Displacement Reactions


Reactivity series


Elements can be arranged in order of reactivity.


Source: /


The most reactive elements are the group one alkali metals, which are so reactive they explode upon contact with water. The least reactive elements are the noble gases and the precious metals. E.g helium and gold.


Reactivity vs discovery.


The lower the reactivity generally the earlier the discovery date. This is because metals which don’t react are found naturally, while those which are more reactive must be separated from their compounds with equipment.Those which are highly reactive require very complex equipment and scientific methods e.g electrolysis and such have only been discovered recently.


Predict the product of a displacement reaction


If a metal/halogen comes in contact with a compound in which a less reactive metal/halogen is present, the more reactive element can displace the less reactive element taking its place in the compound.


E.g chlorine + sodium bromide → sodium chloride + bromine


In the above example chlorine is more reactive than the bromine in sodium bromide and so takes its place to form sodium chloride.

How to tell if a displacement reaction has occurred

  1. Color change
  2. Formation of bubbles (a gas)
  3. Formation of a precipitate


Oxidation and reduction


Oxidation is loss of electrons

Reduction is gain of electrons


This can be remembered with the memomic: OilRig



Loss of electron



Gain of electrons


Half equations


These show how many electrons an element gains or loses to become an ion during a redox reaction.


To write it, first write the element as an atom then draw an arrow and write it again as an ion and add the change in charge. Make sure to balance the two sides and include any charges


E.g 2Cl- → Cl2 + 2e-




Electrolysis is a process which can be used to split up an ionic compound into its elements, by applying an electrical charge to it through two electrodes.


The compound being split up is called the electrolyte and for the process to work its ions must be able to move freely.


How does it work?

The probes have opposite charges; one is negative one is positive. When placed inside the electrolyte the ions are attracted to the oppositely charged electrode, splitting it up.


At the positively charged electrode, the anions give away their negative charge and become atoms precipitating out of the solution


At the negatively charged electrode, the cations gain a negative charge and become atoms precipitating out of the solution.


However in a solution the water molecules are also broken up into hydroxides and hydrogen so whether hydrogen or the metal is produced depends on how reactive it is.



Anode and cathode

Cathode: This is the negatively charged electrode because it attracts cations.

Anode: This is the positively charged electrode because it attracts anions


Voltaic cells

Also known as Galvanic cells or Electrochemical cells. This process allows us to create electricity and it this process which batteries rely on to function. It is essentially electrolysis but in reverse.


Half cells: the two sides of the cell


What is needed ?



What is it?

Why is it needed?



Two metals made of different materials

The metals provide ions to flow and produce electricity

Zinc, copper


A solution of ions

It allows the ions from the electrodes to flow

Salt solution, vinegar, zinc


Conductive metals attached to either end of the voltaic cell

It lets the ions flow into the device which needs to work.

Copper wires

Salt bridge (optional)

A bridge between the two half-cells, made of an electrolyte

To allow ions to flow and to balance the charges

Sodium and chloride gel inside a u tube.


How is electricity created?


There are two metals electrodes and in both of them their atoms want to gain full outer shells, requiring them to lose electrons. After connecting them with electrons both of begin trying to push their electrons away from themselves and on to the other, however as the two metals are different one is more reactive than the other and it pushes its electrons onto the weaker metal. This creates a flow of electrons. The bigger the potential difference in the reactivity the greater the push in electrons. This push has a force and can be measured in volts.


Anode and Cathode

Cathode: more reactive metal because it’s atoms lose electrons and become positive ions.

Anode: less reactive metal  because it’s atoms gain electrons and become negative ions.


How does the mass of the electrodes change?


Mass decreases: This is the cathode because the each time the atoms become positive ions they break off the cathode and dissolve in the electrolyte.


Mass increases: This is the Anode because as it becomes negatively charged, the positively charged ions in the electrolyte, take the charge and become atoms. This causes them to precipitate out of the solution and coat the Anode.


How are voltaic cells an example of redox?


Reduction takes place at the cathode as it is losing electrons to push them onto the anode.

Oxidation takes place at the anode as it is gaining electrons from the cathode.




What is the purpose of the salt bridge?

Over time the two sides of the the cell become very electrically imbalanced of charges and the electricity can no longer flow, in order for it to be able to flow the circuit must be completed with a  salt bridge. This lets ions flow freely through it so they can move form one half cell to the other so that the positive and negative charges can move around and not concentrate in one area.


The salt bridge works by sucking ions through it to the other side and replacing them with the oppositely charged ions form inside the tube.



Hydrogen Fuel Cell


This a type of battery which uses hydrogen as a fuel to power different things. Its biggest application is in an alternative way of powering cars which is cleaner than existing fuels.


Electrodes: Carbon coated in a catalyst.

Electrolyte: Some kind of hydroxide compound E.g Potassium Hydroxide.

Fuel: Hydrogen




At the anode


Hydrogen fuel enters the cell from a tank. When it comes with the electrode it is stripped of its electrons leaving it as an ion. These electrons then flow through the electrode and power the circuit.


At the cathode

Oxygen enters the cell at the cathode side and picks up the electrons which have gone round the circuit it is now ionized. The negative oxygen ions and positive hydrogen ions then get pulled together and react to form water.



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