Chemical Reactions. Teachers Guide.

Transcription

1 Chemical Reactions. Teachers Guide. Introduction: Almost everything you will ever see in this universe is made of atoms. Each different type of atom is known as a chemical element there are currently 118 known elements; these are listed in the Periodic Table of the Elements, shown below. Atoms combine with each other in different arrangements to form substances known as molecules. A chemical formula shows the type and number of atoms a molecule is made from. For example, the chemical formula of water is H2O, meaning that it has two hydrogen atoms (H) joined to one oxygen (O); sand is SiO2 and has two oxygens for each silicon atom (Si). When mixed with each other in certain conditions, some types of chemicals will combine with each other to form new molecules. The process of combining atoms and molecules is known as a chemical reaction. How do we know whether mixing two substances together will cause a chemical reaction? Many reactions will produce effects that we can observe. In this set of experiments we will investigate several chemicals and observe the changes that occur when they react.

2 Equipment and consumables: Plastic cups Water Popsicle sticks or plastic spoons for mixing Bucket, paper towels and rubbish bags for clean-up Citric acid Baking soda (Sodium hydrogen carbonate) Epsom salts (magnesium sulfate) Thermometer Blackcurrant juice (with natural colouring - Ribena or Ribena concentrate work well) Calcium chloride available from supermarkets (Countdown, New World) and hardware stores (Bunnings, Mitre 10) as the powder in Damp-Rid moisture absorbing products (approximately $8 for 300 g) or as calcium water hardener from swimming pool supply stores (approximately $10 for 1 kg. Urea available from hardware stores (Bunnings, Mitre 10) and garden centres. Approximately $10 for 1 kg. (Optional) Instant ice pack (available from pharmacies and sport stores, approximately $5 each). Bleach White vinegar

3 1. Effervescent reactions (Fizzing and foaming) If the product of a chemical reaction is a gas then we observe effervescence the technical name for fizzing or foaming. As the gas molecules are produced they gather together to form bubbles that rise to the surface. The faster the reaction, the more bubbles are produced. A good demonstration of effervescence can be made by mixing citric acid with baking soda (sodium hydrogen carbonate). In a plastic cup, place 1 tsp of citric acid and 1 tsp of baking soda. Nothing will happen until you add a small amount of water (approximately ¼ of a cup). The reaction will effervesce for about twenty seconds. As powders, the two chemicals do not react as they cannot mix properly. Adding a small amount of water will cause the mixture to effervesce the water does not take part in the reaction, but dissolves both components and allows them to mix, letting the reaction proceed. The gas produced in this reaction is carbon dioxide. 2. Exothermic reactions Many chemical reactions generate heat these are known as exothermic reactions, from the Greek exo (outwards) and thermikόs (thermal). A common example is combustion whether a burning a candle, or operating a car engine. If two substances generate heat when mixed together it is a sure sign that a chemical reaction is taking place. A safe way to demonstrate an exothermic reaction in a classroom is to mix calcium chloride and water. The reaction generates enough heat to noticeably warm the water, but not enough to pose any danger. Add ½ of a cup of water to a plastic cup. Measure the temperature of the water with a thermometer. Pass the water around so that students can note the temperature.

4 Quickly add 3 tbsp of calcium chloride (CaCl 2 ) and mix well. Measure the increase in temperature. Pass the cup around the class again so that students can feel the temperature difference. In this reaction, calcium chloride is dissolved in water the technical term for this is a solvation reaction. Calcium chloride is made of ions (atoms or molecules with a positive or negative charge) in this case Ca 2+ and Cl -. During solvation, the calcium chloride crystals are split apart so that the Ca 2+ and Cl - ions are no longer in contact. The process of splitting crystals apart absorbs a lot of energy; however, even more energy is released by the interaction of water with the separate Ca 2+ and Cl - ions, resulting in a net release of energy in the form of heat. By the end of the reaction each of the ions are surrounded by water chemists show this by writing the name of the ion followed by (aq), from the Latin word for water (aqua). 3. Endothermic reactions Endothermic reactions absorb heat they are the opposite of exothermic reactions. The name comes from the Greek endon (within) and thermikόs (thermal). Heat is required to start an endothermic reaction; this heat is absorbed from the surrounding area, causing a drop in temperature. Details for two class demonstrations are found below: mixing urea and water, and activating an instant ice-pack. Both demonstrations rely on the same principle the ice packs contain ammonium chloride (NH 4 Cl) which is mixed with water when the pack is shaken. Add ½ of a cup of water to a plastic cup. Measure the temperature of the water with a thermometer. Pass the water around so that students can note the temperature. Quickly add 3 tbsp of urea (CON 2 H 4 ) and mix well. Measure the decrease in temperature. Pass the cup around the class again so that students can feel the temperature difference. Pass an instant ice-pack around the class for students to examine. Measure its temperature and then activate it (usually by squeezing and then shaking). Measure

