Kinetics Flashcards
During a reaction, what decreases and what increases with time
Concentration of reacants decreases with time
Concentration of products increases with time
When calculating the rate of reaction, what is shown by the instantaneous slope or the derivative of the curve (tangent) created
In general the rate will decrease with time
How would you work out the rate of reaction
Using the derivative of the product over the derivative over time
How can you work out the rate from the reactant
Using the negative of the equation which uses product
What is a derivative
The derivative of a function measures the sensitivity of a function to changes in its variables
e.g. the derivative of an objects position with respect to time measures the velocity of the object (the rate of change of an objects position with time)
The process of finding a derivative is called
Differentiation
The derivative of a function y with respect to a variable x
= dy/dx
This is the Haber process: N₂(g) + 3H₂(g) → 2NH₃(g)
Knowing that the rate for the products will be positive, whereas the rate for the reactants will be negative, using the equation propose an expression for the rate of the reaction
Na = stoichiometric coefficients
What is rate law
The experimentally measured rate often varies with concentration according to the straightforward power: rate law
Describe how you would write rate law
Rate = k [A]ⁿ [B]ⁿ
Where k is the rate constant
the powers ‘n’ are the orders of reaction with respect to the reactant (not always the stoichiometric coefficient)
Adding the two ‘n’ values will give us the overall order the the reaction
Experimentally it is ususally found that the rate constant increases as
This relationship is shown empirically through
the temperature increases
Arrhenius Equation
What is Arrhenius equation
A is the pre-exponential factor (ame units as rate constant)
Ea is the Activation energy
What is Arrhenius Plot
The Arrhenius equation implies that if Ln(K) is plotted against 1/T
The gradient = -Ea/R
The intercept = Ln(A)
What is the activation energy
It is the energy barrier which must be overcome for the reaction to proceed
When two molecules collide in the right orientation they will only react if the energy of the collision exceeds the activation energy
If the value of the rate constant, K1, is known at a particular temperature, T1, the Arrhenius equation can be used to calculate the value, K2 at any other temperature, T2
Describe the equation for this
What is it meant if a reaction is described as elementary
A reaction is described as elementary if it takes place in a single step with one transition state and no intermediates
Why are the stochiometric coefficients important in an elementary reaction
For an elementary reaction, the stochiometric coefficients in the chemical reaction give the orders of the reaction with respect to each reactant
What is a reaction mechanism and what does the rate of this reaction depends on
Is a set of elementary reactions which predicts the steps involved in a complex reaction
This is known through the rate law not being the same the the overall reaction
This is how we know the the rate determining step is
Often (though not always) the rate determining step corresponds to the elementary reaction with the highest activation energy
How would we demonstrate this on an energy profile
Double bump
Reactants higher in energy than products
For the elementary reaction: A + B → P + Q
How would you work out equilibrium constant
Products / Reactants
Experimental data usually consists of measurements of the concentration of a reactant or product at different times
It is useful to have an expression that shows how concentration varies with time
This is done through
The integrated rate equation for the reaction
The integral is a mathematical way of evaluating the area
In order to find the variation in the concentration of the species A and P with time we use the integrated form of the rate law
What is the intergrated rate equation for first order reactions
n order to find the variation in the concentration of the species A and P with time we use the integrated form of the rate law
What is the intergrated rate equation for second order reactions
