Chemical Reactions

Chapter 7.1

Objectives

Recognize some signs that a chemical reaction is taking place.

Explain chemical changes in terms of the structure and motions of atoms and molecules.

Describe the differences between endothermic and exothermic reactions.

Identify situations involving chemical energy.

Signs of a Chemical Reaction

Release of gas

Change in color

Change in odor

Release of energy

Release of Gas

http://webpub.allegheny.edu/employee/a/asheffie/images/AEScannon.JPG

Change in Color http://genchem.chem.wisc.edu/demonstrations/Images/15precip/pbi.jpg

Change in Odor http://www.julieleung.com/archives/cheesewhey.jpg

Release of Energy http://upload.wikimedia.org/wikipedia/commons/thumb/3/36/

Large_bonfire.jpg/250px-Large_bonfire.jpg

Explain why the following are examples of a chemical reaction.

Lighting a match Mixing vinegar and baking soda Grilling a burger Food rotting

Chemical Reactions

Involves rearrangement of atoms. The number and types of atoms are conserved.

Chemical Change Example: In a car or boat engine,

isooctane (C8H18) and oxygen (O2) react to form water (H2O) and carbon dioxide (CO2).

Chemical Reactions

Reactant – a substance that undergoes a chemical change.

Product – a substance that is the result of a chemical change.

Predict which atoms will be found in the products of the following reactions. Mercury (II) oxide, HgO, is heated and

decomposes Hg and O

Limestone, CaCO3, reacts with hydrochloric acid, HCl Ca, C, O, H, and Cl

Table sugar, C12H22O11, burns in air to form caramel C, H, and O

Energy and Reactions

Energy must be added to break bonds. You need a spark to react gasoline and

oxygen so that you can break apart the bonds within the gasoline and oxygen.

Energy is released when forming bonds. After you have broken the bonds between

the gasoline and oxygen, they will form new bonds to create carbon dioxide and water and energy will be released.

Energy and Reactions

The chemical energy of reactants is equal to the chemical energy of the products.

Chemical energy – the energy stored within atoms and molecules that can be released when a substance reacts

Energy and Reactions

Exothermic reaction – a reaction that transfers energy from the reactants to the surroundings usually as heat. Energy is released.

Taken from http://jchemed.chem.wisc.edu/jcesoft/cca/cca3/STILLS/GLYCER/GLYCER/64JPG48/7.JPG

Energy and Reactions

Endothermic reaction – a reaction in which energy is transferred to the reactants usually as heat from the surroundings. Energy is absorbed.

Taken from https://secure.edventures.com/oscart/catalog/images/WIZ_1060-06.jpg

Critical Thinking:

Calcium oxide, CaO, is used in cement mixes. When water is added, heat is released as CaO forms calcium hydroxide, Ca(OH)2.

What signs are there that this is a chemical reaction?

Which has more chemical energy, the reactants or products?

Explain your answer.

1. Identify which of the following is a chemical reaction

a. Melting icephysical

b. Burning a candleChemical reaction

c. Rubbing a marker on paperphysical

d. Rusting ironChemical reaction

2. List four forms of energy that might be absorbed or released during a chemical reaction.

Heat, electricity, light, sound2. List three signs that could make you

think a chemical reaction might be taking place.

Change in color, release of gas, release of energy, change in odor

4. Classify the following as exothermic or endothermic.

a. Paper burning with a bright flame* exothermic

b. Plastics becoming brittle after being left in the sun* endothermic

c. A firecracker exploding

* exothermic