During a reaction at constant temperature the concentration of the reactants decreases and so the rate of reaction gradually slows down.
For an endothermic reaction, one that absorbs heat, increasing the temperature may produce large increases in the rate of reaction. A 10°C rise can double the rate while a 40°C rise can produce a 50- to 100-fold increase in the rate.
The collision theory is used to explain these effects. Increasing the concentration of a liquid means there are more particles per unit volume, therefore there are more collisions and more successful collisions. Increasing the temperature makes the particles move much faster, resulting in more collisions per unit time and more successful collisions; consequently the rate increases.
Factors:
Reactions happen repeatedly but not always at the same speed. Certain variables affect the overall speed of the reaction and the number of collisions that can occur. The variables depend on what state of matter the reaction is taking place in whether it is gas or liquid or solid, some have a larger number of factors then other states of matter.
Concentration: If there is more of a substance in a reaction, so there is more molecules of that substance within that system. This means there is a greater chance that molecules will collide and speed up the rate of the reaction; because there are a lot more molecules of that substance in the system so this increases the ration of collision of molecules in this system.
If there is less of a substance, so less molecules, there will be fewer collisions and the reaction will most probably occur at a significantly slower speed. This diagram illustrates how less acid molecules will mean fewer collisions between acid molecules and the marble chips.
Temperature: When the temperature is raised of a reaction, the molecules move around much more (because they have more energy given in the form of heat). When they bounce around more, they are more likely to collide. The collision rate between molecules goes up. When the temperature is lowered, the molecules are slower (because they have less energy) and collide much less. This temperature drop decreases rate of reaction.
Pressure: Pressure affects the rate of reaction, especially in the gas state of matter. When you increase the pressure, the molecules have less space for them to move around freely in. The larger concentration of molecules affectively increases the number of collisions. If you decrease the pressure, molecules don't hit each other as often. Lower pressure decreases the rate of reaction.
Surface area: If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So the larger the surface area of the solid, the faster the reaction will be.
Summary of factors:
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Temperature: A reaction at a higher temperature increases rate of reaction because of the induced energy through heat.
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Concentration: If the concentration of the reactant increases therefore collisions of molecules increase which speeds up the rate of reaction.
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Light: Light is a form of energy. It may affect the rate maybe even the course of a reaction. For example when methane reacts with chlorine in dark, the reaction rate would be very low. It can be speeded up when the mixture is put under diffused light. In bright sunlight, the reaction is explosive. http://en.wikipedia.org/wiki/Rate_of_reaction
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A catalyst: A catalyst increases rate of reaction by providing an alternative pathway with lower activation energy.
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Surface Area: In some systems if surface area is increased the rate of reaction increases. Larger surface area compared to the volume, therefore means a faster reaction will occur.
Diagram:
Equipment list:
- Conical Flask
- Bung + Rubber Pipe
- Measuring cylinder
- Limestone
- Weighing scales
- Hydrochloric Acid
- Plastic Tub
Method:
The first step was to weigh the limestone chipping and get it as close as possible to one gram. The once I had collected my equipment and put on the goggles and apron I measured out the correct volumes of acid and later water. Then I mixed/poured the acid and/or water into the conical flask. Then I filled a measuring cylinder with 100ml of water and filled a plastic tub with water to the top. I had to submerge the measuring cylinder into the tub as quickly as possible without loosing water I used my hand to cap the cylinder. Once submerged the rubber tube was inserted into the cylinder. Following this I dropped the limestone into the conical flask and started the timer and applied the bung on to the flask as fast as I could. Results were recorded by how much was water was lost every 30 seconds out of the measuring cylinder and into the tub of water.
Safety Precautions:
- Safety goggles – Stop any acid damaging the eyes
- Aprons were used – Stop acid from attacking skin incase of spillage
- Hands washed – Hands were washed to free them of any chemicals after the experiments
- Spillages were wiped clean
Preliminaries:
After Preliminary Evaluation:
I have tested 2 fair extremes of the reaction with a high concentration of acid and a low one. Also I have used two different time scales; I conclude that I will use a large time scale and that the factor of concentration will increase by 5ml acid every attempt, with the attempts for every measurement for fair and accurate and reliable results.
Evaluation:
Although keep the temperature constant, kinetic theory is relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react.
Conclusion:
Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is the collision theory again. The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen. If there are a higher number of collisions in a system, more combinations of molecules will occur. The reaction will go faster, and the rate of that reaction will be higher.
