Rate of reaction Module 5 Flashcards
What is the reaction rate?
The change in the amount of reactants or products per unit time
Units are mol dm^-3 s^-1
Ways to follow the rate of reaction?
Measure the volume of gas produced
Measure the loss in mass as a gas is produced
Use colorimetry to measure colour change of a reaction
Measure the pH change of a reaction
How do you work out reaction rate from a concentration- Time graph?
Find the point of time where you want to know the reaction rate, then draw a tangent from that point on the curve
Work out gradient of that tangent using (change in y)/(change in x)
Must be positive value
How do you find the initial rate and what can you use it for?
Draw the tangent from were time=0
Can be used by changing concentrations to work out what order chemicals are
What does a clock reaction do?
Measure how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants.
There will be an observable endpoint when desired amount of product has formed, so the quicker the end point reached the faster the initial rate of reaction
What assumptions do you need to make when doing a clock reaction?
The concentration of each reactant doesn’t change significantly over time period of clock reaction
Temperature stays constant
When the endpoint is seen, that the reaction hasn’t proceeded too far
Explain the iodine clock reaction?
H2O2 (aq) + 2I- (aq) + 2H+ (aq) = 2H2O (l) + I2(g)
Small amount of Sodium Thiosulfate solution and starch are added to an excess of H2O2 (Hydrogen peroxide), and Iodide ions in acid solution (H+)
Starch is an indicator and turns blue/black in presence of iodine
Sodium thiosulfate instantly reacts with any Iodine that forms making it go back to iodide ions
So once the set amount of Sodium thiosulfate is used up solution will go blue/black, so can vary iodide or H2O2 concs to give you different times
What does the order of reaction tell you?
How the reactants concentration affects the rate
How do you know what order a reactant is?
0 order if you double reactants conc and rate stays the same
1st order if you double reactants conc and the rate doubles
2nd order if you double reactants conc and the rate quadruples (power of 2)
What’s the overall order of a reaction?
Sum of all the orders of all different reactants
What’s the only way to find orders of reaction?
Experimentally
What must you make sure to do when trying to find order how each reactant affects the rate?
Measure one by one, so make sure the other is constant and in excess
What’s the shape of a Concentration-Time graph of a 0 order reaction?
Straight diagonal line going down from y axis
What’s the shape of a Concentration-Time graph of a 1st order reaction?
Curved line with diagonal gradient from y axis, reaching an asymptote near x axis
What’s the shape of a Rate-Concentration graph for a 0 order reaction?
Horizontal line from Y axis, as changing concentration doesn’t affect rate
What’s the shape of a Rate-Concentration graph for a 1st order reaction?
Positive gradient for y=x, through the origin
Rate is proportional to concentration
What’s the shape of a Rate-Concentration graph for a 2nd order reaction?
Positive curve going through the origin, Rate is proportional to concentration^2
How do you draw a Concentration-Time graph?
Concentration on Y axis
Time on X axis
How do you draw a Rate-Concentration graph?
Rate on Y axis
Concentration on X axis
How do work out what order reactants are from a table which gives you their concentrations in the experiment and the initial rate of reaction?
Find 2 experiments where everything is the same expect the concentration of one reactant, and see how the rate changes between experiments to work out what order that reactant is
Will have to use the fact that you know other reactants order of reaction to work out some of them aswell (normally order 0)
How would you write the rate equation if A + B = C + D?
Rate (mol dm^-3 s^-1) = k x (A)^m x (B)^n k= rate constant m= order of reaction of A n = order of reaction of B (A)= concentration of A (B)= concentration of B
What does a bigger rate constant mean?
The faster the reaction
Do you include a reactant in the rate equation if it’s order 0?
No because anything to order 0 is 1
How do you work out the rate constant?
Insert all your values into rate equation, and rearrange to find k
What’s an easy way to find the rate constant of a first order reaction?
Draw the Rate-concentration graph
The rate constant is equal to the gradient
What is half life?
The time taken for half the reactant to be used up
What’s the half life for a first order reactant?
Constant, will always be the same amount of time for reactant to half
Meaning it’s half life can be read off it’s Conc-Time graph
How do you work out the rate constant of a first order reaction if you know it’s half life?
k = ln2 / half life Units = s^-1
What’s the half life for a second order reaction?
Not constant
What’s the rate determining step?
Is the slowest step in a multi step reaction, which determines the overall rate
Rules for determining if a reactant is involved in the rate determining step?
If it’s in the rate equation it must affect the rate, so it must be in the rate determining step
If a reactant isn’t in the rate equation then it can’t be in the rate determining step, neither can anything derived from it
What does the order of reaction in the rate equation tell you about the rate determining step?
1st order means 1 molecule
2nd order means 2 molecules
Can be used other way round, so look at rate determining step and the amount of molecules to work out what order of reaction it is
What does the rate equation normally tell you?
Which chemicals and how many molecules are reacting in the first step
For a reaction to happen, what needs to occur?
Particles need to:
Collide with each other
Have enough energy to react ( at least the activation energy)
Have the right orientation
What does increasing the temperature do the reaction rate?
Particles have more energy, speed up, collide more per unit time
More reactant particles have required activation energy so greater proportion of the collisions will result in reaction per unit time
Therefore temperature increases rate of reaction
What’s the Arrhenius equation?
k = A x e^((-Ea)/RT))
k = rate constant Ea= activation energy (j mol^-1) T= temperature in kelvin R = Gas constant = 8.31 A = the pre exponential factor, another constant
What’s the Arrhenius equation in ln form?
ln k = -Ea/Rt + ln A
How do you use ln form of equation to create a Arrhenius plot and find activation energy and the pre exponential factor?
Plotting ln K on y axis so ln K is substituted for y
Plotting 1/T on x axis so 1/T is substituted for x
y = mx + c
LnK = -Ea/R x + lnA
Creating a graph with a gradient of -Ea/R and a y intercept of lnA
Calculate the gradient and make it equal to -Ea/R then solve for Ea
Find the y intercept and make it equal to ln A and solve for A, or just put Ea into equation
(Make sure graph is well proportioned, doesn’t need to show Y intercept)
