Section 7 The Chemical Industry Flashcards
(21 cards)
What ways can you measure the rate of a reaction?
pH: if one of reactants/products is acid/base you can follow reaction by monitoring reaction mixture’s pH. You can convert pH data into units of concentration using [H+] = 10-pH.
Gas Volume: using gas syringe. Units are Volume time-¹
Loss of mass. Using balance units = mass time-¹
Colour change: using colorimeter higher absorbance means higher concentration
Titration: take small samples of reaction mixture at regular intervals and titrate.
How can you calculate reaction rate from a concentration-time graph?
Draw tangent to curve of part of graph you want to find reaction rate of. Calculate rate by gradient using formula change in y ÷ change in x.
Describe how clock reactions can be used to calculate initial rate.
Clock reaction has an easily observable end point. The quicker the reaction finishes, the faster the initial rate.
Most notable example is the iodine clock reaction
Small amount sodium thiosulfate solution and starch to excess hydrogen peroxide and iodide ions in acid solution.
Sodium thiosulfate reacts instantly with any iodine that forms. Once all sodium thiosulfate used up iodine stays in solution and starch turns blue-black. This is the end point.
Varying conc of iodide ions or hydrogen peroxide will give different times for colour changes.
What is the rate equation?
For a general reaction: A + B > C + D , the rate equation is:
Rate = K[A]m[B]n
m and n = orders of reaction. Tell you how concentration of A or B affects the rate.
K = rate constant, the bigger it is , the faster the reaction. The rate constant = always same for a specific reaction at a particular temperature.
How do orders tell you how a reactants concentration affects the rate?
If [A] changes and rate stays the same, order if reaction with respect to A = 0.
If rate is proportional to [A], then the order of reaction with respect to A is 1. E.g. if [A] doubles, rate will double.
If rate is proportional to [A]², then order of reaction with respect to A is 2. E.g. if A doubles, rate will 2², so 4 times as fast.
Concentration-time graph for zero, first and second order reactions?
Zero: conc decreases linearly over time. As rate of reaction independent to concentration.
First: concentration decreases exponentially over time. The rate is directly to concentration.
Second: steeper initial followed by levelling. Faster than first order as rate proportional to square of concentration.
Rate concentration graphs for zero, first and second order reactions.
Zero: horizontal lind means changing concentration doesn’t change the rate.
First: straight line through origin.
Second: curved line.
How can order be found from half lived on concentration-time graphs?
Zero: graph= straight line so half life decreases as the reaction goes on.
First: Rate proportional to concentration, so half life is constant as reaction goes on.
Second: Rate proportional to square of concentration so half life increases as reaction goes on.
Formula to calculate rate constant from half life?
K = ln(2) / t¹/²
Units for K: S‐¹
t¹/² = half life
Rate determining step?
slowest step in a multi-step reaction
the order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in the rate determining step
e.g. if rate = k[H2][ICl]
then rate determining step must involve 1 molecule of H2 and one molecule of ICl.
costs involved in producing a chemical?
- raw materials
- fuel/energy
- fixed costs e.g. staff wages
- cost of disposal of by-products
reactions with high atom economy and percentage yield = best as fewer raw materials and have less waste products
Temperature in an industrial reaction?
Advantage = reactions go faster at higher temp
disadvantage= high temp make reaction more expensive due to the cost of fuel
pressure in an industrial reaction?
advantage= higher pressure makes gaseous reaction faster
disadvantage= to create a high pressure, gas must be pumped into reaction vessel. Uses lots of energy and expensive. High pressure can be dangerous, so reaction vessel must be made out of strong material which is expensive.
Catalysts in industrial reactions?
advantages= right catalyst can speed up reaction at relatively low temp, saves money on fuel. good investment as dont get used up.
disadvantages= industrial catalysts can be expensive. If a catalyst is in same state as reactant, must be removed after reaction adding an extra step to the industrial process.
Effect of catalyst, temp and pressure on Kc?
catalyst= doesnt change Kc
Temp= changes Kc
pressure+ doesnt change Kc
Risks of producing a chemical?
some chemicals are highly flammable and carry risk of explosion. they must be stored and handled correctly
some chemicals ae harmful e.g. chlorine is toxic
some chemicals damage environment e.g. SO2 contributes to acid rain
what methods can be used to find concentration of a substance in a reversible reaction at equilbrium?
Colorimetry= used to find absorbance of coloured substance at equilibrium and use calibration curve to calculate the conc of the coloured ions.
pH= use pH probe to find pH of equilibrium mixture. use pH calculations to find the conc.
titrations can NEVER be used as disrupts equilibrium
Nitrogen?
diatomic
group 5 nitrogen atoms share 3 pairs of electrons
triple bond = hard to break to N2 very unreactive
Oxides of nitrogen?
NO= colourless gas
N2O= sweet smell, colourless
NO2= brown gas, sharp odour and toxic
Test for ammonium?
Add NaOH and gently heat. Ammonia will be given off, turning damp red litmus paper blue.
Test for nitrates?
Warm with NaOH and aluminum foil/devarda’s alloy
aluminium reduces the nitrate ions to produce ammonia gas, turning damp red litmus paper blue.
