Chemical Reactions 2.4

Chemical property: The ability of a substance to undergo a specific chemical change. Words that signify a chemical change: Burn Corrode Rust Decompose ExplodeFerment (Tarnish)

Classify the following properties of the element sodium as chemical or physical properties. Easily cut with a knife Reacts vigorously with water Doesn’t dissolve in oil Melts at 97.7 °C Is malleable (can be hammered flat) Metallic silver in color P C P P P P Conducts electricity P Floats in water P Emits a yellow light when burned. C Forms a gray coating when exposed to air. C

Chemical Reaction: During a chemical change, the composition of matter always changes. A chemical change is also called a chemical reaction. solid (s)(s) + CaC 2 Reactants Products 2H 2 OC2H2C2H2 + Ca(OH) 2 (l)(l) (g)(g) (p)(p) liquid gasprecipitate subscriptcoefficient Yields Calcium Carbide Water Acetylene Calcium Hydroxide + Chemical equation: +

Part I: You and a partner or two will obtain the following items: Safety glasses for each person in your group One 50mL beaker One stirring rod One Dixie cup Five pellets of calcium carbide (in the Dixie cup) 1.Put on your safety glasses 2.Place 1 drop of phenolphthalein in the beaker 3.Fill a beaker of water ¼ full of water 4.Use the stirring rod to gently mix the water and phenolphthalein

Observations: On your note paper, record the following: The color of the pellets The color of the water / phenolphthalein mixture The relative temperature of the beaker (touch the outside of the beaker with your fingers)

Perform the following procedure and write down at least three clues that a chemical reaction has taken place. 1.Make sure your safety glasses are on 2.Place three pellets in the beaker 3.Observe the reaction 4.When the reaction stops, feel the outside of the beaker and note the relative temperature.

Part II 1.Make sure your safety glasses are on 2.Pour the contents of your beaker into the waste bucket 3.Rinse out your beaker and fill half-full with water 4.Put one drop of phenolphthalein in the beaker 5.Stir the mixture with your stirring rod

Perform the following procedure and write down at least one more clue that a chemical reaction has taken place. 1.Make sure your safety glasses are on 2.Obtain a couple of matches from your teacher 3.Place two pellets in the beaker 4.Wait 15 to 20 seconds 5.Slowly bring the lit match up to the lip of the beaker Clues that a reaction took place: Color change Evolution of a gas Formation of a precipitate Discharge of an odor A transfer of energy: Heat Light Sound

Are the following examples evidence of chemical change? As ice freezes, it releases heat into the air. An iron nail gets rusty over time. CO 2 bubbles are released when a can of 7-up is opened. When droplets of water coalesce around dust particles, precipitation occurs. Leaves changing color as winter approaches. The sky turning from blue to red as the sun sets. P C P P C P A sugar cube disappears when it dissolves in water. P Salt water turning the Statue of Liberty green. C When alcohol reaches its boiling point, bubbles form. P

Law of Conservation of Mass: During any chemical reaction, the mass of the products is always equal to the mass of the reactants. States that in any physical change or chemical reaction, mass is conserved. ? What will be the final mass of the reactants?

Solve the following problem. A) 50.4 grams of water is changed through electrolysis into 5.6 grams of hydrogen. How many grams of oxygen are also formed? 44.8 g Hint:Think conservation of mass. 2H 2 O2H 2 + O g 5.6 g ? g

Solve the following problem. B) When TNT (dynamite, C 7 H 5 N 3 O 6 ) explodes, the products are carbon, nitrogen, water, and carbon monoxide (CO). How many grams of TNT are required to produce 3 grams of carbon, 3 grams of nitrogen, 3.2 grams of water, and 7 grams of CO? 16.2g 2C 7 H 5 N 3 O 6 + 7CO7C + 3N 2 + 5H 2 O ? g 3 g 3.2 g 3 g 7 g C)What scientific law did you use to solve problems A and B? Answer: Conservation of mass.