Rates, Equilibrium and pH Flashcards
How is rate of reaction calculated?
Change in concentration of reactant or product / time
What is the rate equation of a 0 order reaction?
Rate = k
If a reaction is: A + B + C ➝ products and the orders of reaction of A, B and C are x, y and z respectively, what is the rate equation and the overall order of reaction?
Rate = k[A]^x [B]^y [C]^z , overall order = x+y+z
If a reaction is second order with respect to C, and the concentration of C is tripled, what happens to the rate?
The rate increases by 9x (3^2)
What is the equation for k using half lives?
k= ln2/half life, for first order reactions only as the half life is constant
What is the shape of the concentration-time graph for 0 order, first order and second order reactions with respect to A?
0 order- linear
1st order- quadratic
2nd oder- exponential
What is the shape of the rate-concentration graph for 0 order, first order and second order reactions with respect to A?
0 order- flat line
1st order- linear
2nd order- quadratic
In a clock reaction, what is the rate expressed as?
- Rate = k x 1/t
- The quicker the reaction is the faster the rate
What is the rate determining step?
- The slowest step of a multi step reaction
- The moles of each reactant in the rate determining step is equal to the order of reaction with respect to that reactant
- Eg. slow step= CH3Br ➝ CH3+ + Br-
- Rate = k[CH3Br]
What is an intermediate in a multi step reaction?
A species formed in one step of a multi-step reaction that is used up in a subsequent step of the reaction, it doesn’t appear in the overall reaction
What is effect of temperature on the rate constant?
- An increase in temperature gives the molecules more kinetic energy, this means that more molecules collide with an energy greater than or equal to the activation energy and therefore the rate of reaction increases
- When temperature increases, the concentrations of the reactants stays the same and the rate increases, this means that k must increase to maintain the rate equation
For most reactions what does an increase by 10ºC do to k?
Doubles it
What is the Arrhenius equation?
- k = Ae^-Ea/RT
- k = rate constant, A = pre-exponential factor (a constant), Ea = activation energy, R = gas constant (8.314), T = temp (kelvin)
What does the Arrhenius equation tell us?
- There is an exponential relationship between the rate constant and the temperature
- Rate constant increases as temperature increases
What is the Arrhenius plot?
- lnk = lnA - Ea/R x 1/T
- lnk is on the y axis and 1/T is on the x axis
- This means we can think of 1/T as x and therefore -Ea/R as m or the gradient
- When 1/T is 0, lnk = lnA meaning lnA is the y intercept of the graph
How do you calculate the Ea from an Arrhenius plot?
- Gradient = -Ea /R
- Find gradient by taking two points on graph and calculating change in y/change in x
- Gradient will be negative
- Gradient x R = -Ea
- -Gradient x R = Ea
- As gradient is negative, a negative x negative gives a positive so Ea will always be positive
- Units of gradient are K as lnk ahas no units and 1/T has units 1/K, this means that y/x = no units/ 1/K =K
- Units of R are Jmol-1K-1
- K x Jmol-1K-1 = Jmol-1, units of Ea are Jmol-1 from the plot
What is it called when the rates of forward and reverse reaction are equal?
Equilibrium
On an equilibrium graph, where is the point of equilibrium?
Where the concentrations of products and reactants remain unchanged
How could a chemist determine the concentrations of reactants and products at equilibrium?
- Using a titration
- Using a colorimeter
What is a homogenous equilibrium?
An equilibrium where all reactants and products are in the same physical state
What is a heterogenous equilibrium?
An equilibrium where the species making up the reactants and products are in different physical states
In heterogenous equilibrium are the concentrations of solid substances and liquids included in the expression for Kc?
No, in heterogenous equilibrium only aqueous solutions and gases are included
Is water included in the expression for Kc?
- When it is a pure liquid, it is not included if the equilibrium is heterogenous, it is only included if it is in the gaseous state
- Water is always included in homogenous equilibrium
How do you calculate mole fraction of substance A?
Number of moles in substance A/Total number of moles
