Reactions take place --> atoms rearranged + bonds broken/made --> energy is given out (exothermic) or taken in (endothermic).
It is important to understand energy changes so that chemists can control how fast a reaction happens:
- Too fast= hazardous --> explosions
- Too slow= time consuming --> loss of profit
Thus, catalysts are used to control reactions.
Molecular models help to develop theories on behaviour of atoms.
Energy changes 3A
Refer to I/GCSE Chemistry, Exothermic: Releases energy to surroundings e.g. respiration, burning
- The energy of products is less than energy of reactants, b/c the product releases energy (shown by the energy level diagram)
Example of exothermic reaction
Magnesium + Chlorine --> Magnesium Chloride (+ energy released)
Mg + Cl2 -->MgCl2 + energy released
Endothermic: Takes in energy from surroundings
- The energy of products is more than energy of reactants b/c the product takes in energy (shown by the energy level diagram)
Example of endothermic reaction
Sodium HydrogenCarbonate + Citric acid --> Sodium Citrate (+ energy taken in)
NaHCO3 + C6H8O7 -->Na3C6H5O7 (+ energy taken in)
Hydrogen as fuel
Interests in hydrogen as fuel because it is an exothermic reaction and there are no toxic waste products:
H2 + O2 -->H2O (+ energy released)
Refer to I/GCSE Chemistry, It is an exothermic reaction because more energy is given out when bonds form, than energy given out when bonds break.
- The energy given out keeps the mixture hot so that the reaction can continue (the heat is used to break the bonds between the atoms which allow the atoms to recombine and make new bonds).
However, at room temperature, H2 + O2 don’t react. This is because the intra-molecular forces/ bonds are not broken and thus the atoms are not free to form new bonds with each other.
Hence heat is required to break these intra-molecular bonds and free the atoms.
- Chemical bonds are springs
- The springs need to be stretched in order to snap
- Energy from your body is required to snap the springs in half- this refers to adding heat energy
- If you abruptly let go of the spring, the energy makes the spring snap back to normal- this refers to energy released during bond formation
When it comes to I/GCSE Chemistry, The amount of energy required for bonds to break, and, the amount of energy released from bond formation, determines the overall energy changes, and whether the reactions are endo/exothermic.
Energy change calculations
1. Energy needed to BREAK bonds:
(2 x 434) + 498= 1366 KJ
2. Energy GIVEN OUT in bond formation:
4 x 464= 1856 KJ
3. Energy CHANGE: (energy needed- energy released)
1366-1856= -490 KJ
If it shows a negative sign, the bond is exothermic.
If it shows a positive sign, the bond is endothermic.
You got it now!