Rate Equations 2 Flashcards
What is the rate determining step?
The slowest step in a multi-step reaction
What is the principle of mechanisms
• Can have one step or a series of steps
• Each step can have a different rate
• The overall rate is decided by the step with the slowest rate- the rate determining step
What is another word for the rate determining step?
Rate-limiting step
What side of the rate equation affects the rate?
The reactants
What can the rate equation be used for?
Working out mechanisms of a chemical reaction
What are the principles of the reactants in affecting the rate
• If a reactant appears in the rate equation, it must affect the rate
• This reactant or something derived from it must be in the rate determining step
• If a reactant doesn’t appear in the rate equation, then it won’t be involved in the rate determining step (and neither will anything derived from it)
What else can appear in rate equations and what is the effect of this thing
• Catalysts
• They can appear in rate equations too
• so they can be in rate determining steps
What are some important points to remember about rate determining steps and mechanisms?
• The rate determining step doesn’t have to be the first step in a mechanism
• The reaction mechanism can’t usually be predicted from just the chemical equation
What provides information about the rate determining step and how
• Orders of reactions
• The order of a reaction with respect to a reactant shows the number of molecules of that reactant that are involved in the rate determining step
• So if a reactions second order with respect to X, there’ll be two molecules of X in the rate determining step
What may be involved in rate determine steps?
An intermediate
What can help you work out the reaction mechanism
The rate determining step
What does an Arrhenius equation do?
• Links K (reaction constants) with temperature and Activation energy
Define activation energy
Ea: the minimum amount of kinetic energy particles need to react
Give the formula for the Arrhenius equation
K= Ae^ -Ea/RT
Give the units of the Arrhenius equation
• K = rate constant
• Ea= Activation energy (J)
• T = temperature (K)
• R = gas constant (8.31 Jk-1 mol-1)
• A= Arrhenius constant ( another constant)
What are the principles of an Arrhenius equation
• As the activation energy (Ea) gets bigger, K gets smaller
• So, a large Ea will mean a slower rate. If a reaction has a high activation energy, then not many of the reactant particles will have enough energy to react. So only a few of the collisions will result in the reaction actually happening, and the rate will be slow
• The equation also shows that as the temperature rises, K increases.
• Higher temperatures mean reactant particles move around faster and with more energy so they’re more likely to collide and more likely to collide with at least the activation energy, so the reaction rate increases
Re-arrange the Arrhenius equation to make the activation energy (Ea) the subject of formula
• In K = In A - Ea/RT
• Ea/RT = In A - In K
• Ea = (In A - In K) x RT
How can an Arrhenius plot be used to find the Ea and the Arrhenius constant
• Use the equation to create an Arrhenius plot by plotting K against 1/T on a graph
• The line of best fit for the points on the Arrhenius plot will be a straight line graph with a gradient of -Ea/R
• Once you know the gradient, you can use it to find both the activation energy and the Arrhenius constant
• The values of K and T used to draw the Arrhenius plot comes from experiments. To gather the data you repeat the same experiment at several different temperatures
How do you convert Kj to j
1KJ = 1000J
How can e on the calculator be found
Shift, alpha, x10^x
Predict the effect on the rate constant of decreasing the temperature of the reaction to 600C
Decrease in temperature will decrease the value of rate constant
What equilibrium constant is used when dealing with an equilibrium where all the substances are gases
Kp
What is total pressure of a gas mixture
The total pressure of a gas mixture is the sum of all the partial pressures of the individual gases
What is partial pressure
In a mixture of gases, each individual has exerts its own pressure
Define mole fraction
The proportion of a gas mixture that is made up of particular gas
Give the equation for the mole fraction for a gas in a mixture
Number of moles of gas/ total number of moles of gas in the mixture
Give the equation for the partial pressure of a gas
Mole fraction of gas x total pressure of the mixture
What is Kp
Kp is the equilibrium constant for a reversible reaction where all the reactants and products are gases
Using the equation
aA(g) + bB(g) -> dD(g) + eE(g)
Give the equation for the equilibrium constant for this reaction
Kp = (PD)^d x (PE)^e/ (PA)^a x (PB)^b
How do you calculate Kp
Using the partial pressure of the gases in the equation and calculate the units
How does temperature, pressure, concentration and catalysts affect Kp
Temperature
• If you increase the temperature, the equilibrium shifts in the endothermic direction
• If you decrease the temperature, the equilibrium shifts in the exothermic direction
• Changing the temperature changes how much product is formed at equilibrium, changes mile fractions and their partial pressures
Pressure
• If you increase the pressure, the equilibrium shifts to the side with fewer moles of gas
• If you decrease the pressure, the equilibrium shifts to the side with more moles of gas
Concentration
• Just changing the concentration doesn’t change Kp. The equilibrium will shift to keep it the same
Catalyst
• Adding a catalyst won’t affect Kp. It just gets the system to equilibrium more quickly
