Additional Science Chemistry part two

How Fast?

Reaction Rates

• Different reactions happen at different rates 
• reactions that happen slowly have a low rate reaction e.g the chemical weathering of rocks is very slow.
• Reactions that happen quickly have a high rate of reaction e.g explosions are very fast reactions as they happen very quickly.

Rate of Reaction  Rate=cm /time   

The Collision Theory

• For a reaction to take place the particles that are reacting have to collide
• If they collide, with enough energy then they will react (activation energy)
• The minimum amount of kinetic (movement) energy that two particles need if they are going to react when they collide is the activation energy.

The four main variables that can change the rate of reaction are:

• The Concentration of the substance reaction
• The Temperature
• The Size or/ surface area of the particles
• A Catalyst being present

Concentration (The Pressure)

• Increasing the concentration of reactants it increases the number of collisions between particles increasing the rate of reaction.

• Reactions increase                  Reacting particles                     if you have 100 people   when pressure                 get pushed closely together             they are more likely to     is increased.                 so there is a more chance of           bump into each other if in                                               successful reactions               a class room compared to                                                                                                           a field.                                                                                                   

The Temperature

• Higher temperature - particles move faster - [articles collide more often and quickly - more successful collisions.

• As the temperature increases the time it takes decreases and the rate increases.

• When there is a reaction the higher the temperature the more energy the particles have. When the particles have more energy the more quicker so they have more successful collisions. 

The Surface Area

• The bigger the surface area the higher the rate of reaction. If we break a solid into smaller pieces we get more area and a faster reaction. 

  Particles collide                 successful                 reactions take                                       with enough energy              collisions                        place

    Particles collide                successful                 reactions take                                   without enough energy           collisions                        place

lots of successful             reaction                 rate of reaction                                                  collisions                goes quickly                    is high

Catalyst

• A catalyst is a substance which speeds up the reaction
• They are chemically uncharged at the end of the reaction
• When the reaction has finished you would have exactly the same mass of the catalyst as you had at the beginning. 
• Enzymes are biological catalyst.

Advantages

• Make the reaction happen quicker, react at a lower temperature both of these reduce energy costs. 
• Never get used up completely so can be reused.
• Good for suitable development.

Disadvantages

• Expensive to buy as most precious metals, different reactions need different catalysts.
• catalysts can be poisoned by impurities and stop working.

Sum it up;

• As the temperature, pressure (concentration) or catalyst is added the time taken decreases.

• The rate of reaction increases.

• This is because there is more energy so particles reactions have more successful collisions.

Exothermic Reactions

• An exothermic reaction , Heat is Given Out
• Chemicals heat up the surroundings
• The temperature of the reaction RISES example: respiration, neturaslisation and combustion

If in the reaction the temperature drops its an Exothermic Reaction as the heat has then been given to surroundings.

Endothermic Reactions

• In a Endothermic reaction, Heat is Taken In
• The surroundings feel colder
• The temperature of the reaction FALLS example: thermal decomposition, photosynthesis

If in a reaction the temperature increases it is an Endothermic Reaction as the heat from the surroundings has been taken in.

Energy and Reversible Reactions

Some chemical reactions are reversible, in other words they can go in either directions. 

  If a reaction is Exothermic in one direction                                                                         when it is reversed it is Endothermic in the                                                                         other direction.

When a reversible reaction occurs in a closed system (i.e no reactants are added or taken away) an Equilibrium is achieved - in other words, the reaction goes at the same rate in both directions.

Energy Changes

The science behind Hot and Cold packes

• Physical process go in two ways                                                                                       - either produce heat (Hot Pack)                                                                                   - or absorb the heat (Cold Pack) 
• Some packs use physical processes of dissolving when these substances dissolve ibn water they give off or absorb heat.                                                                                                                                                                                                                     e.g Heat Packs have a metal disk that when snapped activates the pack to begin the             dissolving process.                                                                                                            
• This causes a supersaturated solution to crystallize which produces heat.