Chemical reactions result in chemical changes Chemical changes

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 • Chemical reactions result in chemical changes. – Chemical changes occur when new

• Chemical reactions result in chemical changes. – Chemical changes occur when new substances are created. – The original substance(s) reactants change into new substance(s) products See pages 202 - 203

WRITING CHEMICAL EQUATIONS 1) A word equation: Nitrogen monoxide + oxygen nitrogen dioxide

WRITING CHEMICAL EQUATIONS 1) A word equation: Nitrogen monoxide + oxygen nitrogen dioxide

WRITING CHEMICAL EQUATIONS 2) A symbolic equation: 2 NO(g) + O 2(g) 2 NO

WRITING CHEMICAL EQUATIONS 2) A symbolic equation: 2 NO(g) + O 2(g) 2 NO 2(g)

WHAT DOES ALL THAT STUFF MEAN? A symbolic equation: State of matter 2 NO(g)

WHAT DOES ALL THAT STUFF MEAN? A symbolic equation: State of matter 2 NO(g) + O 2(g) 2 NO 2(g) Coefficients - Indicate the ratio of compounds in the reaction. - Here, there is twice as much NO and NO 2 than is O 2. - Letters indicate the state of each compound. -(aq) = aqueous/dissolved in water (s) = solid (l) = liquid (g) = gas

COEFFICIENTS VS. SUBSCRIPTS H 2 + O 2 H 2 O Subscripts tell us

COEFFICIENTS VS. SUBSCRIPTS H 2 + O 2 H 2 O Subscripts tell us how many atoms of each element are present in the reactants and products We use coefficients in front of compounds to “balance” reactions 2 H 2 + O 2 2 H 2 O Now count the atoms on both sides of the equation to make sure they are equal H=4 O=2

PRACTICE Before we continue you need to be able to count atoms in chemical

PRACTICE Before we continue you need to be able to count atoms in chemical equations Complete the ‘How Many Atoms’ worksheet We’ll take it up and continue in 15 minutes

THE LAW OF CONSERVATION OF MASS Chemical change means new compounds are created, BUT

THE LAW OF CONSERVATION OF MASS Chemical change means new compounds are created, BUT John Dalton, 200 years ago, realized that atoms simply rearrange themselves during chemical reactions. No new matter is created or destroyed; atoms are just rearranged. All of the matter in the reactants = all of the matter in the products. Number of each atom in reactants = number of each atom in products.

THE LAW OF CONSERVATION OF MASS This law was developed by Antoine and Marie-Anne

THE LAW OF CONSERVATION OF MASS This law was developed by Antoine and Marie-Anne Lavoisier in the 1700 s. The law of conservation of mass states: In a chemical reaction, atoms are neither created nor destroyed. Therefore: Mass of reactants = mass of products Fuel in (gas and O 2) Reactants EQUALS Exhaust out Products

THE NEXT STEP… BALANCING EQUATIONS A skeleton equation shows the formulas of the elements/compounds.

THE NEXT STEP… BALANCING EQUATIONS A skeleton equation shows the formulas of the elements/compounds. A skeleton equation shows atoms, but not quantities of atoms. K + O 2 K 2 O A balanced chemical equation shows all atoms and their quantities Balancing ensures that the number of each atom is the same on both sides of the reaction arrow. Always use the smallest whole-number ratio. 4 K + O 2 2 K 2 O

BALANCING EQUATIONS Word equation: methane + oxygen water + carbon dioxide Skeleton equation: CH

BALANCING EQUATIONS Word equation: methane + oxygen water + carbon dioxide Skeleton equation: CH 4 + O 2 H 2 O + CO 2 To balance the compounds, count how many atoms of each element occur on each side of the reaction arrow C=1 H=4 O=2 C=1 H=2 O=3 To balance, try to add coefficients that make the atoms on both sides of the equation equal CH 4 + 2 O 2 2 H 2 O + CO 2 C=1 H=4 O=4

STEPS FOR BALANCING Trial and error will work but can be very inefficient. 1.

STEPS FOR BALANCING Trial and error will work but can be very inefficient. 1. 2. 3. 4. Balance compounds first and elements last. Balance one compound at a time. Only add coefficients; NEVER change subscripts. If H and O appear in more than one place, attempt to balance them LAST. 5. Polyatomic ions (such as SO 42 -) can often be balanced as a whole group. 6. Always double-check after you think you are finished.

PRACTICE BALANCING • Use the molecular sets to practice balancing equations

PRACTICE BALANCING • Use the molecular sets to practice balancing equations