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Heat Capacity
2007-07-05 07:59:00
Heat Capa cityWhen a substance is heated, its change in temperature(?T) is proportional to the quantity of heat energy it absorbs(q), thus:q/?T = constantThe value of this constant is different for each substance and is called that substance's 'heat capacity'.Specific Heat CapacityThe specific heat capacity(c) of a substance is the amount of heat energy required to raise 1 gram of that substance by 1 degree kelvin.c = q/(mass * ?T)Molar Heat CapacityThe molar heat capacity(C) of a substance is the amount of heat energy required to raise the temperature of 1 mole of that substance by 1 degree kelvin.C = q/(moles * ?T)UnitsHeat energy is in joules, temperature is in degrees kelvin, and mass is in grams, thus c has units of J/(K*g) and C has units of J/(K*mol).
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Enthalpy
2007-07-02 05:11:00
(See Energy first.)What is enthalpy?Enthalpy(H), or 'heat content,' is a variable that describes the thermodynamic potential(hence 'heat content') of a system.The enthalpy of a system is defined as its internal energy plus the product of the pressure on it(P) and its volume(V).H = E + PVHow is it used?In many reactions the only type of work done is that of a newly formed gas expanding and pushing back the atmosphere. This is called PV work, and is equal to the product of the pressure of the atmosphere and the change in volume of the gas.w = -P?VThe value is negative because the gas(system) is doing work on, and thus losing energy to, the atmosphere(surroundings).In reactions such as these, if the pressure is constant (as is most common) it is often more meaningful and convenient to describe the change in enthalpy of a system rather than the change in its internal energy. This is because by using enthalpy we can avoid having to consider PV (and thus all) work done by the system...
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Energy
2007-06-30 15:36:00
When we wish to observe a change in something we can describe that thing as a 'system'. Everything else relevant to our observations of that system is it's 'surroundings'.-The internal energy(E) of a system is the sum of the potential and kinetic energies of all its particles. When energy is transferred between a system and its surroundings it is always transferred in the form of work and/or heat.-Heat(q) is the energy transferred between a system and its surroundings due to a difference in their temperatures.-Work(w) is the energy transferred when a force moves an object.A change in internal energy of a system will be equal to the energy transferred to/from its surroundings in the form of heat and/or work. This statement is described in the equation:?E = q+wwhere ? = 'change in'.q, w, and ?E can have positive or negative values depending on whether the system is gaining or losing energy. (loss is -, gain is +)The 'first law of thermodynamics' or the 'law of conservation o...
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Nuclear Charge
2007-06-25 03:31:00
The nuclear charge, Z, of an atoms nucleus is the total charge of all the protons it contains (same as atomic number.) Lower orbital electrons can in effect "shield" higher orbital electrons from some of the positive charge of the nucleus, changing the Z that higher orbital electrons experience. The effective nuclear charge, Zeff, is the net positive charge of the nucleus as perceived by an electron.
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Periodic Table
2007-06-25 03:11:00
Click image for full size version.See also:Peri odic Table Trends
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The True Food Chain
2007-06-25 03:01:00
From the Simpsons... It's vaguely related to science!(I'm just testing image posting.)
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Periodic Table Trends
2006-09-18 05:37:00
GeneralAtomic SizeAtomic size increases down the groups as more protons are added to the nucleus and higher electron energy levels are filled.It decreases from left to right across the periods as the same electron energy level is filled and the larger nucleus pulls the electrons closer. (Electron shielding lowers Zeff less than increasing numbers of protons in the nucleus raises it)Ionization EnergyIonization Energy (IE) is the energy needed to remove an electron from an atom. It decreases with increasing atomic size because the further away from the nucleus the less Zeff.ElectronegativityElectronegativity (EN) is the ability to attract electrons and naturally follows the same pattern as IE, however this pattern is broken with the noble gasses which already have a stable electron configuration.GroupsGroup 1 (Alkali Metals)-They have the lowest molar masses in their period and the largest atomic size; this results in low density.-They only have 1 valence electron which is far from th...
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Bridge Bonds
2006-09-17 16:13:00
A bridge bond is a covalent bond through which 3 atoms share 2 electrons.For example in a B-H-B bridge bond an electron from one of the B atons and one from the H atom would be shared between all 3 atoms. Each of the 3 atoms would consider itself the owner of the 2 electrons in terms of filling its valence shell.
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Colligative Properties of Solutions
2006-09-14 16:24:00
(Coll igative means collective.)Prop erties of a solution are influenced by the number of solute particles present.1. VP (Vapor Pressure) is lowered.2. BP (Boiling Point) is elevated.3. FP (Freezing Point) is lowered.4. OP (Osmotic Pressure) is increased.Nonvolatile Nonelectrolyte Solution s .(Volatility is the tendency of a substance to become a gas.)(Electrolytes are a mixture of ions that when dissolved in solution conduct a current.)1. VP is lowered.The amount of volatile liquid relative to the total surface area of the mixture is smaller, thus the ratio of particles leaving to entering is less than that found in a pure sample of the volatile liquid.The relationship between amount of solute and VP (known as Raoult's law) is:VPsolvent = Xsolvent * VPpure solvent(X= mole fraction)If we want to know the magnitude of the change in VP we can obviously see that:VPpure solvent - VPsolvent = change in VPFurther:Xsolvent + Xsolute = 1rearrangingXsolvent = 1 - XsolutethusVPsolvent = Xsolven...

