Chemistry GCSE Unit 2 ii

Reactions go at different rates:

1) One of the slowest is the rusting of iron.

2) A moderate speed reaction is a metal reacting with acid to produce a gentle stream of bubbles.

3) A really fast reaction is an explosion, where its all over in a fraction of a second.

RATE OF REACTION DEPENDS ON FOUR THINGS:

1) Temperature

2) Concentration

3) Catalyst

4) Size of Particles.

Three ways to measure the speed of a reaction:

Rate of reaction = Amount of reactant used or amount of product formed                                                                              Time

1) Precipitation- This is when the product oif the reaction is a precipitate which clouds the solution.

2)Change in Mass(Usually Gas Given Off)-Measuring the speed of a reeaction that produces a gas can be carried out on a mass balance.

3) The volume of Gas Given Off- This involves the use of a gas syringe to measure the volume of gas given off.

Reaction rates are explained perfectly by collision theory.Which says the rate of reaction depends on how often and how hard the reacting particles collide with each other.

More collisions increases the rate of reaction:

1) Higher temperature increases collisions.

2) Higher Concentration( or pressure) increases collisions.

3) Larger Surface area increases collisions.

4) Catalysts increase the number of successful collisions.

Faster Collisions increase the rate of reaction- 

Faster collisions are only caused by increasing the temperature.

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

1) Catalysts lower the activation Energy:

The activation 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.

2) Solid Catalysts Work Best When They Have A Big Surface Area:

Catalysts are usually used as a powder or pellets or a fine gauze. This gives them a large surface area to enable the reacting particles to meet up and do the business.

3) Catalysts Help Reduce Costs In Industrial Reactions.

In an Exothermic Reaction, Heat is Given Out.

An exothermic reaction is one which gives out energy to the surrondings, usually in the form of heat and usually shown by an rise in temperature.

1) Burning Fuels-The bestexample of an exothermic reaction is burning fuels-also called combustion.

2) Neutralisation Reactions-Neutralisation reactions ( acid + alkali) are also exothermic.

3) Oxidation reactions- Many are exothermic. Addition of sodium to water produces heat so it must be exothermic. The sodium emits heat and moves about on the surface of the water as it is oxidised.

In an Endothermic Reaction, Heat is Taken In.

AN ENDOTHERMIC REACTION IS ONE WHICH TAKES IN ENERGY FROM THE SURRONDINGS, USUALLY IN THE FORM OF HEAT AND USUALLY SHOWN BY A FALL IN TEMPERATURE.

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants A+B-><- C+D.

Reversible Reactions will reach Dynamic Equilibrium-

1)If a reversible reaction takes place in a closed system then a state of equlibrium will always be reached.

2) Equilirbrium means that the relative % quantities of reactants and products will reach a certain balance and stay there.

3) It is in fact a Dynamic Equilibrium which means that the reactions are still taking place in both directions.

Changing Temperature and pressure to get More Product-

1)In a reversible reaction the 'position of equilibrium' depends very strongly on the temperature and pressure surronding the reaction.

2)If you deliberately alter the tempereature and pressure you can move the 'position of equlibrium' to give more product and less reactants.

This is an important industrial process. It produces ammonia (NH^3),Which is used to make fertilisers. 

Nitrogen and Hydrogen are needed to make Ammonia

1) The nitrogen is obtained easily from the air, which is 78% nitrogen (and 21% oxygen)

2) The hydrogen comes from a natural gas or from other sources like crude oil.

3) Because the reaction is reversible-it occurs in both directions-not all of the nitorgen and hydrogen will convert to ammonia. The reaction reaches a dynamic equilibrium.