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Topic 3: Energy changes

Brief intro

 

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

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 a fuel

 

Interests in hydrogen as fuel because it is an exothermic reaction and there are no toxic waste products:

 

H2 + O2 ->H2O (+ energy released)

 

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.

Spring theory

  1. Chemical bonds are springs
  2. The springs need to be stretched in order to snap
  3. Energy from your body is required to snap the springs in half- this refers to adding heat energy
  4. 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

 

Determining reactions

 

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

 

Bond Energy changes for the formula masses (KJ)
H-H 434
O=O 498
O-H 464

Bonds broken (reactants):

There are two H-H bonds are broken (2H2)

There is one O=O bond broken (O2)

 

Bond formation (products):

There are four O-H bonds (2H2O)

 

  1. Energy needed to BREAK bonds:

(2 x 434) + 498= 1366 KJ

 

  1. Energy GIVEN OUT in bond formation:

4 x 464= 1856 KJ

 

  1. 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.

How fast? 3B

 

Molecular collisions

 

H2O and O2 gas molecules are constantly colliding – but not all the collisions lead to reactions. This is because the molecules only have enough energy to merely bump into each other; they don’t have enough energy for the collisions to break bonds. Bonds between atoms (intra-molecular) must be broken before new bonds can be formed.

  • If every collision lead to a reaction->explosive reaction

 

Activation energies

 

Bonds cannot break though weak collisions. Thus, an external source of energy needs to be provided (activation energy) to the molecules so that they gain enough energy to break the bonds.

 

Activation energy (AE) – the minimum amount of energy required for the collision to break the bonds between atoms, and start the reaction.

  • Low AE means molecules need less energy to break bonds
  • High AE means molecules need more energy to break bonds
  • If the energy provided is lower than AE, the reaction won’t start; nothing will happen

 

‘High-jump bar’ Theory

 

If the high-jump bar (HJB) is low, many athletes will be successful in jumping over it.

Likewise, if the AE is low, a high proportion of collisions will release enough energy to break bonds.

  • The reaction will be fast despite low temps. This is b/c the low temp. provides more than enough energy to surpass the AE boundary

 

If the HJB is high, fewer athletes will be successful in jumping over it.

Likewise, if the AE is high, a lower proportion of collisions will release enough energy to break bonds.

  • Heating gives the reaction more energy->increases no. of successful collisions

 

Catalysts

Catalysts have three jobs:

  • Speed up reactions (not used up in process)
  • Lowers the AE
  • Provides alternative routes in energy graphs (dotted red line)

 

The overall energy change of the product doesn’t change but the AE is lowered so that there is an increase in the no. of successful collisions to break the bonds.

 

Explosives

 

Primary explosives have a low AE -> more sensitive to heat/ energy stimulus-> more dangerous

Secondary explosives have a higher AE-> less sensitive->less dangerous

 

Detonators used to be used with electric current, but not anymore b/c electromagnetic radiation makes it unsafe to handle. As a result, fibre optic cables with lasers have been introduced (safer).