporate the water and leave the rest to evaporate slowly to get pure crystals of the salt
What is crystallisation?
When you are trying to get a pure solid crystal of salt evaporate the water to make the solution more concentrated and let the rest evaporate slowly
Why can’t you make soluble salts using a metal or an insoluble base with alkalis like sodium, potassium or announce hydroxides?
- you can’t add a metal or insoluble base with alkalis like sodium, potassium or ammonium hydroxide because you can’t tell when the reaction has finished and you can’t just add an excess to the acid and filter out what’s left
How do you make soluble salts using an alkali?
- you have to add exactly the right amount of alkali to just neutralise the acid
- use an indicator to show when the reaction is finished
- repeat using the exact same volume of alkali and acid so the salt isn’t contaminated with indicator
- evaporate off the water to crystallise the salt
When can you use a precipitation reaction to make salt?
When the salt you want is insoluble
How do you make an insoluble salt?
- pick two solutions that contain the ions needed e.g to make lead chloride you need a solution with lead ions and one with chloride ions (preferably soluble like lead nitrate solution and sodium chloride solution)
- once salt has precipitated out you filter it from solution, wash it then dry it on filter paper
What are all group 1 compounds like sodium chloride?
Soluble
What can precipitate reactions be used to remove?
Poisonous ions like lead from drinking water.
Calcium and magnesium ions can also be removed this way as they make water hard which stops soap lather.
To treat effluent.
What is effluent?
Sewage
What does electrolysis mean?
Splitting up with electricity
What happens if you pass an electric current through an ionic substance that’s molten or in solution?
It breaks down into the elements it’s made of in a process called electrolysis
What is an electrolyte?
A liquid that conducts the electricity in electrolysis.it contains free ions - they’re usually the molten or dissolved ionic substance.
What conducts electricity in electrolysis?
The free ions from the molten substances
What does an electrical circuit need to be complete?
A flow of electrons
What generally happens during electrolysis?
Electrons are taken away from ions at the positive electrode and given to other ions at the negative electrode. As ions gain or lose electrons they become atoms or molecules and are released.
What does the electrolysis reaction involve?
Oxidation and reduction
What is reduction and oxidation and how can you remember them?
Reduction is the gain of electrons.
Oxidation is the gain of oxygen or a loss of electrons.
Don’t always include oxygen.
Oxidation
Is
Loss
Reducation
Is
Gain
Explain the electrolysis of lead bromine
When a salt e.g lead bromine is molten it will conduct electrify.
+ve ions are attracted to the -ve electrode and they gain electrons in reduction. Lead is produced here by accepting two electrons to become one lead atom.
-ve ions are attracted to the +ve electrode and they lose elections in oxidation. Bromine is produced here by two bromine ions losing one electron each becoming one bromine molecule.
As well as molten substances what else can you use in electrolysis?
Electrolyse solutions
What is an example of there being more than two free ions in the electrolyte?
If a salt is dissolved in water there will be some H+ and OH- ions as well.
If metal ions and H+ ions are present at the negative electrode what will happen?
The metal ions will stay in the solution if the metal is more reactive than hydrogen because the more reactive an element the keener it is to stay as ions so hydrogen will be produced unless the metal is less reactive than it
If OH- and halide ions like Cl-,Br-,Ir are present at the positive electrode what will happen?
Then molecules of chlorine, bromine or iodine will be formed. If no halide is present then oxygen will be formed.
When common salts like sodium chloride is dissolved in water and electors is what are the three useful products it produces?
Hydrogen, chlorine and sodium hydroxide
What happens during the electrolysis of sodium chloride solution?
- at negative electrode two hydrogen ions accept two electrons to become one hydrogen molecule. Reduction.
- at positive electrode two chloride ions lose their electrons and become one chlorine molecule. Oxidisation.
- the sodium ions stay in the solution because they are more reactive than hydrogen
- hydroxide ions from water are also left behind so sodium hydroxide is left in the solution
What is the main thing to make sure of about half equations?
The number of electrons is the same for both half equations
What are the half equations for the electoysis of sodium chloride?
Negative electrode: 2H+ +2e- –> H2
Positive electrode: 2Cl- –> Cl2 + 2e-
Or 2Cl- -2e- –> Cl2
What is the half equation for the electoysis of molten lead bromide?
Ne: Pb2+ + 2e- –> Pb
Pe: 2Br- –> Br2 + 2e-
What are two useful products form the electrolysis of sodium chloride solution?
- Chlorine has many uses in bleach and plastics
2. Sodium hydroxide is a strong alkali used in the chemical industry to make soap
How is electrolysis used with aluminium?
To remove aluminium from its ore.
- aluminium is an abundant metal it is always found naturally in compounds
- main ore is bauxite and after mining and purifying a white powder is left
- this is pure aluminium oxide (Al2O3)
- aluminium has to be extracted from it using electrolysis
How does cryolite help in the electoysis of aluminium oxide?
Al2O3 a very high melting point of over 2009 so melting it would be expensive.
Instead it’s dissolved in molten cryolite.
This brings the temperature down to about 900 so it’s cheaper and easier.
What is a less common ore of aluminium called?
Cryolite
What are the electrodes made from in the electrolysis of aluminium oxide?
Carbon (graphite) which is a good conductor of electricity
In the electrolysis of aluminium oxide what firms at each of the electrodes?
Aluminium forms at the negative electrode and oxygen forms at the positive one
What are the half equations for the electrolysis of aluminium oxide?
Ne: Al3+ + 3e- –> Al
Pe: 2O2- –> O2 + 4e-
Why do the positive electrodes in the electrolysis of aluminium oxide need to be replaced every now and again?
Because the oxygen that forms there react with the carbon in the electrode to produce carbon dioxide so the positive electrode gradually gets eaten away and needs replacing
What does electroplating do?
Uses electrolysis to coat the surface of one metal with another metal e.g might want to elector plate silver onto a brass cup to make it look nicer
How does electroplating work?
- the negative electrode is replaced with the metal object and the positive one is the pure metal you want to plate it with
- the electrolyte needs to contain ions of the plating metal as well (ions that plate the metal come from this but the positive electrolyte keeps the solution topped up)
Example of how to electroplate silver onto a brass cup
The brass cup would be the negative electrode to attract the posits silver ions and a lump of pure silver would be the positive electrode and keep them in a solution of silver ions e.g silver nitrate
What are two uses of electroplating?
- For decoration because silver is attractive but expensive. Much cheaper to plate a boring brass cup with silver than to make it out of solid silver but it’s just as pretty
- For conduction as metals like copper conduct electricity well so are used to plate metals for electronic circuits and computers
