Everything Science Grade 11

CHAPTER 7. ENERGYCHANGES IN CHEMICAL REACTIONS 7.4

• When BaCl 2 and KNO 3 dissolve in water, they take in heat from the surroundings. The
  dissolution of these saltsis endothermic.


• When CaCl 2 and NaOH dissolve in water, heat is released. The process is exothermic.


• The reaction of H 2 SO 4 and NaOH is also exothermic.

7.4 Examples of endothermic and exothermicreactions

ESBBR

There are many examples of endothermic and exothermic reactions thatoccur around us all thetime.

The following are just afew examples.

1. Endothermic reactions
   
   
   • Photosynthesis
     Photosynthesis is the chemical reaction that takes place in plants, whichuses energy from
     the sun to change carbon dioxide and water intofood that the plant needs to survive, and
     which other organisms(such as humans and other animals) can eat sothat they too can
     survive. The equation for this reaction is:
   
   
   
   

6 CO 2 + 12H 2 O + energy→ C 6 H 12 O 6 + 6O 2 + 6H 2 O

Photosynthesis is an endothermic reaction because it will not happen without an external

source of energy, whichin this case is sunlight.

• The thermal decomposition of limestone
  In industry, the breakdown of limestone into quicklime and carbon dioxide is very impor-
  tant. Quicklime can beused to make steel fromiron and also to neutralise soils that are
  too acid. However, thelimestone must be heated in a kiln at a temperature of over 900 ◦C
  before the decomposition reaction will take place. The equation for thereaction is shown
  below:

CaCO 3 → CaO + CO 2

2. Exothermic reactions
   
   
   • Combustion reactions - The burning of fuel isan example of a combustion reaction, and
     we as humans rely heavily on this process forour energy requirements. The following
     equations describe the combustion of a hydrocarbon such as methane (CH 4 ):
   
   
   
   

Fuel + Oxygen→ Heat + Water + Carbon Dioxide

CH 4 + 2O 2 → Heat + 2H 2 O + CO 2

This is why we burn fuels for energy, because the chemical changes thattake place during

the reaction release huge amounts of energy, which we then use for things like power and

electricity. You should also note that carbon dioxide is produced during thisreaction. Later

we will discuss some of the negative impacts of CO 2 on the environment. The chemical

reaction that takes placewhen fuels burn therefore has both positive andnegative conse-

quences.

FACT

Lightsticks or glow-

sticks are used by divers,

campers, and for decora-

tion and fun. A lightstick

is a plastic tube with a

glass vial inside it. To

activate a lightstick, you

bend the plastic stick,

which breaks the glass

vial. This allows the

chemicals that are inside

the glass to mix with the

chemicals in the plastic

tube. These two chemi-

cals react and release en-

ergy. Another part of a

lightstick is a fluorescent

dye which changes this

energy into light, caus-

ing the lightstick to glow!

• Respiration
  Respiration is the chemical reaction that happens in our bodies to produce energy for our
  cells. The equation below describes what happens during this reaction:

C 6 H 12 O 6 + 6O 2 → 6 CO 2 + 6H 2 O + energy

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