Controlling Reactions

Types of reactions


Exothermic reactions

Definition: An exothermic reaction is a reaction which Releases energy in the form of heat, making the surrounding environment hotter.. This requires a small activation energy to get it started.


Endothermic reactions

Definition:  An exothermic reaction is a reaction which Absorbs energy in the form  of heat, making the surrounding environment colder. Requires a large activation energy to get it started.

Combustion reactions


Definition: Combustion reactions are reactions where a hydrocarbon reacts with oxygen (burns) to produce water and carbon dioxide.


To have fire you need three elements: Oxygen, heat and fuel. As shown by the fire triangle

To put out a fire you need to remove on of the three elements, an effective way is to smother it e.g with a pot, in order to remove the oxygen.


Bond energy


Breaking and making bonds

The bonds holding atoms together in compounds are full of potential energy.


When breaking bonds, energy is absorbed. As the bonds won’t just break themselves, you have to put energy in to do it.

When making bonds energy is released. When bonds form the compound reaches a lower energy level, so there is excess energy.


Calculating bond energies

Energy needed to break the reactants bonds is the sum of all the bond energies needed to break the individual bonds.


Energy released when making the products bonds is the sum of all the bond energies needed to break the individual bonds plus the excess energy.


To find if a reaction is exothermic or endothermic, find the difference in energy needed to break the reactants bonds and energy needed to make the products bonds.


Balancing equations


To balance an equation, simply make sure that each element is has the same number on either side of a reaction equation. To do this you can multiply the compounds by a number to increase the number of elements on one side.


Unbalanced equation: Be2C   +H2O Be(OH)2 +   CH4


To start figure out which elements are unbalanced, you can do this by making a table. When there is an element inside brackets, this is a polyatomic ion, so to find how many elements multiply the elements inside by the subscript number on the right. Just like multiplying brackets in algebra.



2x Be

1x Be

1x C

1x C

2x H

6x H

1x O

2x O

All except for carbon is unbalanced.


To balance it use trial and error to find the correct coefficient which makes it balanced.


Balanced equation: Be2C   +4H2O 2Be(OH)2 +   CH4


Reversible reactions


Not all reactions are one way, there does exist some reactions which can go both ways.

The symbol for a reversible reaction is ⇌



Even if both sides start unbalanced, as the reaction progresses more product will be created until both sides react at the same speed simultaneously creating product and reactants. This point is called the Equilibrium and when it’s reached the reaction will appear have stopped, as the quantity on either side is not changing.


Le Châtelier’s principle


This states: “If one side of a reaction is changed the equilibrium point will shift to counteract the change and restore order”


Pressure - If the pressure is increased in a reaction, the equilibrium will shift towards the side which has less molecules, as this side takes up less space and hence has less pressure.


If pressure is increased the equilibrium will shift towards the reactants as there as there are less molecules

If pressure is increased there will be no change in the reaction as there’s the same number of particles on either side.

Although there is the same number of molecules on either side as the right is a gas it takes up more space so increasing pressure shifts the equilibrium towards the reactants


Temperature - If temperature increases it will shift towards the side which can absorb heat in order to counteract it. This side is which ever side is endothermic.


Concentration - If more of a certain element is added to one side increasing its concentration the equilibrium will shift towards the side which has less of that element to counteract that change.


Catalysts- Adding this speeds up the reaction but it doesn’t shift the equilibrium as both sides speed up equally.


Haber process

One of the most notable examples of  reversible reaction is the Haber process.


What is it?

State: Haber process makes ammonia from air.

Describe:The Haber process is a  way of synthesizing ammonia( a fertilizer ) from the air, and is the primary way of producing ammonia today. It works by reacting Nitrogen and oxygen to create ammonia. The equation for the haber process is: N2+3H2 ⇌2NH3


Why is it needed?

Plants need nitrogen in order to grow fast, but they are unable to extract it from the air as it has strong covalent bonds holding it together the nitrogen molecules N-N, so instead they extract it form ammonia which weakly bonds nitrogen and hydrogen, so they are able to use it. However ammonia isn’t abundant in nature, so Haber created the haber process in order to synthesize it from the air. Under extreme pressure and heat, they are able to react nitrogen and hydrogen in order to create Ammonia.







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