Rates of reaction Flashcards
Rate of reaction definition?
The change in concentration of a reactant or product in unit time
What is rate of reaction measured in?
mol dm^-3 s^-1
What does higher pressure do to the rate of reaction?
Increases it, as more frequent collisions
How can a chemical reaction be monitored?
Colour change
Change in mass
Volume of gas produced
Conductivity
Rate of reaction formula?
Change in concentration/ change in time
How can you measure the rate of reaction at a certain point on a concentration- time graph?
Find the gradient of the tangent
Catalyst definition?
A substance that increases the rate of a chemical reaction, without being used up in the process. By providing an alternative route for the reaction with lower activation energy
What’s a heterogeneous catalyst?
One in a different physical state from the reactants, most frequently gaseous reactants, and solid catalyst
What’s a homogenous catalyst?
One in the same physical state as the reactants, normally aqueos of gaseous state
What are the sustainable and economic importance of catalysts?
Reduce energy consumption, and therefore reduced effect on environment
Also less energy used in heating reaction, so increased profits
How to draw a Boltzman distribution?
Curve starts at (0,0)
Goes up in straight line then slopes down, then slopes off
X axis= kinetic energy
Y axis= Number of particles
Label the activation with a vertical line at tail
What does the area under a Boltzman distribution represent?
Total number of particles in the system
Which proportion of molecules have more than the kinetic energy and therefore can react?
The area after the EA line
What happens to the Boltzman distribution if you increase the temperature?
Peak gets lower, and moves to a higher energy
Tail will there be higher when passing the EA
What happens to the proportion of molecules with at least the activation energy when the temperature increases?
Increases
What happens to Boltzman distribution when you add a catalyst?
EA decreases
Why does increasing the concentration of reactant or pressure of reactant increase rate of reaction?
More particles per unit volume, so more frequent collisions
How does grinding the solid reactant into a powder increase the rate of reaction?
Increased surface area of contact, so more frequent collisions
What is meant by activation energy, and why do most chemical reactions require one?
Minimum energy required to start a reaction (by the breaking of bonds), and most reactions require bonds to be broken in reactant molecules
Definition of rate equation for the reaction A + B = C ?
Rate = K ((A)^m)((B)^n)
where m and n are the order of reaction (0, 1 or 2)
Rate= rate in mol dm^-3 s^-1
K= rate constant
A and B are the concentrations of the chemicals in mol dm^-3
Definition of order of reaction?
The power to which the concentration of reactant is raised in the rate equation
Definition of rate constant?
The constant that links the rate of reaction with the concentrations of the reactants raised to the power of their orders in the rate equation
Zero order reaction on a Rate-conc graph?
Horizontal line
1st order reaction on a Rate-conc graph?
starts at origin and is diagonal line
2nd order reaction on a Rate-conc graph?
Starts at origin, is curved line, gradient increases
Definition of half life?
The time taken for the concentration of a reactant to reduce by half?
How to determine what order a reaction is on a conc-time graph?
0 order- diagonal line going down
1st order- curved line going down, half life is constant
2nd order- steeper curved line going down, half life isn’t constant
How to calculate k from the half life?
k = ln2/ (Half life)
Only for first order
Definition of initial rate of reaction?
The change in concentration of a reactant or product per unit time at the start of a reaction (time = 0)
Found with a tangent
Why is initial rate of reaction a good measure?
Concentrations of all reactants known
Temperature is known and controlled
How to work out orders of reaction from various experiments in a table?
Look at 2 experiments where all concentrations are the same except 1, and see what change this has done to the initial rate
If you conc has doubled and initial rate is the same then is order 0
If rate has doubled is 1st order
If rate has multiplied by 4 then its 2nd order
Can use fact that you’ve worked out zero order, to work out other unknowns
Put values into rate equation to work out k
What happens to rate constant if temp is increased?
It is increased, as a greater proportion of collisions will be successful
What happens to rate constant if a catalyst is added?
It will increase, as catalyst provides an alternative route with lower activation energy so greater proportion of collisions will be successful
What’s a clock reaction?
Where the time is measured for a specific change to occur, time is inversely proportional to the rate of reaction
What is the rate determining step?
The slowest step in a reaction mechanism of a multi step reaction
How to tell if a reaction is single step?
If the reactants in the normal balanced equation match the rate equation
What’s the Arrhenius equation?
k= Ae^(Ea/(RT)
k = rate constant A= frequency factor e= mathematical quantity Ea= activation energy R = gas constant T= temp in kelvin
Logarithmic form of Arrhenius equation?
ln K = -Ea/RT + ln A
-Ea/RT = gradient
ln A = intercept
