Rates of Reaction

Different Speeds of Reaction

Different chemical reactions take place at different speeds.

Very fast:

• explosion of a petrol-air mixture
• precipitation reactions
• fireworks going off
• coal burning

Moderately fast:

• reaction of metals or carbonates with dilute acids

Slow: 

• rusting of iron in air
• reaction of magnesium with cold water
• oil forming
• silver tarnishing

How do we calculate the speed of reaction? 

During a chemical reaction, the reactants get used up as products are formed. We can measure the speed of reaction by measuring the amount of a reactant used up per unit time, i.e.

Speed of reaction= amount of reactant used up/time taken

The speed of a reaction can also be measured in terms of the amount of a product formed/obtained per unit time, i.e. 

Speed of reaction= amount of product formed/time taken

For a chemical reaction that produces a gas, the speed of reaction can be found by measuring the volume of gas produced per unit time, i.e.

Speed of reaction= volume of gas produced/time taken

Measuring Speed of Reaction from Changes in Volume

The speed of a reaction can be found by measuring the following quantities at regular time intervals:

• volume of gas produced by the reaction
• the mass of the reactant that remains

Measuring Speed of Reaction from Changes in Mass

The speed of a reaction can also be found by measuring the changes in mass of a reaction mixture. This method works best for reactions which produce gases such as carbon dioxide. 

Factors Affecting Speed of Reaction

Many factors affect the speed of a chemical reaction. These include:

• the concentration of the reactants
• the pressure of the reactants (if the reactants are gaseous)
• the particle size or surface area of the reactants
• the temperature at which the reaction is occurring

Concentration:
Increasing the concentration means there's more of the reactants in the same volume. Thus with more particles there will be more collisions, and there will be higher chance of effective collisions, thus increasing the rate of reaction.

Pressure:
This is just like concentration, but it's more to do with gaseous reactants. With higher pressure--more particles--more collisions--higher chance of effective collisions--increase rate of reaction.

Surface area:
The bigger the surface area, the more chance of collisions, ditto above. Same thing. It's all about having more collisions, thus more effective collisions which increase rate of reaction. E.g. if you had a piece of metal, and you cut it into many smaller pieces, it would react faster with acid because the acid will have more area of the metal to collide with.

Temperature:
The higher the temperature, the more thermal energy is transferred to kinetic energy for the particles, thus they move faster and collide more often. If they have more energy, it is likelier that they will have minimum activation energy therefore there will be more effective collisions. And for a change, the rate of reaction increases... :P


For a reaction to occur between 2 particles: 

1. the reacting particles must collide with each other
2. they must collide with a certain minimum amount of energy known as the activation energy

In this way, collisions between reacting particles result in the formation of product particles. These collisions are known as effective collisions. 

Thus in a reaction between hydrogen and chlorine, only fast-moving molecules with energies equal to or greater than the activation energy will react on collision to form hydrogen chloride. 

• In general, when any factor increases the rate of effective collisions between reacting particles, it will also increase the speed of reaction. 

Energetics

4.13 understand the use of ΔH to represent molar enthalpy change for exothermic and

endothermic reactions

4.14 represent exothermic and endothermic reactions on a simple energy level diagram

4.15 recall that the breaking of bonds is endothermic and that the making of bonds is

exothermic

Just remember this! To break smth, obviously you need energy, so you need to take it in, hence breaking of bonds is endothermic. When substances form bonds, it's usually to become more stable, and thus they release energy. E.g. reactive elements like sodium react with chlorine to form sodium chloride, your common table salt, and this is a very stable ionic compound, and the reaction is exothermic. (less energy, less violently reactive...makes sense right?)

http://chemistry.about.com/cs/generalchemistry/a/aa051903a.htm

Heat Changes in a Reaction

heat change/enthalpy change: the amount of energy involved in a reaction, measured in kilojoules (kJ) which is 1000 joules, and is represented by the symbol DH. (D is the Greek letter for 'delta', which means change. H means energy content. 

For an exothermic reaction DH is negative. This is because the chemicals have lost energy to the surroundings. 

For an endothermic reaction, DH is positive. This is because the chemicals have gained energy from the surroundings. 

Examples:

1. When one mole of carbon is burnt in excess oxygen, 349 kJ of heat is produced. This is an exothermic reaction. So DH=-349kJ. 

 carbon + oxygen à carbon dioxide

C(s) + O₂ à CO₂ (g)   DH=-349kJ

2. When one mole of hydrogen reacts with one mole of iodine, 52 kJ of heat is absorbed from the surroundings. This is an example of an endothermic reaction. DH=+52 kJ

 hydrogen + iodine à hydrogen iodide

H₂ (g) + I₂ (g) à 2HI (g)   

Energy Level Diagrams for Exothermic and Endothermic Reactions

Energy level diagrams--convenient ways to express energy changes in a reaction

Exothermic Reaction 

Consider an exothermic reaction, heat energy is lost to surroundings. This means that total energy of the products is less than that of the reactants. 

The energy level diagram of an exothermic reaction

The difference between the energy levels of the products and the reactants is equal to the amount of energy given out by the reaction. 

i.e.  DH=H_products –  H_reactants  (as the energy of the reactants is larger, DH becomes negative. --> taking away a larger value from a smaller one gives a negative result. e.g. 2-4=-2) 

NOTE: USE THE SAME EQUATION FOR ENDOTHERMIC REACTIONS. It is always: 

 DH= H_products –  H_reactants

Endothermic Reaction

Since an endothermic reaction absorbs heat from the surroundings, the products will have more energy than the reactants. The difference between the energy levels of the products and reactants is the energy absorbed during the reaction.

The energy level diagram of an endothermic reaction

Activation Energy "E_a" - The minimum energy required to initiate (start) a chemical reaction. Both endothermic and exothermic reactions require activation energy.


So when particles collide and react, this is called an effective collision. This only happens when the particles have the minimum activation energy, if not, they may collide but it wouldn't result in a reaction.

Exothermic Reaction	Endothermic Reaction
Gives out heat to the surroundings	Takes in heat from the surroundings
Causes an increase in temperature	Causes a decrease in temperature
Has a negative DH	Has a positive DH
Has products lower in energy than the reactants	Has products that have higher energy than the reactants

Exothermic Reaction

Endothermic Reaction

Gives out heat to the surroundings

Takes in heat from the surroundings

Causes an increase in temperature

Causes a decrease in temperature

Has a negative DH

Has a positive DH

Has products lower in energy than the reactants

Has products that have higher energy than the reactants

This is a great, concise video to summarise everything, watch!