Topics: Chemical reaction, Chemistry, Reaction rate Pages: 9 (2989 words) Published: October 22, 2013
Controlled assessment- How concentration affects the rate of reaction between sodium thiosulfate and hydrochloric acid. SA): Strategy
Scientific knowledge
Sodium thiosulfate (Na2S2O3), also spelled sodium thiosulphate, is a colourless crystalline compound that is more familiar as the pentahydrate,Na2S2O3·5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.” Sodium thiosulphate has a melting point of 48 degrees C. It is freely soluble in water. It contains five molecules of water as water crystallization. At 48 degrees C, the sodium thiosulfate melts; at 215 degrees C, it loses all its five molecules of water; and above 220 degrees C, it is converted into sodium tetrasulfide. The collision theory briefly: For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, surface area, temperature and the addition of catalysts. 

Different reactions can happen at different rates. Reactions that occur slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, rusting is a slow reaction: it has a low rate of reaction. Burning and explosions are very fast reactions: they have a high rate of reaction.

For a chemical reaction to occur, the reactant particles must collide. But collisions with too little energy do not produce a reaction. The particles must have enough energy for the collision to be successful in producing a reaction. The rate of reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction. Collision theory is that for two molecules to react they have to collide with each other with sufficient energy for them to react. This is their activation energy. If the experiment looks at concentration then it means there is more probability of an acid molecule and a thiosulphate molecule as there are physically more of them If the experiment looks at temperature the molecules will be moving faster so will have more collisions so more chance of hitting and also will be more likely to have the required activation energy to react.

In a chemical reaction, the reactant particles can only react with each other when they bump into one another. According to collision theory when molecules collide, bonds between their atoms can break, and then new bonds can form to make new molecules. The molecules in gases and liquids are moving constantly, and millions of collisions take place every second. But only a small number of these collisions lead to the formation of product. For a collision to be 'successful', the particles involved must possess enough energy, called the activation energy, to break some of the existing bonds. Any change that increases the number of collisions per second, or increases the energy of the particles that are colliding, will increase the rate of reaction.

There is a minimum amount of energy which colliding 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 no reaction occurs. The faster the particles are going, the more energy they have. Fast moving particles are more likely to react when they collide. The collision theory tells us that the larger the surface area, the faster the reaction. So the higher the...
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