Science is simply common sense at its best. ~Thomas Huxley Chemical Change Unit A: Science is simply common sense at its best.  ~Thomas Huxley

When a chemical change occurs during a chemical reaction: - it is always accompanied by energy changes - energy can be released slowly (battery) or quickly (explosion) - different substances with different properties are formed

Chemical reactions can be useful because of: 1) The substances they produce: Reactions that produce gas When a large volume of gas is formed quickly from a small amount or solid or liquid, an explosion occurs (ex. Ignition of sodium azide in airbags) Slow release of gas from solids or liquids can also be useful (ex. Air pocket in batter and dough form when baking) Reactions that produce solids When 2 liquids are combined, a solid can be formed quickly (precipitate) or over time (epoxy/glue)

Chemical reactions can be useful because of: 2) How they occur: Endothermic reactions ABSORB HEAT Exothermic reactions RELEASE HEAT

Energy Changes in Reactions: Exothermic reactions – reactions that release energy in the form of heat, light, electricity Important exothermic reactions: 1) COMBUSTION: a substance reacts rapidly with oxygen to produce new products and heat energy C(s) + O2(g) → CO2(g) + energy 2) OXIDATION: a substance reacts slowly with oxygen to produce new products and heat energy (i.e. cellular respiration) C6H12O6 (aq) + 6O2(g) → 6CO2(g) + 6H20(l) + energy

Energy Changes in Reactions Endothermic reactions – reactions that absorb energy in the form of heat, light, electricity Important endothermic reactions: 1) PHOTOSYNTHESIS: a process that capture’s the Sun’s light and allows a plant to use this energy to make sugar molecules. CO2(g) + 6H20(l) + solar energy → C6H12O6 (aq) + 6O2(g)

JUST A FEW REMINDERS: States in chemical formulas are indicated by the following symbols: (s) solid (g) gas (l) liquid (aq) aqueous – dissolved in water All elements are solids except for: Gases: H2(g) , N2(g) , O2(g) , F2(g) , Cl2(g) , noble gases Liquids: Hg(l) , Br2(l) All ionic compounds at SATP are solids If (aq) solutions are reacting, use your solubility rules to determine if the products are (aq) or (s) Molecular compounds can be solid, liquid or gas at SATP General form of all reactants is: reactants → products

Counting Atoms How many atoms are present? 3 Ba3(PO4)2 4 (NH4)2SO4

LAW OF CONSERVATION OF MASS In 1789, Antoine Lavoisier stated that the total mass of a reaction system never changes: “In a closed system, the total mass of the products equals the total mass of the reactants” At this time, there are no known exceptions to the Law of Conservation of Mass When a theory has a wide application and has no known exceptions, it is given the status of “Law”

Law of Conservation of Mass In a chemical reaction, matter is not created or destroyed. If you start with 10g of reactants you will end up with 10g of products If you have 2 hydrogen atoms combine you will end up with 1 hydrogen molecule (but there are still 2 hydrogen atoms present) + → H + H → H2(g)

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So how much zinc will be produced?? 104 g 64 g + 192 g = 152 g + 104 g 256 g = 256 g

Writing Chemical Equations Unit A: Section 3.2 A little knowledge is a dangerous thing. So is a lot. Albert Einstein

Writing Chemical Equations Word equation: uses the “chemical names” hydrogen + oxygen → water Formula equation – uses the “chemical formulas” of reactants and products to represent a reaction O2(g) + H2(g) → H2O(l) Skeleton equation –a formula equation that shows the identities of the substances involved in the reaction and which elements are present Doesn’t show the correct proportions/number of elements on each side Balanced Equation – integers called “co-efficients” are used to show equal numbers of each element present on both sides O2(g) + 2H2(g) → 2H2O(l)

Steps in Writing an Equation Write the chemical symbols and or formulas for the reactants and products. Check for diatomic elements. Balance the equation using coefficients. Make sure atoms are equal on both sides. Ex: methane and oxygen react to form carbon dioxide and water ___CH4(g) +___O2(g) → ___CO2(g) + __H2O(g)

Balancing Equations 1) Write the chemical formulas for the reactants and products including states CH4(g) + O2(g) → CO2(g) + H2O(g)

Balancing Equations 1) Write the chemical formulas for the reactants and products including states Reactants Products CH4(g) + O2(g) → CO2(g) + H20(g)

Balancing Equations 2) Start by balancing everything except H and O. In other words, start by balancing the atoms of C (carbon): Reactants Products CH4(g) + O2(g) → CO2(g) + H20(g) C = 1 H= 4 O= 2 H= 2 O=3 There is one atom of carbon on each side of the equation. Therefore, the carbon atoms are already balanced.

Balancing Equations 3) Now balance the atoms of H in the equation Reactants Products CH4(g) + O2(g) → CO2(g) + H20(g) C = 1 H= 4 O= 2 H= 2 O= 3 2 Add a coefficient! 4 / / 4 Now there are 4 atoms of hydrogen on the reactant side and on the product side. Therefore, the hydrogen atoms are balanced.

Balancing Equations 4) Finally, balance the atoms of oxygen (O) in the equation Reactants Products CH4(g) + O2(g) → CO2(g) + 2H20(g) C = 1 H= 4 O= 2 O= 4 2 Add a coefficient! / 4 Now there are 4 atoms of oxygen on the reactant side and on the product side. Therefore, the oxygen atoms are balanced.

Balancing Equations CH4(g) + 2O2(g) → CO2(g) + 2H20(g) Is the equation balanced??? CH4(g) + 2O2(g) → CO2(g) + 2H20(g) Reactant side = 1 atom of carbon, 4 atoms of hydrogen and 4 atoms of oxygen Product side = 1 atom of carbon, 4 atoms of hydrogen Yes, the equation is balanced!