Intermolecular Forces
2006-09-12 22:58:00
Ion-Dipole Forces The attraction between an ion and a pole of a polar molecule(dipole).BE(Bond Energy) = 40-600 kJ/molDipole-Dipole ForcesThe attraction between oppositely charged poles of polar molecules.BE = 5-25 kJ/molHydrogen BondWhen an H atom is bound to a small and highly electronegative atom with lone pairs (N,O or F) the electrons are drawn away from the H atom making this particular covalent bond very polar. The H atom has a relatively strong + charge and the N, O or F atom has a relatively strong - charge. The + H atom of one of this type of molecule forms very stong bonds with the - N, O or F atom of another of the same type of molecule. The H atom is specifically attracted to the lone pairs of electrons of the N, O and F atoms.BE = 10-40 kJ/molCharge-Induced DipolesLone pair electrons are in constant motion around their atom in what we picture as a cloud of negative charge. A nearby electric field can distort the cloud by pulling towards a positive charge or pushing it...
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Ask me anything!
2006-09-12 22:15:00
If you have any questions about Chemistry just ask!I am happy to help anyone requiring assistance. From primary school to university, no question is too big or too small.Simply send me an email or comment on this post with your question.If you know of anyone who could use some chemistry help, feel free to direct them here.
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Hess's Law
2006-09-12 21:50:00
Hess's law of heat summation states that the enthalpy change of an overall process is the sum of the enthalpy changes of the individual steps of the process.We can select a number of steps with known enthalpy changes that lead us from an initial state to a final state, even if in reality the process from may not progress by the path we have chosen.ExampleIf we know how much energy it takes to heat a can of soup to 100 degrees C and how much is released when it cools by 10 degrees C we can work out how much energy it takes to heat the soup up to 90 degrees C by adding the two together. Even though in reality if we heated the soup to 90 we wouldnt first heat it to 100 then cool it by 10, we can still imagine doing it this way in order to work out how much energy was needed.

Solutions and Solubility
2006-09-12 21:01:00
Like dissolves LikeWhen one substance dissolves in another, the intermolecular bonds in the solute must break, as must those in the solvent. The breaking of these bonds requires energy (endothermic). The seperate solvent and solute molecules then join together and release energy (exothermic).If the strength of the bonds broken is too much greater than of those formed the substance will not dissolve.Examples1.When NaCl dissolves in water the H-bonds between the water molecules break as do the ionic bonds between Na+ and Cl- ions. However the ion-dipole bonds that form between the H2O and the ions are very strong and so release much energy. This release of energy is close enough to the energy needed to break the initial bonds in the substances that the reaction can occur.If NaCl was mixed with oil, the bonds between oil and ions would be ion-induced dipole, far too weak relative to the ionic bonding of NaCl to allow it to dissolve.2.When octanol is mixed with water it does not dissol...
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Conduction and Band Theory
2006-09-12 13:17:00
Band Theo ry Band theory is an extension of MO(molecular orbital) theory.The more atoms that join together in a molecule the more MOs are created.With the number of atoms in a sample of metal (moles * Avogadro's number) a large number of MOs are formed and thus the energies of the MOs are so closely spaced that they form a continuum or band.According to band theory the lower energy MOs are occupied by the valence electrons. These make up the valence band.The empty MOs of higher energy make up the conduction band.ConductionConductorsIn conductors such as metals there is no gap between the valence band and the conduction band. This means that electrons can enter the valence band when they receive even a tiny quantity of energy. Once into an orbital in the conduction band electrons can move through the sample freely.This is why electrons in metals are considered to be completely delocalised.When heated the movement of atoms (heat causing vibration) interferes with the flow of electrons ...
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Gases
2006-09-12 04:52:00
Gas LawsBoyle's Law At a constant temperature and pressure, the volume occupied by a fixed amount of gas is inversely proportional to the applied (external) pressure.V = (1/P) * constantCharles's LawAt a constant pressure the volume occupied by a fixed amount of gas is directly proportional to its absolute temperature.V = constant * TCombined Gas LawCombining the relationships in Charles's and Boyle's gives the combined gas law which is:V = constant * (T/P)The relationship between pressure and temperature for a gas is:P = T * constant(At constant volume, the pressure exerted by a fixed amount of gas is directly proportional to the absolute temperature.)Avogadro's LawAt fixed temperature and pressure, equal volumes of any ideal gas contain equal numbers of particles (or moles).Thus we can see that:At a fixed temperature and pressure, the volume occupied by a gas is directly proportional to the amount (mol) of a gas. SoV = constant * nThe Ideal Gas LawCombining all these relatio...
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