Chemical Reactions

• Reactants – Substances present prior to start.

• Products – Substances produced.

• Rearrangement of atoms-none created-none destroyed

• Law of conservation of mass: -mass is never created or destroyed in

an ordinary chemical reaction.

• Chemical Equation: describes a chemical reaction.

- symbols and formulas

- before and after picture

Reactants

Products

MgO + C → Mg + CO

- reactants and products separated by plus signs

- arrow points from reacts. to products.

• Rules:

– Must use actual (correct) formulas

– Must follow conservation of mass (balanced)

MgO + C → Mg + CO

Balancing equations

• Need same number of atoms of each element on each side of equation

• Done using coefficients only

3Cu + 8HNO → 3Cu(NO ) + 2NO + 4H O

Coefficients Subscripts

3 23 2

•Never!!!!!!• Change subscripts to balance an equation!!!

• Will change substances.

• Start with the most complicated element

• Save an element that is all alone for last

• Practice!!!!!!

Fe O + C → Fe + CO

C H + O → CO + H O

2 3 2

2 6 2 2 2

Combustion

• Reacting with oxygen in the air to form CO2 and H2O

• Is one form of Oxidation

• Also called burning

• Many of common fuels are hydrocarbons– Contain only hydrogen and carbon

Oxidation in Our Bodies

• Our energy comes from oxidation of our food– Oxygen comes from the air, through our

lungs, to our blood.– The hydrogens and carbons in our food get

oxidized to CO2 and H2O• Very similar to combustion but more controlled and

no flames (lower temperatures)

Incomplete Combustion

• Happens when not enough time or oxygen– In automobile engines, second-hand smoke

• Most common products are CO and soot (C)– CO is a very poisonous gas

• Combines with hemoglobin not allowing oxygen to combine

• Very difficult to get it off once it is on• Makes your heart work harder

Gas Concentrations

• Can be expressed in % or fractions

• For low concentrations, ppm is used– Stands for parts per million

• % is parts per hundred

– How many molecules of the gas in one million molecules of air

– Used frequently for pollution, etc.

CO As an Air Pollutant

• Very dangerous indoors – faulty furnaces, etc.– Many people have installed CO detectors

• Limit outside is 9 ppm– Helped by the use of catalytic converters on

autos• Catalyzes the conversion of CO to CO2

– Some cities must use oxygenated fuels during winter – in Utah?

Tarnishing and Rusting

• Other oxidation reactions from gases in the air

• Silver reacts with H2S in the air to form Ag2S (tarnish)

• Corrosion comes from oxidation of iron from oxygen in the air to form rust (Fe2O3)

– Water greatly accelerates rusting– Costs US alone more than $10 billion/year

H5 – C2

• 16 - 21, 25, 26, 29, 45, 46, 48, 50, 51, 56, 58, exercise 2.3 (in chapter)