fighting-frost-with-ice.pdf

fighting Frost with ice SHUTTERSTOCK...

fighting Frost with ice SHUTTERSTOCK By Brian Rohrig F rost! As the seasons change, from winter to spring So, while frost is what we observe as an indication of dropping temperatures, it’s actually the cold temperatures that farmers have to and summer to fall, farmers around the world fight. Farmers use two common methods to keep their orchards warm: struggle to protect their crops from frost damage. spraying their crops with water to form a mixture of water and ice around the buds or fruit, or using mobile gas heaters to do the same to In the spring, vulnerable buds that die in a late frost the frost as it forms. can mean no fall crops, and an early frost in the fall can It might seem odd to use ice to keep plants from freezing, but the mean a fully grown crop dies on the vine. goal is to keep the temperature at or above freezing. It’s a matter of energy and what happens during phase changes. Frost occurs when water vapor in the air deposits on a At the molecular level, the flow of energy is complicated. Heat is surface when the outside temperature drops below 0 °C released when water freezes or fuels burn, but releasing energy may (32 °F). Because no liquid form of water is present, this first require an initial input of energy. What we observe is the net result of all the associated molecular- or atomic-level shenanigans. is an example of deposition—a phase change directly from a gas to a solid without going through the liquid USING ICE TO PROTECT AGAINST FROST DAMAGE phase. Snowflakes are made this way too, but where In 1920, a young researcher in Washington State, Daniel James “DJ” Crowley, suggested that the cranberry farmers in the area turn over- they form on dust particles, frost grows on solid surfac- head sprinklers on their cranberry harvest before an early frost. The es such as plants, trees, windows, and streetlights. farmers were skeptical, but they did as he suggested. And it worked. At the cellular level plants are predominately water. What did DJ Crowley know that prompted him to make this suggestion? As unusual as it might sound, water, or rather a mixture of water and When plant cells freeze, they burst. This is because ice, can protect crops from dropping temperatures. When water and water expands when it freezes. Plants can withstand a ice exist at the same time the temperature remains at 0 °C (32 °F) until the freezing process is complete, no matter how far the environmental few hours at temperatures below —0.6 °C (31°F), but a temperature dips. few minutes at –2 °C (28 °F) can result in plant death. What is the connection between phase changes and energy? ChemMatters | www.acs.org/chemmatters 5  Intermolecular Forces SHUTTERSTOCK H-bonds Interactions between atoms Intramolecular Forces Phase changes are driven by intermolecular forces of attraction between molecules. Like interpersonal relationships between you and your friends, intermolecular forces happen between one water molecule and its neighbors. In water, these intermolecular forces are called hydrogen bonds, or H-bonds for short. And while they are called bonds, they are much weaker than intramolecular interactions, which occur between the individual water molecule’s oxygen and hydrogen atoms. Remember, water has a dipole, that is, the more electroneg- Covalent Bonds ative oxygen atom in the water molecule pulls the electrons Interactions between atoms towards itself. This attraction of electrons toward the oxygen atom creates a partially negative charge on the oxygen. And, because the exothermic. Heat is exiting the system: the prefix exo- is Greek for outside. We electrons are pulled away from the hydrogens, they become perceive this as a hot reaction: think campfire. partially positively charged. When energy is absorbed from the environment, we call this endothermic. An H-bond is a very specific kind of intermolecular force Heat is being taken in: the prefix endo- is Greek for inside. We perceive this as a that occurs between a molecule that contains hydrogen atoms cold reaction: think ice cube melting in your hand. To break the hydrogen bonding bonded to highly electronegative atoms like oxygen. By con- interactions between water molecules in the solid phase, the ice pulls energy from vention, H-bonds are shown as dotted lines between the partial