Additional Science C2 Topic 8

Topic 8

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  • Created by: Victoria
  • Created on: 15-05-12 16:42

Rates of Reaction

The rate of reaction depends on four things:

1. Temperature
2. Concentration- or Pressure for gasses
3. Catalyst
4. Size of Particles- or Surface area

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Measuring rates of reaction

Three ways to measure speed of a reaction:
Rate of Reaction= Amount of reactant used or amount of product formed / Time

1. Precipitation
When a product of a reaction is a precipitate which clouds a soloution. You could time this untill it went clear. People may debate when it had turned clear. Not very accurate.

2. Change in mass
Measuring the speed of a reaction that produces gass can be done using the mass balance. The quicker the reading on the blance drops the faster the reaction. This is the most accurate of the three methods because the mass balance is very accurate.

3. The volume of gas given off
You measure this using a gass syringe. The more gass given off during a time interval the quicker the reaction. Quite accurate.

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Collision Theory

The rate of reaction simply depends on how often and how hard the reacting particles collide with each other. The idea is the particles have to collide to react.

1. Higher temperature increases collisions- When temperature is increased the particles move quicker if they are moving quicker there will be move collisions.

2.High concentration
If a solution is more concentrated it means there are more particles which means there will be more collisions.

3. Larger surface area
if one of the reactants is a soild then breaking it up into smaller pieces will increase its surface area. There will be more useful collisions.

4. Catalyst
A catalyst works by giving the reacting particles a surface to stick to. They increase the number of successful collisions.

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Catalysts

A catalyst is a substance which changes the speed of a reaction, without being changed or used up in the reaction.

Catalysts lower the activation energy
the activiation energy is the minimum amount of energy needed for a reaction to happen. Catalysts lower the activation energy of reactions making it easier for them to happen. This means a lower temperature can be used.

Solid Catalysts work best when they have a big surface area
Catalysts are usually used as a powder or pellets. This gives them a very large surface area to enable reacting particles to meet up.

Enzymes are catalysts produced by living things
Living things have thousands of chemical reactions going on inside them. These reactions need to be carefully controlled to get the right amount of substances. living things produce enzymes which act as a biological catalyst to speed up useful chemical reactions in the body.

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Energy transfer in reactions

Energy must always be supplied to break bonds
During a chemical reaction old bonds are broken and new bonds are formed.

Energy must be supplied to break existing bonds so bond breaking is an endothermic process.
Energy is released when new bonds are formed so bond formation is an exothermic process.

An exothermic reaction is one which GIVES OUT energy to surroundings. Usually in the form of HEAT.
Most common reaction.

An endothermic reaction is one which TAKES IN energy. Usually in the form of heat. Shown by a fall in temperature.
These are very un common.

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Reversible Reactions

A reversible reaction is where the products of the reaction can react with each other and then convert back to their original state.

Equilibrium means that the relative quantities of reactants and products will reach a certain balance and then stay there.
Dynamic equilibrium means that the reactions are still taking place in both directions but the overall effect is nothing because the forward and reverse reactions are cancelling each other out.

Changing temperature and pressure to get more product
In a reversible reaction the position of the equilibrium depends on the temperature and the pressure of the reacting mixture.
If you deliberatly alter the temperature and pressure you can move the position of the quilibruim to give more product and less reactants.

Temperature- If you raise the temperature the endothermic reaction will increase to use up the extra heat. If you reduce the temperature the exothermic reaction will increas to give out more heat.

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The Haber Process

Nitrogen and Hydrogen are needed to make Ammonia.
Nitrogen is obtained for the air and Hydrogen comes from natural gas or from other sources like crude oil. Because the reaction is reversible it occurs in both directions not all of the Nitrogen and Hydrogen will convert to ammonia. The reaction reaches a dynamic equililbrium.

The reaction is reversible so there is a compromise to be made:
Higher pressures favour the forward reaction. So the pressure is set as high as possible to give the best % yeild.
The forward reaction is exothermic the increasing temperature will move the quilibrium the wrong way. So the yield would be better at a lower temperature.
The thing is... Lower temperature the slower the reaction. So the temperature is increased anyway to get a faster reaction.
450c is a comprimise between maximum yeild and speed of reaction.
The ammonia is formed as a gass and as it cools it turns to liquid and is removed.
The unused Hydrogen and Nitrogen is recycled.

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Ammonia

Ammonia is used for ammonium nitrate fertiliser.
Ammonium = Nitrate + Ammonia
Used for fertiliser because it gives nitrogen from two sources. Plants need nitrogen to make proteins. Ammonium nitrate is much more effective then organic fertilisers (pig poop)

There are also big problems with artificial fertilisers. If nitrate fertilisers wash into streams they can cause mega growth, death or decay. Its called Eutrophication.

If too many Nitrates get into drinking water it can cause health problems especially for young children. Nitrates prevent blood from carrying oxygen properly, babies can die from it.

To help avoid this Nitrates need to be applied by farmers carefully. For example not to apply too much and not to apply if it is likely to rain soon.

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