# GCSE C2 Chapter 4

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• Created by: emma998
• Created on: 09-03-14 15:47

## Rate of reactions

• The rate of a reaction measures the speed of a reaction
• Rate of reaction = amount of product fromed / time
• Rate of reaction = amount of reactant used / time
• An average rate of reaction can be found by measuring time taken for a specific amount of product to be formed or for a specific amount of reactant to be used up
• Graphs can be used to measure the amount of product formed or amount of reactant used on the Y axis aginst the time taken on the X axis
• To find the rate of reaction at a given time is the gradient of the slope
• The steeper the gradient the faster the rate of reaction
• The Y axis can represent the amount of product formed, the amount reactant released, change in colour, change in pH, change in concentration or pressure or change in temperature
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## Collision theory

• Collision theory states that for a reaction to occur the particles must collide with enough energy to change into new substances.
• The minimum energy required is the activation energy for the specific reaction
• Factors that affect the rate of reaction are:
• Surface area
• Temperature
• Concentration / pressure
• Using a catalyst
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## The effect of surface area

• The larger the surface area of solids in a reaction the more collisions in a given time
• Braking large pieces of solid into small pieces exposes new surfaces for collisions meaning more collisions will occur in a given time increasing the rate of reaction
• So powder reacts faster than large lumps of a substance
• The finer the powder the faster the rate of reaction
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## The effect of temperature

• Increasing the temperature causes an increase in rate of reaction:
• Increasing the temperature increases the energy of particles
• This means they move around much quicker and with more energy
• More frequent collisions
• More collisions with the activation energy (more successful collisions)
• Faster rate of reaction
• In most reactions an increase of 10C will double rate of reaction and a decrease in 10C will halve rate of reacion
• This is why we refrigerate foods because it decreases the rate of reaction so the growth of bacteria is greatly reduced. Keeps food fresh for longer
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## The effect of concentration and pressure

• Increasing the concentration or pressure will increase the rate of reaction:
• Particles in fluids move around randomly because of the energy they have
• If the concentration or pressure is increased there are more particles in the same volume
• Therefore there are more frequent collisions
• Rate of rection increases
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## The effect of catalysts

• Catalysts increase the rate of reactions by lowering the activation energy
• Therefore more collisions result in a reaction
• They are not used up in the reaction
• Only work for a specific reaction
• Catalysts that are solids are usually in foms that have large surface areas maximising rate of reaction
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## Catalysts in action

• Catalysts can be expensive to produce however they can be economical because they can be used over and over again
• They are used in many industrial processes to speed up reactions
• This helps to reduce costs and energy, preservelimited resources and reduce pollution
• Cataylsts are often made from transition metals and their compounds which can be toxic and can cause harm if they get into the environment
• Finding new and better catalysts is a major source of research
• Nanoparticles would provide excellent cataylsts because of their huge surface area
• Biological catalysts such as enzymes are also being researched as these can bring aout reactions at normal temperatures and pressures reducing energy requirements and reducing the impact on the environment
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## Exothermic and endothermic reactions

• When chemical reactions take place, energy is required to break the bonds between the compounds and energy is released when new bonds are formed
• Exothermic reactions release energy into the surroundings causing temperature increase:
• Oxidation
• Combustion
• Neutralisation
• Endothermic reactions take energy in from the surroundings or require a constant supply of energy:
• Thermal decomposition
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## Energy and reversible reactions

• In reversible reactions the forward and reverse reactions involve equal and opposite energy transfers
• If a reaction is exothermic in one direction it is endothermic in the other
• The amount of energy released in the exothermic reaction is equal to the amount of energy taken in by the endothermic reaction
• Blue crystals       =    white crystals
• CuSO4.5H2O     =          CuSO4        +  5H2O
•    Hydrated  Anohydrous
• Copper sulphate =  Copper sulphate + water
• When water is added to anhydrous copper sulphate the reaction is exothermic
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## Using energy transfers from reactions

Exothermic reactions can be used to heat things e.g in hand warmers:

Reaction can be reversible if made using the crystallisation of a salt:

• Once hand warmer has been used
• Place in boiling water and the salt will re-dissolve into the solution
• Hand warmer becomes warm again
• Can be used many times

Reaction may also be irreversible:

• Reactants are used up and cannot be used again
• These use reactions - oxidation of iron or reaction of calcium oxide with water

Endothermic reactions can be used to cool things e.g sports injury packs

Some contain ammonium nitrate and water:

• The two substances are kept seperate
• Upon usage the substances are allowed to mix (by bursting bag seperating them)
• The reaction which occurs is endothermic taking in energy from surroundings
• This reaction is reversible but not in cold pack because they cannot be seperated
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