18.1-18.5 Flashcards
How can rate be measure in experiments
- the change in pH of a reaction
- the amount of mass lost
- volume of gas produced
- colorimeter
How do you measure the rate of reaction if there is a pH change
- could be change in H+ ions used up or produced over time
- use a pH meter to measure the pH of a reaction at regular intervals
How do you measure the rate of reaction if there is an amount of mass lost
- USE FOR PRODUCTS THAT PRODUCE A GAS
- place reaction on balance and measure the mass lost as gas is lost
- use mole calculations to work out number of moles of gas lost
How do you measure the rate of reaction if there is a volume of gas produced
- measure the amount of gas produced using a gas syringe over a specific time
- use ideal gas equation to work out the number of moles of gas produced
How does measuring the rate of reaction using a colorimeter work
- colorimeter measures absorbable of light by a coloured sample
- the more concentrates a sample is the darker its colour and so the more light absorbed
What is a calibration curve
- a graph making up a range of known different concentrations
How do you find a rate from a graph
- from the gradient
= change in y
_______________
Change in x
How do you find the rate on a curved line
1) draw a tangent that meets the curve at a specific point
2) extend the line right across the graph
3)work out the gradient
What is the initial rate
- the rate right at the start of the reaction
How do you work out the initial rate from a curved line
- work out the gradient of the tangent at 0 minutes
What do clock reaction tell us
- how long it takes for a reaction to occur
What do clock reaction simplify
- simplify the initial rate method
What is the disappearing cross experiment
- an experiment monitored where a black cross is underneath the reaction vessel and over time the cross will disappear and you can no longer see it through the beaker.
- there is a colour change= end point
Complete the sentence:
The quicker the clock reaction…
- the faster the initial rate of reaction is
What 3 assumptions do we make for clock reactions
1) the temperature of the reaction remains constant
2) concentration of reactants doesn’t change significantly during the time period of reaction
3) reaction has not proceeded too far when the end point is seen
Based on our assumptions for rate of reaction what can we infer
- that rate of reaction REMAINS constant during the time period you are measuring
- the rate of the clock reaction is a good estimate of the initial rate of reaction
Describe the iodine clock experiment
1) add sodium thiosulfate and starch( which acts as an indicator) to excess hydrogen peroxide
2) sodium thiosulfate reacts immediately with the I2, that is produced in this reaction
3) when there is no more sodium thiosulfate left then the I2 reacts with starch to give a deep blue/black colour
What does varying the concentration of iodine and/or hydrogen peroxide while keeping everything else constant do to the experiment
- results in the time take for the blue/black colour to appear to change
What does the rate equation link together
- rate with concentration
What is the rate equation
Rate= k [A]^a [B]^b
Rate= moldm^3s^-1
K= rate constant (units vary)
A/B= concentration of substances
a/b= orders of the reaction
What do orders of reactions tell us
- how the concentration of the substance affects the rate
What effect does a zero order have on rate
- change in concentration has NO EFFECT on the rate
E.g if [A] doubles the rate doesn’t change
What does a 1st order mean
- changes in concetration has a proportional change on rate
E.g if [A] doubles then the rate doubles
What does 2nd order mean
- changes in concentration has a squared proportional change on the rate of
E.g if [A} doubles then rate quadruples
What is the only way we can determine orders
- by experiments
What is the rate constant
- a number that allows us to equate rate and concentration
What happens to rate constant (k) if temperature increases and why
- the rate constant (k) will also increase
- to balance the equation
Complete the sentence:
The rate constant is only fixed at a…
- particular temperature
What does a large value of k mean
- the faster the rate of reaction
What happens to the rate and concentration when there’s an increase in temperature
- the particles have more kinetic energy and they collide more often, which increases rate but the concentrations remain constant
What can rate-concentration graphs help with
- identify the order
What is the rate order of these graphs
- ZERO ORDER
- change in concentration doesn’t change the rate
What is the rate order for these graphs
- 1st order
- as the rate on a shallow curve change sin equal amounts so the rate-concentration graph shows a straight diagonal line
- change in concentration changes the rate equally
What is the rate order for these graphs
- 2nd order
- rate on a steep curve graph changes in unequal amounts
- rate-concentration graph shows a curved line
- changing the concentration changes the rate squared
What is half life
- the time it takes for half of the reactants to be sued up
What can half life be used to calculate
- the rate constant
What equation do you use to calculate the rate constant from a half life graph
K = ln2
—— UNITS: s^-1
t1/2
ln2= natural log on calculate
t1/2= half life
How do we use an initial rate experiment to work out the rate of equation
1) repeat the experiment several times but change the concentrations of A,b & C one at a time in each experiment whilst keep the others the same
2) Wouk ou intial rate for each experiment and calculate it by using graph
3) record concentration reactants used for each experiment and their initial rates in a table with respect to each reactants and write a rate equation
Answer this question:
Rate= 7.88moldm^-3s^-1
Answer this question
Rate= k[A]^2 [B]
What is the rate the determining step
- the slowest step in a multi-step reaction
What does the rate determining step determine
- the overall speed of the reaction
In chemical reactions, how do we try to increase the speed of a reaction
- using a catalyst
- or change the temperature (increase it)
Complete the sentence:
Reactants that appear in the rate equation affect…
- the rate of reaction
What are the reacts shows inn the rate equations
- the reactants in the rate determining step
If a reactant is in your rate equation but doesn’t appear in the overall equation, what must it be
- a catalyst
Which step is the rate determining step
- the step where you finally get all the elements in your rate equation
How can reaction mechanism be found
- from the rate determining step
How does rate determining step show us the reaction mechanism
- the mechanism that matches the ratio in the rate equation
What does the Arrhenius equation link
- activation energy & temperature to the rate constants
What happens to the rate constant as the activation energy gets smaller and why
- it gets bigger
- because as the activation energy drops the rate of reaction increases so many more particles have enough energy to react when they collide
What happens to the rate constant when we increase the temperature and why
- the temperature increases, as the particles have more kinetic energy and are more likely to collide with at least the activation energy which increases the rate of reaction
What is the Arrhenius equation
K= Ae^ -Ea/RT
K= rate constant
Ae= Arrhenius constant
-Ea= activation energy (J)
R= gas constant (8.314)
T= temperature (K)
Answer this question:
Calculate the activation energy in kJmol^-1 of a reaction at 330K and a rate constant of 1.3 x 10^-4 s^-1. Assume the Arrhenius constant is 4.55 x 10^13 and the gas constant is 8.31
Ea= 110781Jmol^-1
What are Arrhenius plots used for
- to find out the activation energy and Arrhenius constant
What are the axis of Arrhenius graphs
- x axis= 1/T
- y axis= lnK
How do you work out the activation energy in an Arrhenius graph
- by find the gradient of the line
Then you get -Ea/R= number - then you need to rearrange o make Ea the subject
Ea= -(number x R0
How do you work out the Arrhenius constant from a graph
- substitute the value of the gradient and the coordinates of any point on the line into the equation
-substitute into the lnK= (-Ea/R) + lnA equation as if it is a y=mx+c equation - y= lnK
- mx= (-Ea/R)
- c=lnA
- put in the e button on calculator and that’s the exponential
