Chemical Reactions

7.1 DESCRIBING REACTIONS

Changes in Substances

Physical change – altered appearance but same compositionWater to steamGlass bottle to broken glass

Chemical change or reaction – results in a different chemical compositionCoal to ashesMilk to sour milk

+

Coal OxygenCarbon Dioxide

Chemical Reaction

Reactants Products

C + O2 CO2

(Solid) (Gas) (Gas)

+

Hydrogen Oxygen Water

Chemical Reaction

Reactants Products

2H + O2 H2O

(Gas) (Gas) (Liquid)

+

Chemical Reactions

Chemical reaction: rearranging chemical bonds of substances to form a different substance

Show with chemical equation Reactant(s) Product(s)

Coefficients show the amount of each item

2H2 + O2 2H2O

Law of Conservation

Atoms are not created or destroyed during a chemical reaction

All of the reactant atoms have to be in the product

Not Correct: H2 + O2 H2O

(two) (one)

Correct: 2H2 + O2 2H2O

(four) (two) (four) (two)

+

Chemical Reaction

+

CH4 + O2 CO2 + H2O2 2

Balanced!Balanced!

Law of Conservation

In a chemical reaction, energy is not created nor destroyed but can change between chemical energy (in a chemical bond) and heat energy

Moles

Mole – a unit like a dozenDozen = 12 itemsMole = 6.02 X 1023 items

Mole of the element Na contains

6.02 X 1023 Na atomsMole of CO2 contains 6.02 X 1023

molecules of O=C=O

Moles in Chemistry

I help you get the right amount of

reactants for your chemical reaction.

Moles in ChemistryA chemical mole is something pest-y

like me!

Mole – unit that contains 6.02 X 1023 items.

Moles

3 H2 + N2 2 NH3 you need 3 moles of hydrogen and 1 mole of

nitrogen to make 2 moles of ammonia gas with no reactants left over

amu of H2 is 2 so 1 mole mass of H is 2 grams and 3 moles of H is 6 grams

amu of N is 14.01 X 2 so 1 mole of N is 28.02 grams of nitrogen

amu of NH3 is 14.01 + (3 x1) = 17.01 so 1 mole of NH3 is 17.01 grams; 2 moles is 34.02 grams

6 g of H + 28.02 g of N 34.02 g of NH3

Moles

Moles of elements in a compound: H2O requires two H for every one O so 2 moles of H atoms and one mole of O atoms makes one mole of H2O

3 H2 + N2 2 NH3 (balanced chemical equation): :you need 3 moles of hydrogen and 1 mole of nitrogen to make 2 moles of ammonia gas with no reactants left over (fastest reaction)

Moles in ChemistryHow do you get a mole in chemistry!

1 mole mass = ___ grams

___ = substance’s amu

7. 2 TYPES OF REACTIONS

Types of Chemical Reactions

Synthesis: A + B C (requires energy input)Decomposition: DE D + E (releases energy)Replacement JL+ HK LH+JKOxidation/Reduction = loss/gain of e-

Oxidation: lose by gaining an O or losing a HReduction: gain by gaining a H or losing an O

Reversible Reactions

Important chemical reaction in the body CO2 + H20 H2CO3 H+ + HCO3

-

Reversible reactions go in the direction of high concentration to low concentration

7.3 Energy Changes in Reactions

Heat is a form of energyChemical energy is energy stored in

chemical bonds.Chemical reactions involve the breaking of

chemical bonds in the reactants and the formation of chemical bonds in the products.

Energy in Chemical Reactions

Activation energy – amount of energy needed to break the bonds of the reactants to start the reactionReactants must meet in order to reactWhen molecules collide, bonds are broken

Heat of reaction – energy difference between reactant and productsExothermic reactions release heatEndothermic reactions absorb heat

Rate of Reaction

Reactions with low activation energy go faster

Higher temperature increases reaction rateAmount of reactants affect rate

2 hydrogen molecules in 200 oxygen molecules will only produce 2 water molecules

Catalyst – a substance whose presence speeds up a reaction

time

ener

gy

Energy of reactants

Energy of products

Exothermic Reaction

Energy of Activation

Heat OffHeat Off

Heat of Reaction (released)

Endothermic Reactionen

ergy

time

Heat of Reaction (absorbed)Energy of

reactants

Energy of productsEnergy of

Activation

Chemical Catalysts

Catalyst – speeds up a reaction but does not take part in the reaction and is not used up during the reaction

Enzyme – a catalyst in a living organismEnzymes digest foodEnzymes cause the blood in a wound to clot

CO2 + H20 H2CO3 H+ + HCO3-

CO2 + H20 H2CO3 H+ + HCO3-

The End