1.9 Rates Flashcards
Hydrogen peroxide solution decomposes to form water and oxygen.
2 H2O2(aq) → 2 H2O(l) + O2(g)
(a) Explain why the reaction is fastest at the start.
Higher concentration of reactants at start so there are more successful collisions
Explain why rate of reaction is faster at a higher temperature
more particles have energy greater than or equal to activation energy and so there are more frequent successful collisions
Number of molecules decreases
Particles are spread further apart
Fewer collisions between gas particles so fewer successful collisions
Consider experiments 1 and 2: [B constant]
[A] increases × 3: rate increases by 32 therefore 2nd order with respect to A
Consider experiments 2 and 3:
[A] increases × 2: rate should increase × 22 but only increases × 2
Therefore, halving [B] halves rate and so 1st order with respect to B
Rate equation: rate = k[A]2[B]
What is meant by the term order of reaction with respect to A?
Power of concentration term in rate equation
When the concentrations of A and B were both doubled, the initial rate increased by a factor of 4. Deduce the overall order of the reaction.
2
Units for the rate constant
mol^-1 dm^3 s^-1
This can vary depending on the powers of the concentrations in rate equations
The rate equation for the hydrogenation of ethene
C2H4(g) + H2(g) C2H6(g)
is Rate = k[C2H4][H2]
At a fixed temperature, the reaction mixture is compressed to triple the original pressure.
What is the factor by which the rate of reaction changes?
A 6
B 9
C 12
D 27
The rate of the reaction initially is:
Rate1=𝑘[C2H4][H2]
When the pressure is tripled, the concentrations of C₂H₄ and H₂ are also tripled, so the new rate will be:
Rate2=𝑘(3[C2H4])(3[H2])=9⋅𝑘[C2H4][H2]
answer is B
Define the term overall order of reaction.
The sum of powers to which the concentrations are raised in the rate equation
What are the units of the rate constant for a third order reaction?
A mol dm–3 s–1
B mol–1 dm3 s–1
C mol2 dm–6 s–1
D mol–2 dm6 s–1
Rate=k[A] ^n
where n = 3
mol/dm ^3 s ^−1 =k×mol^dm^−9
re arrange for k to get answer D
A student investigated how the initial rate of reaction between sulfuric acid and magnesium at 20 °C is affected by the concentration of the acid.
The equation for the reaction is
H2SO4(aq) + Mg(s) MgSO4(aq) + H2(g)
(a) The student made measurements every 20 seconds for 5 minutes. The student then repeated the experiment using double the concentration of sulfuric acid.
b) other than temperature and pressure, that would need to be kept constant in this investigation.
Measure the volume of gas / mass of the container + contents
use a gas syringe
b) mass of magnesium /
surface area of magnesium
equipment used to measure volume of gas
gas syringe
measuring cylinder
(c) The student noticed that the temperature of each reaction mixture decreased during each experiment.
Suggest how the student calculated the temperature values in the table above.
measure temperature at start and end of reaction and find the mean
A student investigates the effect of temperature on the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid.
Na2S2O3(aq) + 2 HCl(aq) → 2 NaCl(aq) + SO2(g) + S(s) + H2O(l)
(e) Suggest, by considering the products of this reaction, why small amounts of reactants are used in this experiment.
SO2 is a toxic gas as it is poisonous
Sodium thiosulfate reacts with dilute hydrochloric acid as shown.
Na2S2O3(aq) + 2 HCl(aq) → 2 NaCl(aq) + SO2 (g) + S(s) + H2O(l)
The initial rate of the reaction between sodium thiosulfate and hydrochloric acid can be monitored by measuring the time taken for a fixed amount of sulfur to be produced.
Describe an experiment to investigate the effect of temperature on the initial rate of this reaction.
Include
* a brief outline of your method
* how you will measure the time taken for a fixed amount of sulfur to be formed
* how you will present your results in graphical form
- use a disappearing cross to see when solution is done reacting
- put HCL / sodium thiosulfate into container with cross and add
add second reactant and start timing. - repeat at different temperatures
- record time for cross disappear using suitable intervals
- calculate 1/time, as this is measure of rate
- plot of rate against temperature
What is likely to decrease the accuracy of the experiment?
A Rinsing the flask with acid before each new experiment.
B Stirring the solution throughout each experiment.
C Using the same piece of paper for each experiment.
D Using different measuring cylinders to measure the volumes of acid and sodium thiosulfate.
A
State how initial rate is obtained from graph of concentration of product against time
At time equals zero, calculated gradient
If a graph is a straight line through there origin, what did this day about the order
It is first order
If initial rate is plotted against a concentration of a solution squared and the graph turns out to be a line of best fit thorough origin, what is the order
Second
b) state why different volumes of water added to B,C, D and E
Moles of propane = 0.025
Total volume = 0.09 dm^3
Conc of propane = 0.2778
b) to make volumes constant for all mixtures so volume of propanal is proportional to concentration
What should you say about concentrations in large excess in rates
Concentration stays constant and does not affect rate
Concentration much larger than those that aren’t zero order
Why use colorimeter
Use colorimeter to minimise human error in timing
Rate=k[H^+]^C where C is a constant. It is later found out that [H^+] is zero order, state the units of rate constant
mol dm^-3 s^-1