your hand, making your hand feel cold. negative oxygen side of the water molecule to the partial During a phase change, the temperature of the system remains the same. The positive hydrogen side of a different water molecule. temperature won’t change until all of one phase has been converted to the other. During a phase change from solid water (ice) to liquid water, When two phases are present at the same time, energy is being absorbed and some of the H-bonds between water molecules are broken. released. The energy required to melt ice or solidify water is 334 Joules per gram This takes energy from the environment. Conversely when (J/g). This quantity is called the heat of fusion. The same amount of energy is water solidifies, energy is given off as additional H-bonds form released when water turns to ice, as is absorbed when ice turns to water. It just between water molecules. depends on whether we’re breaking or making H-bonds. When energy is released into the environment, we call this It is water’s heat of fusion and the associated temperature plateau that keeps the buds or fruit from freezing. As long as the ice that forms on the plants is kept wet, the temperature of the fruit inside will remain at or above 0 °C (32 °F). Also, as the ice freezes, the heat that is released as H-bonds form between water molecules, helps H-Bonds in water keep the encased fruit warm. DJ Crowley’s knowledge of phase changes helped saved the cranberry crops in the 1920s, and his technolo- gy is still used today. USING COMBUSTION REACTIONS TO PROTECT AGAINST FROST It might seem strange to think about heating an orchard using a gas heater. But high-powered heaters with jet-like blowers can be pulled behind a tractor to do just that. Another option are to use upright heaters like those you might have seen on restaurant patios to extend the Gaseous Water (Steam) Liquid Water Solid Water (Ice) outside dining season. Along that same idea, some No H-bonds farmers use carefully placed barrels throughout a vineyard or orchard and light small wood fires. 6 DECEMBER 2023 | ChemMatters  heating/cooling curve Heat of Fusion (∆Hfus) 0 °C Temperature Solid Solid and Liquid Liquid Heat Added SHUTTERSTOCK Heat Removed All of these heating options are based on the same have both an endothermic and an principle: Burn a carbon-based fuel to warm the air and exothermic aspect to break bonds Exothermic Reaction melt the frost just enough to keep it wet because mixtures and rearrange them. Energy is of solid and liquid water stay at 0 °C (32 °F). always required to break inter- Combustion reactions are common. We use them to molecular attractions or bonds heat our homes, cook our dinners, and power our cars. between atoms. Activation Energy Unlike phase changes, the chemical composition of our First, energy must be absorbed fuel changes when it's burned. This means intramolecular by the methane and oxygen mol- Energy Reactants forces—bonds between atoms—are broken and new ones ecules. One way to visualize this are formed. is to think of two magnets. If they So, does the heat come from the breaking of the bonds are stuck together, it will require or the forming of the bonds? energy to pull them apart. ΔH = (-) Products Let’s examine a common reaction, the combustion of Then to form the products—CO2 SHUTTERSTOCK methane (CH4), which occurs every time you light a Bun- and H2O—new bonds must be sen burner or a gas stove in your home. When methane formed between the carbon and reacts with oxygen, it is converted to carbon dioxide and the oxygens in carbon dioxide Reaction Progress water. (CO2) and the hydrogens and the oxygen of water (H2O). When CH4(g) + 2 O2(g) a CO2(g) + 2 H2O(g) bonds form, energy is released. Thinking about the magnets again, if the magnets get close enough that they are attracted to each other, they may click together with Overall, we know that this reaction is exothermic, be- an audible snap. The snap is energy being released in the form of sound waves. For cause Bunsen burners are hot! But all chemical reactions atoms, energy in the form of heat is released when a bond forms. The difference between the energy needed to break apart the bonds (pull the mag- nets apart) and the heat released to make new bonds (sound energy released by the magnets attracting each other) is called the enthalpy of reaction or ΔH (delta-H) when