C6-Rate of Reaction


Calculating the Rate of Reaction

Rate of Reaction = amount of reactant used OR product formed : time

Tips about how to measue the amount of reactant used :

  • If one of the products is a gas, measure the mass of the reaction mixture, in grams (g), before and after the reaction takes place and the time it takes for the reaction to happen. 
  • The mass of the mixture will decrease. 
  • The units for the rate of reaction may be given as g/s.
  • The amount of reactants can also be measured in moles (mol).
  • When the reaction takes place the amount of the reactant remaining decreases.
  • The concentration of the reactant is calculated as the amount divided by the volume of the reaction mixture (mol/dm3)

When measuring the amount of products formed, you can use a gas syringe is the product is a gas and the tie it takes for the reaction to happen. The units used would be cm3/s.

Measuring the time it takes for a reaction to become opaque or change colour - 

Rate of reaction = 1 : time taken for solution to change colour

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Collision Theory

Chemical Reactions only occur when the reaction particles collide into each other with sufficient energy. The minimum amount of energy needed to cause a chemical reaction is called the activation energy.

Overall, there are 4 factors that affect the rate of reaction : 

  • Temperature
  • Concentration
  • Surface Area
  • Catalysts

Temperature : When the temperature is higher, the particles have more energy and collide more often. So more collisions are successful. 

Cocentration : When you increase the concentration there is a larger amount of partilcles in the same volume, so there are more frequent collisions due to the fact that there is less free space between particles. Increasing the pressure of reacting gases also increases the frequency of collisions but in a different way - there is the same amount of articles but the volume has decreased.

Surface area : A powder would react faster that a solid piece of reactant because more of it is in contact to the surrounding. If a powder is used more particles are exposed and available for collisions, so there are more collisions anda faster reaction. 

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Reaction Rate Graphs & Closed Systems

Graphs can be used to show the progress of a chemical reaction.

Remember : 

  • The steeper the line, the faster the reaction.
  • When one of the reactants is used up, the reaction stops = the line becomes horizontal.
  • The same amount of product is formed from the same amount of reactants, regardless the rate. 

Closed systems

In a closed system, no reactants are added and no productrs are removed.

When a reversible reaction occurs in a closed system, an equilibrium is achieved when the rate of forward reaction is equal to the rate of the backward reaction.

The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction. 

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A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. They are never included in the chemical equation for the reaction. 

A catalyst:

  • reduces the amount of energy needed for successful collisions to occur.
  • makes more collisions successful.
  • speeds us the reaction
  • increases the chances of a particles bumping into each other as they provide a surface for the molecules to attach to. 

Enzymes act as catalysts in a biological system. Different reactions need different catalysts, e.g. Th cracking of hydrocarbons uses broken pottery // the manufacture of ammonia uses iron. 

Using catalysts in industries is important because increasing the rates of chemical reactions helps to reduce costs. 

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Reversible Reactions

Some chemical reactions are reversible, they can go forwards or backwards. In a reversible reaction, the products can react to produce the original reactants. 

A  +  B                           C  +  D

This means that A and B can react to make C and D but also C and D can produce A and B.

Hydrated Copper Sulfate (Blue)                                   Anhydrous Copper Sulfate (White)    +    Water

When you heat up the Hydrated Copper Sulfate the water evaporates out of it and sets on the walls of the test tube. Also, the colour of the powder changes from blue to white when the water evaporates. To reverse the reaction we need to add water, when we do this the powder changes back to blue and the water put into the test tube is boiling at the contact (gets really hot). The first reaction is an endothermic reaction, which means that it takes in energy (heat from the bynsen burner used to evaporte the water from the original powder). The second reaction is an exothermic reaction, which means that it emitts energy (heat) into its surroundings. The amount of energy taken in in the endothermic reaction is the same as the amount of energy given out in the exothermic reactions as energy can not be made or destroyed. 

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