Chemical Industry Flashcards
What are different ways of measuring rate of a reaction? (5)
- volume of gas produced
- mass change
- pH
- colorimetry
- chemical analysis using titration
What is rate?
How quickly a quantity of something changes
What is reaction rate?
change in the amount of reactants or products per unit time
How can you work out reaction rate?
By using a tangent on a concentration-time graph
What’s the equation for initial rate?
Amount of reactant used or product formed / time
How do you work out reaction rate from conc-time graph?
Work out the gradient using a tangent
What is the initial rate of reaction?
The rate at the start of the reaction
What do you assume in the initial rates method?
- Conc of other reactants isn’t changing significantly
- Temp stays constant
- Reaction has not proceeded too far
What is the equation for initial rate?
initial rate = amount of reactant used or product formed / time
What is a clock reaction?
A reaction which has a easily observable endpoint
What is the rate equation?
Rate = k[A]^m[B]^n
m and n are orders of reaction
k is rate constant (units of k vary)
What does zero order with respect to [A] mean?
[A] changes and the rate stays the same
What does 1st order with respect to [A] mean?
rate is proportional to [A] (if [A] doubles rate doubles)
What does 2nd order with respect to [A] mean?
Rate is proportional [A]^2 (if [A] doubles rate will be 2^2)
What is the overall order of a reaction?
The sum of all the orders of the different reactants
What does the order of reaction with respect to a particular substance tell you?
Tells you how a reactants concentration affects the rate
How do you work out the orders?
Do initial rates method experiment to see how reactant affects the rate one by one
What is the half life of a reaction?
The time taken for a reactant to half in quantity
What does half life look like for zero order?
half life decreases as reaction goes on
What does half life look for first order?
half life decreases as conc does but is constant
What does half life look like for 2nd order?
Half life increases as reaction goes on
how do you work out rate constant using half lifes?
k = ln2/t1/2
What is t1/2?
half life
When does the rate constant valid?
At a specific temperature for that equation
What do the parts of the Arrhenius equation mean?
K = Ae^(-Ea/RT)
K= rate constant Ea = activation enthalpy T = temperature (K) R = gas constant A = pre-exponential factor
What does a large Ea mean for the arrhenius equation?
k = Ae^(-Ea/RT)
means a slow rate of reaction because higher activation enthalpy means not as many particles will react
What do you plot against each other in the arrhenius plot
lnk against 1/T
What is the rate determining step (rate limiting step)
It is the slowest step in a multi-step reaction
What does the order of a reaction tell you about the molecules in the rate determining step?
shows the number of molecules in the rate determining step
What costs are involved in producing a chemical?
1) raw materials
2) Fuel/energy needed
3) fixed costs (eg. labour)
4) Disposal Costs
What reactions save the most money?
Reactions with high atom economies and high percentage yields
What should be considered when designing an industrial process, to find the right reaction conditions?
rate of reaction
product yield
cost
What are advantages and disadvantages of high temp for an industrial process?
+
The reaction goes faster
-
more expensive because of fuel
Advantages and disadvantages of high pressure for an industrial process?
\+ the reaction goes faster - expensive strong material needs to be used because high pressure is dangerous
Advantages and disadvantages of a catalyst for an industrial process?
\+ faster reaction at lower temps good investment as don't get used up - can be expensive homogeneous catalyst has to be separated from products
What is Kc?
the equilibrium constant
What is dynamic equilibrium?
- rate of forward reaction is equal to rate of back reaction
- so there’s no overall change in conc of reactants and products
What is the equation for Kc?
aA + bB dD + eE
Kc = ([D]^d x [E]^e) / ([A]^a x [B]^b)
Do temperature changes alter Kc?
Yes - Kc is only valid at specific temperatures
Do pressure changes alter Kc?
No
Do catalysts alter Kc?
catalysts have no effect
Do you have to work out the units for Kc?
yes - (work it out for mol/dm)
