GSCE Science - Chemistry - C5

Topics

• Endothermic and Exothermic Reactions
• Chemical Equilibrium
• Electrolysis

Endothermic and Exothermic Reactions

• Endothermic - the substance is cooler than the surroundings so will take in heat

In an endothermic reaction, since the amount of energy is increasing it shows that the energy is being taken in while the reaction is occurring. Examples include combustion, oxidation and neutralisation. Things like burning, the detonation of explosives and reactions of metal with acid are also included.  

ΔH - Postive - change in temperature is positive

• Exothermic - the substance is hotter than the surroundings so will give out heat

In an exothermic reaction, since the amount of energy is decreasing it shows that the energy is being given out while the reaction is occurring. Examples include thermal decomposition. Things, like boiling a kettle, ice melting and dissolving salts in water, are also included.

ΔH - Negative - change in temperature is negative 

Reactions can only occur when there is a sufficient amount of energy so particles can collide. Activation energy is used to give particles that minimum amount of energy. 

Endothermic and Exothermic Reactions

Endothermic

Exothermic

Equilibrium

All chemical reactions have a forward and reverse reaction. The amount of energy in the system determines which of these reactions is favoured. Both the forward and reverse reaction occur at the same time and this is symbolised with a double-headed arrow like this ⇌.

A good example of this reaction in practice is the Haber process. It is the reaction between nitrogen and hydrogen, which makes ammonia.

Nv2 + 3Hv2 ⇌ 2NHv3 or Nitrogen + Hydrogen ⇌ Ammonia

This process is very important in producing enough food for the population. The Haber process basically turns air into fertilizer. This reaction shows chemical equilibrium because when the molecules of nitrogen and hydrogen react producing ammonia, some remain unreacted which can be re-reacted again. 

Electrolysis

Electrolysis is the process of breaking down an ionic compound, as this compound which is known as the electrolyte has to be molten or dissolved in water so the ions are free to move. 

Electrolysis is an oxidation and a reduction reaction occurring at the same time so an ionic compound is split into two atoms. 

• Anode - positive electrolyte
• Cathode - negative electrolyte 
• Electrolyte - broken down compound

Electrolysis

During electrolysis, several steps occur to make the reaction occur:

• 1. The positively charged ions move to the negative cathode (covered with electrons)
• 2. The negatively charged ions move to the positive anode (lacks electrons)
• 3. Both of these are powered by a power pack
• 4. The cathode is supplied by the negative side and the anode is powered by the positive side
• 5. The positive ions and the negative ions are attracted to either electrode because opposite charges attract 
• 6. These ions move through the solution because heat is added giving them energy
  
• 7. The positive ions gain electrons to form an atom, this is reduction - x^+ + e^- → x
• 8. The negative ions lose electrons to form an atom, this is oxidation - y^- + e^- → y