Rate of Reactions - Chemistry

How do we measure the Rate of a Reaction?

The rate of a reaction can be found by measuring the amount of reactant used or the product that is formed (e.g. measure mass loss of a gas or volume of gas in a syringe) over a period of time:

Rate of reaction: Amount of reactant used / product formed
                                                          Time

• In order for a reaction to occur, particles must collide.
• Particles must collide with sufficient energy in order to then react.The minimum amount of energy is called the activation energy.
• Particles with less energy than the activation energy move apart again unchanged after the collision.
• The activation energy is the energy required to break existing bonds. This must happen before new bonds form.
• Reactions that require large activation energies will occur less easily than ones with low values.
• Different reactions have different activation energies.                                     

How can we change the rate of reaction?

The rate of reaction can be changed by one of the following:

Changing the concentration of a solution or the pressure of a gas

• The more concentrated the particles in a solution are, the greater the number of particles in a given volume (per cubic centimetre). 
• As a result, the collision frequency increases. 
• The more often the particles collide, the greater chance there is that the particles with the activation energy, or with greater energy, will collide and then react, i.e. there is an increase in the frequency of sucessful collisions.
• The higher the pressure of a gas, the more often particles with sufficient energy will collide and therefore there will be a faster rate of reaction. 

Changing the Surface Area of a Solid

• Reactions between either a liquid or a gas with a solid can only occur at the surface.
• The greater the surface area, the greater the number of particles available to collide, therefore the more frequently particles will collide and the faster the rate of reaction will be. 

Required Practical

• CaCO_3(s) + 2HCl_(aq) CaCl_2(s) + H₂O_(l) + CO_2(g)

In this practical the reaction between hydrochloric acid and calcium carbonate will be followed using three different sized forms of calcium carbonate: A, B and C.

The rate of reaction will be monitored by measuring mass due to loss from the release of carbon dioxide gas. The mass is recorded every ten seconds until the reaction is complete. A cotton wool plug is placed in the neck of the conical flask to prevent the water from fizzing or escaping. Each experiment will use 5g of Calcium carbonate and 25cm of dilute HCl in excess.

Changing the Temperature of a Reaction

• The rate increases as the temperature increases, because the temperature increases the energy of the particles so that they move faster. This means that the particles collide more frequently.
• Importantly, more energy means that a greater proportion of the particles have at least the activation energy for the reaction and so a greater proportion will react upon collision. In other words, there will be a greater frequency of sucessful collisions. 

Required Practical

Na2S2O3(aq) + 2HCl(aq) → S(s) + SO2(g) + 2NaCl(aq)

• The time taken for the cross to disappear is measured for each of the 5 temperatures. 
• The flatter the conical flask is, the longer it will take for the cross to disappear and vice versa. 
• A systematic error may be that the Sodium Thiosulfate may have cooled down being added to room temperature HCl, and therefore should be conducted in a waterbath for more accurate results. 

The Usage of a Catalyst

Definition: A catalyst is something that speeds up a reaction by lowering the activation energy, providing another route for the reaction to happen, but is not used up.
This means that although the collision frequency has not changed, there is a greater chance that when particles do collide, they will react. 

• E.g. Manganese Dioxide
• Each catalyst is specific to each reaction
• You can prove that the catalyst was not used up, by weighing it before and after the reaction.
• They are good for the environment, because they save energy and pollution, it is the same rate but at a lower temperature.

General Notes

• The units of the rate of reaction is cm^2/s
  
  
• A curved line is created, because initially, the unused concentration and surface area are at the peak, meaning that there are many particles per cubic centimetre, and it can react with the largest amount of surface area. As the reaction progresses, this means that the concentration and surface area are being used up, meaning that the reactants are being used up, and the product is increasing.
• A limiting reagent is the reactant that controls the amount of product created as a result of the reaction. When it runs out, then the experiment ends. Therefore, another reactant is added in excess, to ensure that the limiting reagent is all used up. 

Collision Theory and Activation Energy Definitions

What is CollisionTheory?

Collisiontheory says that a chemical reaction can only
occur between particles when they collide (hit each other).
Particles may be atoms, ions or molecules.

What is Activation Energy?

There is a minimum amount of energy that particles
need in order to react with each other. If the colliding
particles have less than this minimum energy then
they just bounce off each other and noreaction occurs.
This minimum energy is called the activation energy.