6.2.4

Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.

Questions	Answers
Independent Variable	Surface area of CaCO3 chips (i.e. powder, chips #2 etc.)
Dependent Variable	Volume of CO2 produced
Controlled Variables	Volume of HCl, mass of CaCO3, stop watch, inverted measuring cylinder, top pan balance.
Using collision theory explain the following shape of the graphs at the start of the reaction.	The larger the surface area of CaCO3 the faster the rate of reaction. This is because in a solid only the particles on the surface can collide successfully hence if it is powdered more particles can come into contact with the other reactant.
What does the gradient of the graph at any one point represent?	Rate of reaction
What are the units for the gradient of the graph?	cm3/seconds
Discuss the reasons for the differences in the shape of the graphs.	The larger the surface area of CaCO3 the steeper the graph should be, due to an increase in rate of reaction.

6.2.3

Describe the collision theory.

Factors affecting rate of reaction	Description	Diagram
Collision Frequency	Higher the frequency of collision, the higher the probability of successful collision.
Collision Geometry	Due to particles coming in different orientations it is necessary for the collision to take place successfully.
Number of particles with E ≥ Ea	In order for collision to take place particles must have a minimum amount of energy to overcome repulsion between molecules and to break some bonds in the reactants. Only particles with E ≥ Ea will successfully collide.

6.2.2

Define the term activation energy, Ea.

The minimum value of kinetic energy which particles must have before they are able to react.

6.2.1

Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in Kelvin’s.

The kinetic theory states that when temperature of a substance is increased the average kinetic energy of the particles increases too. However if the temperature reaches absolute zero it means there is no average kinetic energy as there is no motion of particles.

In the apparatus shown below, the increase in voltage mimics the rise in temperature while the height of the polystyrene cylinders represents the increase in volume.

6.1.3

Analysing data from rate experiments.

6.1.2

Suitable experimental procedures for measuring rates of reactions.

Experimental Method	Dependent and Independent Variables	Additional Notes	Diagram
Change in volume of gas produced	Independent: time (seconds)  Dependent: volume of gas (cm3)	Gas syringe (appropriate) or displacement of water in an inverted burette/measuring a cylinder (but only for gas with low solubility in water).
Change in mass	Independent: time (seconds) Dependent: mass (g)	Top pan balance however doesn’t work with light weight gases (e.g. hydrogen)
Change in transmission of light: colorimetry/ Spectrophotome-try	Independent:  time (seconds) Dependent: light intensity	Higher the concentration of the coloured compound more light is absorbed thus less is transmitted	 2HI(g) à H2(g) + I2(g) 
Change in concentration measured using titration	Independent: time (seconds) Dependent: concentration	Samples taken from solution. Quenching stops the reaction at the moment it is withdrawn.	H2O2(l) + 2H+(aq) + 2I-(aq) à I2(aq) + 2H2O(l)
Change in concentration measured using conductivity	Independent: time (seconds) Dependent: electrical conductivity	Conductivity meter- electrodes put into solution. Readings converted into concentrations of ions present.	BrO3-(aq)+5Br-(aq)+6H+(aq)à3Br2(aq) +3H2O(l)
Non-continuous methods of detecting change during a reaction: ‘clock reactions’	Independent:  time (seconds) Dependent: observing a change in the reaction (e.g. magnesium ribbon reacting completely)	Where rate of reaction is hard to measure, time is recorded. Reaction must be closely observed.

6.1.1

Define the term rate of reaction.
The decrease in the concentration of reactants per unit time or the increase in the concentration of product per unit time.

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