the reaction is carried out at constant pressure. When a mole of water molecules form from hydrogen and oxygen, the heat released is 286 kilojoules (kJ). And carbon dioxide, because of its double bonds, releases even more energy. When a mole of CO2 forms from carbon and oxygen, 394 kJ of energy is released. A joule of energy is roughly equivalent to dropping a pear from the height of one meter. Imagine what 394,000 pears dropping from waist height might sound like! When methane combusts, more energy is released when the products form than when the methane (CH4) and oxygen (O2) molecules are broken up. This makes the overall reaction exothermic. In an exothermic reaction, the products at the end of the reaction have lower ener- gy than the reactants. This difference in heat energy between the reactants and the products can then be used to save the harvest. Vintners—wine growers—light container fires to warm grapevines to keep them from freezing during a frost. SHUTTERSTOCK ChemMatters | www.acs.org/chemmatters 7  Bonding and energy Luckily, heat doesn’t have to be applied to the whole orchard all at once. Remember, our goal is to have both ice While bonding can be used to keep things warm, other chemical reactions can be and water present at the same time to keep the tempera- very cool too—literally! A very cool reaction involves mixing solid barium hydroxide octahydrate ture around the fruit at 0 °C (32 °F). So, the farmers only (Ba(OH)2·8H2O) and ammonium thiocyanate (NH4SCN) and then resting it on a wet need to warm their orchards enough to melt some of the piece of filter paper on a piece of wood. The liquid slurry that forms gets so cold the frost, keeping both ice and water present at the same time. water in the filter paper freezes to the wood and the beaker! Brian Rohrig is a chemistry teacher living in Columbus, Ohio. His most recent ChemMatters article, “The Chemistry that Keeps Ba(OH)2·8H2O(s) + 2 NH4SCN(s) Trains Moving,” appeared in the April 2023 issue. a Ba(SCN)2(s) + 10 H2O(l) + 2 NH3(g) REFERENCES When a reaction gets cold to the touch, it means the reactants are taking in more Janovich, A. Cranberries. Washington State Magazine, Winter 2021: energy to break bonds than the products are releasing to form bonds. This means https://magazine.wsu.edu/2021/11/08/cranberries/ [accessed Sept the reaction is endothermic. 2023]. The Endothermic Reaction Between Barium Hydroxide and Ammonium Thiocyanate. Science Skool. https://www.youtube.com/ Endothermic Reaction watch?v=PijjjHjjsck&t=45s [accessed Sept 2023]. Exothermic and Endothermic Reactions. American Chemical Society, Science Lesson Video: https://highschoolenergy.acs.org/how-can- Heat of Reaction (ΔH) energy-change/exothermic-endothermic.html [accessed Sept 2023]. Activation Energy Allain, R. We Need to Talk About the Energy in Chemical Bonds. Wired, (Enthalpy) Dec 7, 2015: https://www.wired.com/2015/12/we-need-to-talk-about- the-energy-in-chemical-bonds/ [accessed Sept 2023]. Products Baird, C. S. When Does the Breaking of Chemical Bonds Release Energy? Science Questions with Surprising Answers, June 27, 2013: https://www.wtamu.edu/~cbaird/sq/2013/06/27/when-does-the- ΔH = (+) breaking-of-chemical-bonds-release-energy/ [accessed Sept 2023]. Reactants SHUTTERSTOCK Reaction Progress types of frost Frost happens when water vapor in the air deposits on a surface. Ice occurs when water liquid freezes. Window Frost Hoar Frost Frost Flowers Rime Frost Window frost forms when a Hoar frost forms when water This rare phenomenon happens Rime frost forms when water pane of glass is exposed to vapor freezes and the grains when water is pushed out of vapor, in the form of fog close SHUTTERSTOCK/KELLEY DONAGHY below-freezing temperatures become long hair-like particles the pores of trees. It is not to the freezing point of water, on the outside and warm air on on a surface. Another example actually a frost, despite the comes into contact with surfac- the inside. The frost forms on of deposition, the water name, because it is formed by es that are below 0 °C (32 °F). the warm side of the glass and particles form from the water freezing water. Wind causes the fingers of rime is an example of deposition— in the air. frost to make long plumes of gas forming a solid without ice crystals. going through the liquid phase. 8 DECEMBER 2023 | ChemMatters