CK-12 Physical Science Concepts - For Middle School

http://www.ck12.org Chapter 3. Chemical Interactions

Reactants + Energy→Products

FIGURE 3.34

Note: ∆H represents the change in en-

ergy.

In endothermic reactions, the temperature of the products is typically lower than the temperature of the reactants.
The drop in temperature may be great enough to cause liquids to freeze. That’s what happens in the endothermic re-
action at this URL: http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemreac/energychange
srev1.shtml.

Q:Now can you guess how an instant cold pack works?

A:Squeezing the cold pack breaks an inner bag of water, and the water mixes with a chemical inside the pack. The
chemical and water combine in an endothermic reaction. The energy needed for the reaction to take place comes
from the water, which gets colder as the reaction proceeds.

Photosynthesis

One of the most important series of endothermic reactions is photosynthesis. In photosynthesis, plants make the
simple sugar glucose (C 6 H 12 O 6 ) from carbon dioxide (CO 2 ) and water (H 2 O). They also release oxygen (O 2 ) in the
process. The reactions of photosynthesis are summed up by this chemical equation:

6 CO 2 + 6 H 2 O→C 6 H 12 O 6 + 6 O 2

The energy for photosynthesis comes from light. Without light energy, photosynthesis cannot occur. As you can see
in theFigure3.35, plants can get the energy they need for photosynthesis from either sunlight or artificial light.

Summary

• An endothermic reaction is a chemical reaction in which more energy is needed to break bonds in the reactants
  than is released when new bonds form in the products.


• A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going.


• One of the most important series of endothermic reactions is photosynthesis. The energy needed for photo-
  synthesis comes from light.