REMEMBER: 2O2(g) + 4H2(g) → 4H2O(l) Never change a subscript to balance an equation! Use coefficients instead! O2(g) + H2(g) → H2O(l) Is unbalanced – but you can’t change it the following way! O2(g) + H2(g) → H2O2(l) X Make sure the coefficients are the lowest whole-number ratio possible: 2O2(g) + 4H2(g) → 4H2O(l) This is a balanced formula but these are not the lowest numbers you could use: O2(g) + 2H2(g) → 2H2O(l)

Reducing Coefficients You must simplify coefficients when there is a coefficient in front of EVERY reactant & product and they are ALL divisible by the same number. Can these coefficients be reduced? 4K + 2F2 → 4KF 2K + F2 → 2KF 2Ba + O2 → 2BaO 4Fe2O3 → 8Fe + 6O2 2Fe2O3 → 4Fe + 3O2

Balancing Chemical Equations 1) Write the chemical formulas for the reactants and products including the states of matter Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2(aq) TRY BALANCING IT

Reactants Products Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2(aq Cu = Ag= N= 1 1 1 / 2 1 / 2 1 / 2 2 6 6

https://phet.colorado.edu/en/simulation/balancing-chemical-equations

Balance the Chemical Equation __S8 + ___ O2 ➔__ SO2 ___Fe + ___AgNO3 ➔ ___Fe(NO3)2 + ___Ag Sodium metal reacts with iron (II) chloride, iron metal and sodium chloride are formed.

Practice ___C6H6 + ____O2 ___H2O + _____ CO2 ___NH3+ _____ O2 ____NO + _____H2O

Extra Stuff

Count how many H and F atoms there are? Does it follow the Law of Conservation of Mass?

Coefficients Coefficients are whole numbers placed in front of reactants and products. They are used to balance an equation to show matter is conserved. Ex. 2H2 + O2 = 2H2O

Balancing Equations CH4(g) + O2(g) → CO2(g) + H20(g) 2) Start by balancing everything except H and O. In other words, start by balancing the atoms of C (carbon): CH4(g) + O2(g) → CO2(g) + H20(g) 1 carbon 1 carbon There is one atom of carbon on each side of the equation. Therefore, the carbon atoms are already balanced.

CH4(g) + O2(g) → CO2(g) + H20(g) Balancing Equations 3) Now balance the atoms of H in the equation CH4(g) + O2(g) → CO2(g) + H20(g) 4 hydrogen 2 hydrogen How can we balance them? Now there are 4 atoms of hydrogen on the reactant side and on the product side. Therefore, the hydrogen atoms are balanced. 2 Add a coefficient!

Balancing Equations CH4(g) + O2(g) → CO2(g) + 2H20(g) Finally, balance the atoms of oxygen (O) in the equation CH4(g) + O2(g) → CO2(g) + 2H20(g) 2 oxygen 2 oxygen 2 oxygen So we have 2 oxygen on the reactant side and 4 on the product side. So we need to add another coefficient Now we have 4 oxygen on the reactant side and 4 on the product side, so the oxygen atoms are balanced. 2

Balancing Equations CH4(g) + 2O2(g) → CO2(g) + 2H20(g) Is the equation balanced??? CH4(g) + 2O2(g) → CO2(g) + 2H20(g) Reactant side = 1 atom of carbon, 4 atoms of hydrogen and 4 atoms of oxygen Product side = 1 atom of carbon, 4 atoms of hydrogen Yes, the equation is balanced!

Remember: When counting elements, don’t forget to look at both the subscript and the coefficient. For example: P2O5 = has 2 phosphorus atoms and 5 oxygen atoms 2P2O5 = has 4 phosphorus atoms and 10 oxygen atoms Because there are 2 molecules (indicated by the coefficient) and 2 atoms in each molecule (indicated by the subscript) – So you multiply!!

REMEMBER: Never change a subscript to balance an equation! Use coefficients instead! Make sure the coefficients are the lowest whole-number ratio possible: 2O2(g) + 4H2(g) → 4H2O(l) This is a balanced formula but these are not the lowest numbers you could use: O2(g) + 2H2(g) → 2H2O(l)

Balancing Chemical Equations 1) Write the chemical formulas for the reactants and products including the states of matter Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2(aq) 2) Balance the element (atom or ion) present in the greatest number by multiplying by the lowest coefficient possible (NO3)2(aq) = 2 present (lowest coefficient possible to balance = 2) Cu(s) + 2AgNO3(aq) → Ag(s) + Cu(NO3)2(aq) 3) Repeat step 2 for the rest of the elements Now we have 2 Ag, so balance the other side Cu(s) + 2AgNO3(aq) → 2Ag(s) + Cu(NO3)2(aq) 4) Count elements on each side of the final equation to ensure they balance: 1 Cu(s) = 1 Cu(s) ; 2Ag = 2Ag(s) ; 2 NO3 = (NO3)2(aq) TRY BALANCING IT

Balancing Equations Revisited Law of Conservation of Mass Matter cannot be created or destroyed in a chemical reaction. Atoms are simply rearranged to form new substances. Atoms and ions on = Atoms and ions on the dfreactant side the product side

Writing Chemical Equations Chemical equations use chemical symbols as a shorthand way of showing what happens during a reaction Reactants – the starting materials (before the reactions) Products – substances made during the reaction ( + ) groups the reactants together ( → ) separates the reactants from products, it means “produces”