5 the temperature drop and then pass around the class again so that students can feel the temperature difference. These two endothermic reactions are similar to the exothermic example described in the previous section the process that is occurring is, again, a solvation reaction. The urea molecules and ammonium chloride ions are split apart and surrounded by water. As is the case for exothermic reactions, the process of splitting urea and ammonium chloride crystals apart absorbs a lot of energy; unlike the exothermic examples, the release of energy by the interaction of water with the separate urea molecules and ammonium chloride ions is much smaller, resulting in a net decrease of energy absorbing heat instead of releasing it. For the urea the reaction is: For the instant ice-pack the reaction is: 4. Colour changes A change of colour is an excellent indication that a chemical reaction has occurred. Many common consumer items make use of colour changing reactions because humans have excellent colour vision colour changing test strips are used to determine the ph and water hardness of swimming pools, and are also used in medicine for pregnancy tests and for monitoring kidney function and glucose levels. In this demonstration we will change the colour of blackcurrant juice.

6 Method: 1. Half fill a plastic cup with blackcurrant juice. 2. Add 1 tsp of bleach and stir. Continue adding bleach until the red colour is replaced by yellow. Aim to use as little bleach as possible to achieve the colour change. 3. You can reverse this reaction, and change the colour back to red by adding white vinegar (although you will need to use more vinegar than you used bleach). In this reaction, it is not the active ingredient in bleach, sodium hypochlorite, that is responsible for the colour change. Instead, it is sodium hydroxide included in bleach as a stabiliser. The purple colour in blackcurrants is a natural ph indicator known as anthocyanin which plants use to attract animals and insects. Anthocyanins change colour as acidity changes. The sodium hydroxide is a strong base (alkaline) which lowers the acidity (increasing the ph) and causes the purple colour to become yellow. Vinegar is acidic, lowering the ph and reversing the colour change. 5. Precipitation reactions A precipitation reaction is a process where a molecule is formed that is not soluble in the liquid in which it was created. This is often seen when two different salts are mixed, although each salt, on its own, completely dissolves in water, the combination of salts creates a precipitate an insoluble chemical. A common example of a precipitation reaction is the formation of soap scum. The major chemical found in soap (sodium stearate) forms an insoluble precipitate when exposed to magnesium and calcium ions found in hard water. A simple demonstration of a precipitation can be made using magnesium sulfate (MgSO 4 ) and calcium chloride (CaCl 2 ). In separate plastic cups, dissolve 1 tbsp of magnesium sulfate and calcium chloride in ½ a cup of water. You may need to use warm water to make the magnesium

7 sulfate dissolve (if the solid does not completely dissolve, tip the liquid into another cup and discard the undissolved solid into a rubbish bin). Slowly pour one of the liquids into the other you should see a fine white solid form as they mix. Magnesium sulfate (MgSO 4 ) and calcium chloride (CaCl 2 ) can each be dissolved in water forming Mg 2+ & SO 2- (aq) 4 (aq) ions and Ca 2+ (aq) & Cl - (aq) ions, respectively. However, when they mix, the calcium and sulfate ions combine to form calcium sulfate (CaSO 4 ) which is not in soluble in water solid calcium sulfate then falls out of the water. Chemists use a (s) written after the chemical formula to indicate a solid. The other combination of ions, magnesium chloride, is soluble and remains in the water. The reaction is: