TOPIC 6

6.1.1 Define the term rate of reaction
The change in concentration of the product or reactant per unit time (concnetration of product will increase and concentration of reactant will decrease). The rate of chemical reaction is a measure of the amount of reactants being converted into products per unit amount of time.


6.1.2 Describe a suitable experimental procedures for measring rate of reaction

Method	Description	Diagram
Change of volume of gas produce	This is a suitable method if one of the products is gas. We can collect the gas and measure the change in volume of the gas at a regular time intervals. From this we can produce a graph of change in volume of gas against time. Gas can be collected via a syringe or displacement method.
Change in mass	A lot of reaction involves change in mass. Some reaction where gas is given off, there will be a change in mass. We just have to place our mixture on a balance and we will get a continuous reading of change in mass against time. But in some cases, this won’t work. For example, if hydrogen gas is given, there won’t be a significant change in mass since hydrogen is too light.
Change in transmission of light: colorimetry/ spectrophotometry	This technique can be use if one of the reactants or products is colored and so will give characteristic absorption in visible region. A colorimeter or a spectrophotometer will shine light through the mixture in the reaction. The mixture will absorb some light and we can measure the amount of light that is transmitted using photocell. This method allow continuous readings and the graph of absorbance against time can be plot.
Change in concentration measured using titration	In a reaction, it is possible to measure the concentration of the product or reactant to a known standard. However, this cannot be done continuously since we have to take a sample from the reaction at a regular time intervals to measure its concentration using titration method. For this to work properly, quenching need to be use. When we take out a sample, it is possible that the reaction is sill occurring in the sample causing the concentration to not be accurate by the time we titrate. There we need to add a substance which stops the reaction from occurring in the sample.
Change in concentration measured using conductivity	The total electrical conductivity depends on its total concentration of ions and charges. So when a reaction occurred, the concentration will change and therefore we can measure the change in conductivity. This can be done by using conductivity meter which involves immersing inert electrodes in the solution.
Non continuous of detecting change during a reaction “clock reaction”	For some reaction, it is hard to record a continuous change. Therefore we use this method. We choose a fix point which acts as a arbitrary end point, when our reaction reach this point, we then stop the clock. We can do the same reaction under different concentration to compare and contrast the rate of reaction.

6.1.3

6.2.1

The kinetic theory is the theory that proves that every particles move. They move in random direction and speed (random velocity). Since the particles move, we can say that they posses kinetic energy. However, because every particles move at different velocity, the will have different kinetic energy so therefore we have to take their average kinetic energy. Furthermore, we also know that the average kinetic energy is proportional to the temperature in Kelvin. So if we increase the temperature, we will increase the average kinetic energy possess by the particles and therefore increase the average velocity of the particles.

This is an image of a piece of eqiupment which show us the kinetic theory. When we turn on the machine, it'll cause all little the metal balls inside inside the glass tube to jump around in random direction and speed (velocity). The voltage is like the temperature, it control the avergae kinetic energy.

6.2.2 and 6.2.3

Definition of activation energy, Ea	It is the minimum value of kinetic energy the particles must have before they are able to react. Catalyst lowered this value.
Collision theory	We know that the particles possess kinetic energy which allows them to move. This kinetic energy can cause particles to collide. The energy in these collisions cause some bonds to be broken and made and therefore product are formed. Using the collision theory, we can work out the 3 factors that affect the rate of reaction.
Factor 1- Collision frequency	If the particles collide more frequently, more products will be made. This will increase the rate of reaction. To increase the collision frequency we can increase, the pressure (less volume for the particle to move), temperature (increase KE and also see maxwell boltzman curve) and concentration of the non excess reactant (more paticles but if we increase this the amount of product produced will increase too).
Factor 2- Number of particles with kinetic energy greater than the activation energy.	If there are more particles then the collision frequency would increase. More particles mean more chance of collision. If those particles have kinetic energy which is less than the activation energy then it’ll be useless because they won’t be able to react. So need the particles to have more kinetic energy, we can do this by increasing the temperature because temperature is proportional to the average kinetic energy.
Factor 3- Appropriate collision geometry and orientation	We know that particles more with random velocity, therefore when they collide they will collide at random orientation. In some reaction this is crucial in determining if the reactions will occur or not. If particles collide at an incorrect orientation then reaction might not occur.

