Chemistry - Topic 6 - The Rate and Extent of Chemical Change
Rate of reaction in business means, the faster the reaction the faster you make money.
Factors Affecting Rate
Concentration: The more concentrated, the more particles, the more collisions
Pressure: More pressure (gasses), particles close together, more collisions.
Temperature: High temp, particles have more energy, particles move faster, more collisions (and harder ones- more likely to have activation energy)
Surface Area- smaller pieces, more surface area, more exposed particles, more collisions.
Catalyst: Lowers activation energy, more collisions have Ea
HT Only Be aware we need to be able to find gradient of a tangent to the curve of a graph. This measure rate over time
Make sure you can
Describe practically how to measure rate (measuring rate required practical)
Calculate rate of reaction from supplied data or graphs
Explain in terms of particles and collisions, how concentration, temp, pressure, SA and a catalyst effect rate of reaction and why they slow down (as particles are used up less collisions occur, limiting factor)
FOR HT: calculate the rate of reaction from the gradient of the line/curve on a graph
Measuring rate
The rate can be calculated by measuring the quantity (amount) of a react used or amount (quantity) of a product produced over time
Mean rate of reaction = Quantity of reactant used / time taken
Mean rate of reaction = quantity of product formed / time taken
The amount of reactant or product can be measured by the mass in grams or by the volume in cm^3. Units of rate of reaction may be given as g/s or cm^3/s
Interpretation Graphs
Mean (average) rate of reaction can be calculated from supplied data or graphs- the steeper the gradient the faster the reaction.
e.g if a reaction makes 40^3 gas in 60 seconds
Mean rate = 40cm^3 / 60 (s) = 0.67 cm^3/s
Typical Exam Questions Reaction takes place as shown here and is timed until the cross beneath the flask can no longer be seen: Na2S2O3 (aq) + 2HCL (aq) -------> 2NACL (aq) + H20(l) + SO2 (g)+S (s)
The student is investigating the effect of temperature on rate. Give two variables the student should control to make a fair test. *Any TWO from: vol of (state one or both) or concentration of (state one or both)
State the effect that increasing the temp of the sodium thiosulphate has on the rate. Explain using particle theory. Answer: Rate: Increases, particles have more energy, move faster, more successful collisions (with activation energy)
The reaction at 20C produced 0.32g of sulphur in 64 seconds. Calculate the rate of reaction, use correct units. ANSWER*: 0.32g / 64s = 0.005 g/s
Rates of Reaction-Required Practical 5
Equipment
40g/dm(cubed) sodium thiosulphate solution
2.0m dilute hydrochloric acid
10cm(cubed) measuring cylinder
100cm(cubed)measuring cylinder
Printed black paper cross
Stop clock
Method 1)Use a measuring cylinder to put 10cm(cubed) sodium thiosulphate into the flask
Using the cylinder then add 40cm(cubed) of water to dilute the sodium thiosulphate to a concentration of 8g/dm(cubed)
Then place the flask over a black cross
2)Put 10cm(cubed) of dilute hydrochloric acid into the flask. At the same time swirl and start the stopwatch.
3)Look down through the top of the flask, stopping the clock when you can no longer see the cross. Take care to avoid breathing in fumes
4)Write the time for the cross to disappear in your table
5)Repeat the previous steps but in step 1 use:
20 sodium thiosulphate + 30 water (concentration 16g/dm^3)
30 sodium thiosulphate + 20 water (concentration 24g/dm^3)
40 sodium thiosulphate + 10 water(concentration 32g/dm^3)
50 sodium thiosulphate + no water (concentration 40g/dm^3)
6)Ten repeat the entire investigation 2 more times calculating the mean, and plotting a graph:
Mean time taken for cross to disappear (secs) on the y-axis
The rate of reaction can be found out in this experiment as the sulphur, changes the solution from clear to yellow/cloudy.
Risk Assessment Be careful when looking into the flask-Do not breath in fumes. Also goggles must be worn at all time.