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Experiment 10: Determining Chemical Reactivity A chemical reaction is said to occur when changes are observed. Examples of Chemical Reaction 1. Release of gas 2. Formation of precipitate 3. Change in color 4. Change in mass 5. Change in temperature Elements are more reactive if it is further left of the periodic table or further down. Metal atoms have only a few electrons in their outermost electron shell. Reactivity of metals increases as you go down the group because the outer electrons are further from the nucleus. Activity Series of Metals Rb, K, Na, Ca, Mg, Al, Zn, C, Cr, Fe, Ni, Sn, Pb, H, Cu, Bi, Sb, Hg, Ag, Pt, Au Metals above hydrogen react with acid to produce hydrogen gas. o Examples: Zn + H2SO4 → ZnSO4 + H2 2Na + 2HCl → 2NaCl + H2 Ca + H2O → Ca(OH)2 + H2 Observations A. CuSO4 NH4OH CuSO4 + NH4OH Light blue solution Colorless Dark blue solution CuSO4 + NH4OH → (NH4)2SO4 + Dissolves B. HCl + Fe HCl + Cu HCl + Mg HCl + Al Cu(OH)2 dark blue Observations Release of gas, formation of rust NO REACTION Release of heat, release of gas, disappearance of the substance Release of heat, release of gas HCl HCl HCl HCl + + + + Fe2+ Cu Mg Al → FeCl2 (rust) → → MgCl2 → AlCl3 Observations C. ZnSO4 BaCl2 ZnSO4 + BaCl2 Colorless Colorless Clear solution with white precipitate ZnSO4 + BaCl2 → BaSO4 White precipitate + ZnCl Experiment 11: Types of Chemical Reactions Types of Chemical Reaction 1. Combination – a reaction in which two or more substances combine to form a single product. A + B → AB 2. Decomposition – a reaction in which a single substance breakdown into two or more substances. AB → A + B 3. Single Replacement - a reaction in which one element reacts by taking the place of another element in a compound. A + BC → AC + B 4. Double Replacement – the reaction in which the positive ion from one compound takes the negative ion of another compound and the remaining ions also combine to from another ionic compound. AB + CD → AD + CB A chemical equation is not considered valid until it is balanced. Observations A. 1. Fe nail in a bottle Fe nail + moist 2. H2O + NaOH + phenolphthalein + breath Fe3+ + O2 → No reaction Formation of rust Colorless Dark pink solution Colorless Fe2O3 Iron oxide or rust 2NaOH + 2CO2 → 2NaCO3 + H2 Observations B.1. (NH4)2CO3 + moist RLP + moist BLP 2. CoCl2.6H2O + Heat 3. BaCl2.2H2O + Heat Pungent odor Changed to blue litmus paper No change Red-violet crystals Blue powder + moisture droplets White powder It became lighter (NH4)2CO3 → NH3 + H2O + CO2 CoCl2.6H2O + heat → CoCl2 + 6H2O BaCl2.2H2O + heat → BaCl2 + 2H2O Observations C.1. CuSO4 + Fe 2. KI + Cl2 + starch CuSO4 + Fe → Formation of rust Yellow solution Dark solution FeSO4 + Cu rust 2KI + Cl2 → 2KCl + I2 yellow Observations Cloudy solution with white precipitate Dark red/maroon solution D.1. NaCl + AgNO3 2. KSCN + FeCl3 NaCl + AgNO3 → AgCl + NaNO3 precipitate 3KSCN + FeCl3 → 3KCl + Fe(SCN)3 Deep red The carbon dioxide in our breath reacts with the NaOH solution, which results to a change in its color. Iron is more reactive than copper. The presence of iodine was confirmed by the addition of starch. Experiment 12 – Oxidation – Reduction Reactions Oxidation-Reduction or redox – reactions in which transfer of electrons take place. Oxidation Number o For ionic compounds, the oxidation number is equal to the number of electrons lost or gained. It is the same as the charge on the ion. Examples: NaCl = Na+1; Cl-1 MgBr2 = Mg+2; Br-1 o For covalent compounds: 1. All elements in their free state have an oxidation number of zero. 2. The oxidation number of oxygen is -2 (except in peroxides, -1). 3. Oxidation number of hydrogen is +1 (except in metal hybrides, -1). 4. The sum of oxidation numbers must be equal to zero (all coumpounds are electrically neutral). 5. All elements in Group IA have an oxidation number of +1. 6. All elements in Group IIA have an oxidation number of +2. Oxidation Number of Some Elements Hydrogen +1 Chloride Sodium +1 Bromide Potassium +1 Iodide Silver +1 Sulfide Ammonium +1 Oxide Calcium +2 Magnesium +2 Aluminum +3 Iron +2; +3 Copper +1; +2 Tin +2; +4 -1 -1 -1 -2 -2 Oxidation – loss of electrons which cause an atom or an ion to have either a decrease in negative charge or an increase in positive charge. Involves the gain of oxygen and loss of hydrogen Reduction – gain of electrons which cause the atom or ion to have an increase in negative charge or decrease in positive charge. Involves the loss of oxygen and gain of hydrogen Reducing agent – the substance in the reaction which gives up electrons. –It is the substance that is oxidized. Oxidizing agent – The substance which takes up (or receives) these electrons. –It is the substance that is reduced.