5- Energy Changes

• Exothermic reaction: transfers energy from reacting molecules to the surrounding-

   so the temperature of surrounding increases (gets hotter)  

  • Combustion (burning), oxidation and neutralisation
  • Hand warmers, self heating cans (eg: food or drinks)
• Endothermic reactions: take in energy from their surroundings- the temperature

   of the surrounding decreases (gets colder) 
• endothermic:

  Endothermic reactions are chemical reactions in which the reactants

  absorb heat energy from the surroundings to form products.

  The energy is absorbed from the surrounding into the reaction.

  Energy in the form of heat.

  • Melting ice, evaporation, cooking, gas molecules, and photosynthesis are a few examples.
• exothermic:

  An exothermic reaction is a reaction in which energy is released

  in the form of light or heat.

  The energy is released from the system to its environment.

  Energy is released as heat, electricity, light or sound.

  • Rusting iron, settling, chemical bonds, explosions, and nuclear fission are a few examples.
• exothermic

  
• endothermic

  
• Activation energy (the peak point on the graph) 

  • Reactions can only occur when the particles collide with each other and they have to do that with sufficient energy  
  • The minimum amount of energy that particles must have in order to react

  
• In exothermic reactions, energy is transferred from the chemicals to the surrounds 

  • The energy changes are negative because energy has been transferred from the chemicals 
• In endothermic reactions, energy is gained from the surroundings

  • The energy changes are positive because energy has been gained 
• When we break a chemical bond- this requires energy (endothermic), making bonds releases energy (exothermic) 

  • Every chemical bond has a chemical value and this tells us the energy required to make that bond 
• Step 1. Identify whether you’re breaking or making bonds

  • Breaking bonds = endothermic, making bonds= exothermic

  Step 2: Label the bonds with the given numbers 

  Step 3: at the broken bonds together 

  Step 4: IF there’s a big number at the start of the reaction then u 

  multiply the number number given by the big number, the bond

  made is an exothermic reaction so it’s a negative number 

  Step 5: work it out and you have your answer!

  
• Required Practical: Temperature Changes

  • Independent variable: volume of sodium hydroxide in the solution
  • dependant variable: maximum temp reached
  • controlled variables: volume of hydrochloric acid and concentration of both hydrochloric acid and sodium hydroxide solution 
• Required Practical: Temperature Changes

  
• As you increase the volume of sodium hydroxide solution, the maximum temp reached increases- this is because when you add more particles of sodium hydroxide, they react with hydrochloric acid 

  • This is is an exothermic reaction, more energy is released and so max temp reached increases 

  At a certain volume of sodium hydroxide solution, the max temp reached starts to decrease and now there's too much and not enough hydrochloric acid to react 

  • So amount of energy released has reached a maximum 

  When we add large volumes of sodium hydroxide solution the max temp reached decrease 

  
• Required Practical : Temperature Changes

  Reason for using a polystyrene cup with lead 

  • Reduce heat lost from sides and bottom, polystyrene is a good thermal insulator 
  • Lid reduces heat loss to the air