CHEMICAL REACTIONSChapter 5.1

CHEMICAL REACTIONS CHANGE SUBSTANCES

When sugar, water, and yeast are mixed with flour to make bread dough, a chemical reaction takes place Yeast acts on sugar to form

carbon dioxide and lactic acid

Different from sugar Sugar is sweet, lactic

acid is sour Chemical reactions

occur when substances undergoe chemical changes to form new substances (pg 148)

CHEMICAL REACTIONS CHANGE SUBSTANCES

Production of gas and change of color are signs of chemical reactions In bread, the carbon

dioxide gas produces expands the dough

As the dough bakes, old bonds break and new bonds form

Reactions involving starch and protein make food turn brown when heated (pg149)

CHEMICAL REACTIONS CHANGE SUBSTANCES

Chemical reactions rearrange atoms When gasoline is burned, lots of

chemical reactions occur Isooctane, C8H18, and oxygen, O2,

are the reactants They react and form two

products, carbon dioxide, CO2, and water, H2O

The products and reactants contain the same types of atoms: carbon, hydrogen, and oxygen

Atoms are rearranged as bonds are broken and formed

Mass is conserved (pg 149)

ENERGY AND REACTIONS

Energy must be added to break bonds In each isooctane

molecule, the bonds to carbon are covalent

A double covalent bond holds the two oxygen atoms together

For the atoms in isooctane and oxygen to react, all of these bonds have to be broken

This takes energy (pg149)

ENERGY AND REACTIONS

Forming bonds releases energy Once energy is added to

start the isooctane-oxygen reaction, new bonds form to make the products

When new bonds form, energy is released

When gasoline burns, energy in the form of heat and light is released (pg 150)

ENERGY AND REACTIONS

Energy is conserved in chemical reactions The energy released

during the reaction is stored in the reactants

Stored energy is called chemical energy

Total energy before the reaction is equal to the total energy of the products (pg 151)

ENERGY AND REACTIONS

Reactions that release energy are exothermic The isooctane-oxygen

reaction releases more energy than is absorbed to break the bonds in the reaction

This is an exothermic reaction

Temperature of surroundings rises because energy is released (pg 151)

ENERGY AND REACTIONS Reactions that absorb

energy are endothermic If hydrated barium

hydroxide and ammonium nitrate are combined, the reaction between them absorbs so much energy from the surroundings that the water vapor in the air will condense and freeze to the surface of the flask

Endothermic reaction (pg 151)

ENERGY AND REACTIONS

Some endothermic reactions cannot get enough energy from the surroundings, so energy must be added to get the reaction started Photosynthesis is an

endothermic reaction Plants use light

energy convert carbon dioxide and water to glucose and oxygen (pg 152)