Chemical Industries Flashcards
State the basic chemical rate equation
A + B –> C + D
Rate = k[A]^m[B]^n
What unit is used for rate of reaction?
mol dm^-3 s^-1
Explain zero order rates
Concentration of species has no impact on rate
Shown on rate concentration graph as a horizontal line
Rate = k
Explain first order rates
Concentration of species is directly proportional to rate
Doubling concentration doubles rate
Rate = k[A]
Explain second order rates
Rate is proportional to concentration of species squared.
Doubling concentration increases rate by four times
Rate = k[A]^2
How does a zero order reaction look on a concentration-time graph?
Zero order reaction on concentration-time graph looks linear
How do first and second order reactions look on a concentration-time graph?
Both are curved, second order graphs have a slight kink in the curve
How do you calculate the rate in a concentration-time graph?
Draw tangent, find gradient of tangent
What effect on initial rate would zero, first and second order reactions have on doubling the concentration of a species?
Zero order - no change in initial rate
First order - Initial rate doubles
Second order - Initial rate quadruples (2^2)
Define half life
The time for the initial concentration of the reactants to decrease by half
How does the half life change over the course of a first order reaction?
Half life is constant in a first order reaction
Explain the progress curves and initial rate method for finding out the rate order of a reaction?
Measuring the change volume/concentration/pH of products/reactants to draw a progress curve graph. Gradient of a tangent can be calculated to find rate.
Then find order using initial rate method:
Draw tangent at t=0 for different progress curves, find order graphically (straight line is 1st in rate/conc. graph, straight line against rate/conc.^2 is 2nd, line is flat in 0)
What is the rate determining step?
The slowest step of the reaction, which determines the overall rate. The rate equation includes all species in each stage up to and including rate determining step.
If the forward reaction is endothermic, how will an increase in temperature change the position of the equilibrium?
The equilibrium will shift to the right to oppose the change, Kc will increase
Why is nitrogen highly unreactive?
Triple bonds require lots of energy to break. Bonds are non polar and not easily polarizable, so electro/nucleophiles are not attracted to them, making them less likely to be involved in reactions.
Name three common nitrogen oxides, how they appear and where they come from
NO: a colourless gas turns to NO2 in air. Formed from combustion processes especially from vehicle engines, thunderstorms, denitrifying bacteria in soil
NO2: a brown gas, formed from oxidation of NO in atmosphere
N2O: a sweet smelling colourless gas, formed in the soil by denitrifying bacteria
How would you test for nitrate (V) ions?
Warm the sample with NaOH and Devarda’s Alloy, if present gas present should turn red litmus paper blue.
Write out the reaction of aerobic bacteria in the soil oxidising ammonium ions
NH4+ + 1.5O2 –> NO2- + 2H+ + H2O
Nitrate (III) ion is further oxidised to nitrate (V) ion: NO2- + 1.5O2 –> NO3-
Write out the reaction when anaerobic bacteria reduce nitrate (V) ions when oxygen is low
NO3- (aq) –> NO (g) –> N2O (g) –> N2 (g)
How do catalysts affect the position of equilibrium/Kc?
Catalysts do not affect the position of the equilibrium, they only alter the rate at which the equilibrium is attained
How do concentration and total pressure affect Kc and composition of mixture?
Changes in concentration and total pressure may alter the composition of equilibrium mixtures, but they do not alter the value of Kc, provided temperature does not change
What is the unit of k in:
0 order reactions
1st order reactions
2nd order reactions
0 order: k = mol dm^-3 s^-1
1st order: k = s^-1
2nd order: k = dm^3 mol^-1 s^-1
What are the units in Arrhenius’ Equation?
k = Ae^-Ea/RT T = temperature in kelvin R = gas constant (8.314 J K^-1 mol^-1) Ea = activation energy in J mol^-1 A = frequency factor (constant)
What is the gradient in a graph of ln(k) over 1/T?
Gradient = -Ea/R
Explain the reciprocal reaction time method for finding rate order of a reaction
Measure how long the reaction takes to produce a small fixed amount of a product (reaction time)
Plot graph of 1/T against concentration of solution, showing rate
What are feedstocks?
Reactants made from raw materials that go into a chemical process
What is the difference between co-products and bi-products?
Co-products are desirable secondary products that are formed during the manufacturing process
Bi-products are unwanted secondary products
What is the aim of green chemistry?
Minimise use of feedstocks
Recycle unused solvents and reactants
Minimise energy consumption
