Rate equations 3.1.9 Flashcards
how can you calculate rate from a graph
Straight line graphs :
-rate can be found from the gradient
- gradient = change in y / change in x
Curved graphs :
- rate can be found from the gradient of a tangent on a curved line of best fit
- with curves you just draw a tangent
- extend the line right across the graph and work out the gradient
- gradient = change in y / change in x
what is an order of reaction
an order is the power to which a concentration is raised to in the rate equation. It tells us how the concentration of the substance affects the rate
what is the rate equation
Rate = k[A]^a[B]^b
what is zero order
changes in the conc has no effect on rate
what is first order
changes in the conc has proportional change on rate.
what is second order
changes in the concentration has a squared proportional change on rate
how can orders be determined
Only by experiments .
you can’t work them out by looking at an equation
what is rate constant - k
a constant is a number that allows us to equate rate and concentration
what changes rate constant
It’s only fixed at a particular temperature . If temperature changes so does the rate constant
The larger the value of k the faster the rate of reaction.
why does increasing the temperature increase rate of reaction
as we increase temperature the particles have more kinetic energy and they collide more often . This increases the rate of
how can you work out initial rate from a graph
- take the gradient of the tangent but with initial rate we take it at 0 minutes.
- gradient = change in y / change in x
what is the iodine clock experiment used to measure
The initial rate where the time taken to reach a fixed concentration is measured .
The end point is achieved when one limited reactant runs out resulting in a colour change
What’s the equations for the iodine clock experiment and what happens
H2O2(aq) + 2H+(aq) + 2I-(aq) => I2 (aq) + 2H2O(l)
2S2O3 2- (aq) + I2(aq) => 2I-(aq) + S4O6 2-(aq)
sodium thiosulfate reacts immediately with iodine to form iodide ions .
When there’s no more sodium thiosulfate left the iodine reacts with the starch
Giving us a blue - black colour
How can you determine inital rate from the iodine clock experiment
- Varyinf the conc of iodine and / or hydrogen peroxide (H2O2) and keeping everything by constant will result in time taken for the blue / black colour to appear changing .
- We can then work out the order of reaction
How can the change in pH of a reaction allow us to measure rate
- The pH of a reaction may change over time if H+ ions are used up or produced
- A pH meter can be used to measure the pH of a reaction at regular intervals
- You can then calculate the H+ ion concentration
How can rate be measured in the amount of loss reactions
For reactions that produce a gas :
Place reaction on balance and measure the mass loss as gas is lost
- mole calculations can be used to work out number of moles of gas lost and hence the amount of reactants left
what is the continuous rate method
When we follow one experiment over time recording the change in concentration
How can we interpret a graph from the continuous rate monitoring method
Gradient = rate of reaction
- reaction is the fastest at the start where the gradient is the steepest .
- Rate drops as reactants start to get used up and their concentrations drop
- Graph will eventually become horizontal and gradient becomes zero which represents the reaction having stopped
what’s the method of the practical for the measurement of the change in volume of gas
- Measure 50cm3 of 1.0moldm-3 HCl and add to conical flask
- Set up gas syringe in the stand
- Weigh 0.20g of magnesium
- Add the magnesium ribbon to the conical flask , place bung firmly into the tip of the flask and start timer
- Record volume of hydrogen gas collected every 15 seconds for 3 mins
what’s the reaction for the continuous measurement of change in volume of gas experiment
Mg + HCl => MgCl2 + H2
What happens in reactions where there are several reactants and if conc of one of the reactants is kept in a large excess
-That reactant will appear to not affect rate and will be pseudo-zero order .
- This is because it’s conc stays virtually constant and does not affect rate
What can rate - concentration graphs help us identify
The order
what do zero order graphs look like
It would show a horizontal line
Changing conc doesn’t change the rate
what would a first order graph look like
Shows a straight diagonal line .
Changing conc changes the rate equally
what does a second order graph look like
Curved line going up
Changing conc changes the rate squared
how is the volume of gas experiment used to measure the rate
- Measure the amount of gas produced using a gas syringe . Measure this over a specified time
- Use the ideal gas equation to work out the number of moles of gas produced .
- Use the molar ratio in the equation to work out reactant concentrations
How can we use measuring a colour change to measure rate in experiments
- Use a colorimeter
- It measures the absorbance of light by a coloured sample .
The more concentrated a sample is , the darker its colour hence the more light absorbed
what’s a common example of using colour change to measure rate in experiments
Propanone and iodine
Reaction goes from brown to colourless
I2(aq) + CH3COCH3(aq) => CH3COCH2I(aq) + I-(aq) + H+ (aq)
Brown + colourless => all products colourless
How do we interpret the results in a colour changing experiment to measure rate
We use a calibration graph and absorbance
- First plot a calibration curve
- This is created by making up a range of known different conc of sample (e.g iodine)
- Absorption is measured for each one and results are plotted
EQ : Rate equation for a reaction is rate = k[E]
Explain qualitatively why doubling the temperature has a much greater effect on the rate of the reaction than doubling the concentration of E. (3)
- Reaction occurs when molecules have E > Ea
Doubling T by 10 degrees causes many more molecules to have this E .
Whereas doubling [E] only doubles the number with this E
What is the slowest step called
the rate determining step
which step of a reaction controls the overall rate of reaction
the rate determining step ( slowest step)
how can the arrhenius equation be simplified
In K= In A - Ea / RT
what is the arrhenius equation
k = Ae - Ea / RT
k = rate constant
A = arrhenius constant
Ea = activation energy ( Jmol-1)
R = gas constant (8.31Jmol-1k-1)
T = temperature ( K)
when we increase the activation energy what happens to rate constant
K is going to decrease .
- because we have less particles with sufficient energy for a successful collision to occur in the correct orientation
what happens to the rate constant when we increase the temperature
K is going to increase
- because particles have more kinetic energy so have a higher chance of successful collisions in the correct orientation
what does the arrhenius plot show us
shows how K varies with T and Ea
What’s the equation for a straight line graph using the arrhenius ewuation
ln k = - Ea / RT + ln A
ln k = y
-Ea / R = m
In A = c
what’s the equation for a straight line graph
y = mx + c
y = y axis
m = gradient
x = 1/t
c = intercept
how do you rearrange the arrhenius equation for Ea
Ea = ( ln A - ln k ) RT
how do you rearrange the arrhenius equation for A
A = Ke ^ Ea - RT
How do you rearrange the arrhenius equation for T
T = Ea / R ( ln A - ln k)
EQ : Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction at other times during the experiment (1)
At start , concentrations are known
EQ : State how initial rate is obtained from a graph of the concentration of the product against time (2)
- calculate gradient
- at t=0 or at start of graph
what is the overall order of a reaction
adding all the individual orders together
EQ : explain why doubling the temperature has a much greater effect on the rate of reaction than doubling the concentration of E (3)
- reaction occurs when molecules have E > Ea.
- Doubling T causes many more molecules to have this Energy
- Whereas Doubling conc of E only doubles the number with this energy
