Rate Graphs

Collision Theory Do you remember collision theory? What is the unifying statement?

Collision Theory Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

What things effect the rate of reaction? Temperature Surface Area Concentration Catalyst

Rate Graphs We’re going to look at how you use graphing techniques to prove these relationships. As well as a little maths If rate is defined as change in products over time, then plotting a graph of change over time will help calculate the rate. But we need to measure the products in some way

Things to Measure The measurement itself depends on the nature of the reactant or product: The mass of a substance - solid, liquid or gas The volume of a gas For example, if 24 cm3 of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12 cm3 hydrogen / min.

Effect of Temperature on Rate If the temperature is increased: the reactant particles move more quickly they have more energy the particles collide more often The rate of reaction increases

Measuring Rates: Using mass Another method used to follow a reaction monitors change in mass. CO2 is generated and the mass loss is seen on the scale according to the reaction: Hydrochloric Acid CaCO3 + 2HCl  CO2 + CaCl2 + H2O Marble chips (Calcium carbonate) 245.4

Measuring Rates 245.2

Measuring Rates 245.0

Measuring Rates 244.8

Measuring Rates 244.6

Measuring Rates As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds. 244.4

Measuring Rates As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds. 244.2

Measuring Rates As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds. 244.0

Measuring Rates As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds. 243.8

Measuring Rates As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds. 243.6

Measuring Rates As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds. 243.4

Using this technique you can draw a graph for the experiment. E.g. Mass vs. time Volume gas vs. time Pressure vs. time

Difference in Products If the gradient is… Difference in Y Difference in X And Rate is Difference in Products Difference in Time So long as the x-axis is time, then the gradient will be the rate at that point.

Draw axes using a large portion of the graph paper Add labels for each axis as well as units. Plot points Draw a best fit line Draw a tangent line at the required point. (t=60) Calculate the gradient. INCLUDE THE UNITS Gradient = change in Y change in X Gradient = 5.5 cm3 11 seconds Gradient = 0.5 cm3/sec ~5.5 units = 11 cm3 Amount of gas collected (cm3) 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 ~10.5 units = 105 seconds 10 20 30 40 50 60 70 80 90 100 110 Time (seconds)

Initial Rate Vs. Temperature If you calculate the gradient of a straight line from your first measurement to 0, you can find the initial rate of reaction. Then you can directly graph rate of reaction. This is also good for showing data transformation.

Sally Thinks… Sally thinks that you could look at the total change in mass to work out how much the temperature changes the rate. Is she right? Why or why not?