'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'

Note-This graph could be inaccurate due to 2 reasons. First, the marbles come from different suppliers and sources therefore we didn't control that variable causing inaccuracy. Also, the power marble was smothered so therefore not 100% of it's surface area was in contact causing inaccuracy in the results.

What was the independent variable?	Surface area of the calcium carbonate.
What was the dependent variable?	Change in volume of CO2 gas over time.
What variables were controlled?	Mass of calcium carbonate, volume of HCL, concentration of HCl, temperature of the reaction.
Using collision theory explain the following the shape of the graphs at the start of the reaction	The graph will be steeper at the start compare to later on. This is because at the start, there will be more partcles compare to later on where a lot of the particles have already reacted. More partciles mean more collision frequency therefore faster rate of reaction (therefore the gradient is steeper).
What does the gradient of the graph at any one point represent?	The gradient of the graph at 1 point represent the rate of reaction at that point only. The amount of volume of CO2 release over time at that point.
What are the units for the gradient of the graph?	Cm3/S  (centimetre cube per second)
Discuss the reasons for the differences in the shape of the graphs	Each line represents different surface area of the calcium carbonate. The one with the most surface area (powder) should be the steepest and have the fastest rate of reaction. This is because, if one of the reactant is a solid (calcium carbonate) then surface area will change the rate of reaction. If there is more surface area of calcium carbonate in contact with the HCL, then the collision frequency between calcium carbonate and HCL will increase therefore the rate of reaction will be faster.

6.2.5

Image A

 Image B

Maxwell Boltzman Distribution curve

A Maxwell Boltzman Distribution curves is a graph with, number of particles on the y axis and kinetic energy on the x axis. This graph tell us how many particles posses how many kinetic energy. We know that not all particles possess the same kinetic energy, but there will be a specific amount of kinetic energy that the majority of particles possess. It's the peak in the graph (refer to image). It's also the average kinetic energy point, so therefore it's also the temperature.

Maxwell Boltzman Distribution curve with activation energy

Furthermore, we know that for particles to react, the kinetic energy that they posses must be more than the activation energy. If we refer to image B, the activation energy is marked by the red line. We can see that only some particles posses enough kinetic energy to react.

Maxwell Boltzman Distribution curve with activation energy with change in temperature

We know that temperature=average kinetic energy. If we increase the temperature, we will increase the average kinetic energy. How does this transfer to the distribution curve? When we increase the average kinetic energy, the curve will become flatter and more particles will posses higher kinetic energy than before (refer to images). With that being said, if more particles posses higher kinetic energy, therefore more particles must have kinetic energy over the activation energy and they will be able to react. This is why when we increase the temperature, the rate of reaction is faster. It's because more particles will posses enough kinetic energy to react (posses more kinetic energy than the activation energy). One thing to keep in mind is that we do not put in or take out any particles therefore the number of particles stay constant. What this mean is that the area under the graph remain the same. We can change the temperature as much as we want, the area under the graph will always remain the same.

6.2.6

Catalyst

Catalyst is a substance that change the rate of reaction without itself undergoing permanent chemical change (it could change physically). It works by altering the activation energy of the reaction by providing a different route for the reaction. Most will lower the activation energy but some will increase the activasion energy, they are called inhibitor.

Image C

This image show how the catalyst alter the activation energy by providing a new route.

How catalyst works

Catalys might work by holding the reactant at the right orientation. From before, we know that in some reaction, it is required for the reactants to collide at the correct orientation. Therefore, the catalyst could hold the reactants at the right orientation so their collision will become more effective.

6.2.7

Catalyst with Maxwell Boltzman Distribution Curve

Now that we know what catalyst is and what a Maxwell Boltzman Distribution Curve is. Mixing these two together is very simple.

If we look at image B above. I've mentioned that the activation energy is marked by the red line and only the particles that have kinetic energy more than that red line will react.

If we add a catalyst to a reaction, we know that the activation energy will change because the catalyst will provide an alternative route for the reaction, the activation energy will usually lower.

How does this tranfer into the curve? It's very simple, the red line just change it place. If the Ea lower, then the red line lower (move o the left). If the Ea increase, then the red line increase (move to the right).

If the red line lower (move to the left), this mean that more particles will posses kinetic energy above the red line (Ea) so more particles will be able to react. This increase the rate of reaction.

Image D below explain this concept very well

Image D