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Thermochemical equations are just like other balanced equationsexcept they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations: H 2  (g) + ½ O 2  (g) → H 2 O (l); ΔH = -285.8 kJ HgO (s) → Hg (l) + ½ O 2  (g); ΔH = +90.7 kJ When you write thermochemical equations, be sure to keep the following points in mind: Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H 2 O (l) is formed from 1 mol H 2  (g) and ½ mol O 2 . Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous)...

Estimate the  change in enthalpy , ΔH, for the following reaction: H 2  (g) + Cl 2  (g) → 2 HCl (g) Solution To work this problem, think of the reaction in terms of simple steps: Step 1  The reactant molecules, H 2  and Cl 2 , break down into their atoms H 2 (g) → 2 H(g) Cl 2 (g) → 2 Cl(g) Step 2  These atoms combine to form HCl molecules 2 H (g) + 2 Cl (g) → 2 HCl (g) In the first step, the H-H and Cl-Cl bonds are broken. In both cases, one mole of bonds is broken. When we look up the single bond energies for the H-H and Cl-Cl bonds, we find them to be +436 kJ/mol and + 243 kJ/mol, therefore for the first step of the reaction: ΔH1 = +(436 kJ + 243 kJ) = +679 kJ Bond breaking requires energy, so we expect the value for ΔH to be positive for this step.

A molecule is a combination of two or more atoms that are held together by chemical bonds, such as covalent bonds and ionic bonds. A molecule is the smallest unit of a compound that still displays the properties associated with that compound. Molecules may contain two atoms of the same element, such as O 2 and H 2 , or they may consist of two or more different atoms, such as CCl 4  and H 2 O. In the study of chemistry, molecules are usually discussed in terms of their molecular weights and moles. Ionic compounds, such as NaCl and KBr, do not form traditional discrete molecules like those formed by covalent bonds. In their solid state, these substances form a three-dimensional array of charged particles. In such a case, molecular weight has no meaning, so the term  formula weight  is used instead. Molecular Weight and Formula Weight The molecular weight of a molecule is calculated by adding the atomic weights (in ...

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If you ask a random person to tell you what  color the sun is , chances are he will look at you like you're an idiot and tell you the sun is yellow. Would you be surprised to learn the sun is  not  yellow? It's actually white. If you were to view the sun from the International Space Station or the moon, you'd see its true color. Check space photos online. See the true color of the sun? The reason the sun appears yellow during the day from Earth, or  orange to red at sunrise and sunset , is because we view our  favorite star  through the filter of the atmosphere. The True Color of the Sun If you view sunlight through a prism, you can see the entire range ofwavelengths of light. Another example of the visible portion of the solar spectrum is seen in the rainbow. Sunlight isn't a single color of light, but a combination of the emission spectra of all the elements in the star. All of the wavelengths combine to form white light, which is the net col...

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The sky is blue on a sunny day, yet red or orange at  sunrise and sunset . The different colors are caused by scattering of light in the Earth's atmosphere. Here is a  simple experiment  you can do to see how this works: Blue Sky - Red Sunset Materials water milk transparent container with flat parallel sides flashlight A small rectangular aquarium works well for this experiment. Try a 2-1/2-gallon or 5-gallon tank. Any other square or rectangular clear glass or plastic container will work. Conduct the Experiment Fill the container about 3/4 full of water. Turn on the flashlight and hold it flat against the side of the container. You probably won't be able to see the beam of the flashlight, although you may see bright sparkles where the light strikes dust, air bubbles, or other small particles in the water. This is much like how sunlight travels through space. Add about 1/4 cup of milk (for a 2-1/2 gallon container -- increase the a...

Heterogeneous and homogeneous refer to mixtures of materials in chemistry. The difference between  heterogeneous and homogeneous mixtures is the degree at which the materials are mixed together and the uniformity of their composition. A  homogeneous mixture  is a mixture where the components that make up the mixture are uniformly distributed throughout the mixture. The composition of the mixture is the same throughout. There is only one phase of matter  observed in a homogeneous mixture

You know if you put fresh cut flowers in water it  will help  keep them from wilting. If you have a packet of cut flower preservative from a florist or the store, it  will help the flowers to stay fresh much longer. You can make cut flower preservative youself, however. There are several good recipes, made using common household ingredients. Keys to Keeping Cut Flowers Fresh Give them water. Give them food. Protect them from decay or infection. Keep them cool and out of direct sunlight. The  floral preservative  provides flowers with water and food and contains a disinfectant to prevent bacteria from growing. Making sure your vase is clean will also help. Discard any decaying leaves or flowers, because the freshness of flowers is influenced by the gases and bacteria found on wilted or rotting plant material. Also, don't set your flowers near ripe fruit, because the chemicals from the fruit (such as ethylene) will 'ripen' your flowers. If you can,...

Medicinal chemistry  is a stimulating field as it links many scientific disciplines and allows for collaboration with other scientists in researching and developing new drugs. Medicinal chemists  apply their  chemistry training to the process of synthesizing new pharmaceuticals.