F331- Developing Fuels Revision

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  • Topic: Oxygen, Carbon, Chemical bond
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Developing Fuels
* All gases take up the same volume under the same conditions. AT room temperature and room pressure this volume is 24 dm3. * Number of moles= volume in dm3 24
* Balanced equations can also be used to work out gas volumes. In the equations: 2Na + 2H2O 2NaOH + H2, where 15g of sodium is reacted, you can work out the volume of gas from working out the number of moles in 15g Na = 0.65 mol. Then you see that 2 mol of Na 1mol of H2, Thus 0.65 mol Na 0.325 mol H2. 0.325 x 24 = 7.8dm3. * Entropy tells you how much disorder there is in a substance. It is a measure of ways the particles can be arranged. Solids hare ordered so have low entropy, liquids have some disorder, so more entropy, and gases are random and so the entropy is high. * Substances like disorder. Particles will naturally move to give a substance the maximum possible entropy. * Increasing the number of particles increases the entropy as they are a greater number of ways in which the particles can be arranged. SA mixture of two different types of particles also has higher entropy than the equal number of one type of particle. * Enthalpy Change (ΔH) is the heat energy transferred in a reaction at constant pressure. The units of ΔH are in kJ mol-1. All chemical reactions have enthalpy changes. * The reactions can be exothermic or endothermic. Exothermic reactions give out energy, and thus the ΔH is negative. Endothermic reactions absorb energy, thus ΔH is positive. Combustion is exothermic and thermal decomposition is endothermic. * Enthalpy changes can be calculated using average bond enthalpies. To break bonds, energy is needed, thus ΔH is positive. To make bonds, energy is released, thus ΔH is negative. * The enthalpy change for a reaction is the overall effect of these two changes. If you need more energy to break bonds then is released when bonds are made, ΔH is positive. If it’s less, ΔH is negative. Enthalpy Change of Reaction=Total Energy absorbed to break bonds-Total Energy released to make bonds * Bond enthalpy is related to the length of a bond. The distance between two nuclei is the distance where the attractive and repulsive forces balance each other, this distance is bond length. The stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length, and vice versa. * A C=C bond has a greater bond enthalpy and is shorter than a C-C bond. Four electrons are shared in C=C and only two in C-C, so the electron density between the two carbon atoms is greater in C=C. * There are different types of ΔH depending on the reactions: Standard enthalpy change of reaction; standard enthalpy change of formation; and standard enthalpy change of combustion. * Standard enthalpy change of reaction (ΔHr) is the enthalpy changes when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions in their standard state. * Standard enthalpy change of formation (ΔHf) is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions. * Standard enthalpy change of combustion (ΔHc) is the enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions. * Hess’s Law states that the total enthalpy change of a reaction is always the same, no matter what route is taken.

* Enthalpy Changes can also be found out by using calorimetry. In calorimetry you find how much heat is given out by a reaction by measuring the temperature and then using the equations: q=mc ΔT where q = heat lost or gained, m = mass of water, c = specific heat capacity (4.18 J g-1 K-1), ΔT = change in temperature.

* Experimental Results always include errors. There are two types of errors: systematic and random. Systematic errors are repeated every time you carry out the, and always affect your result in the same way, they’re due to the experimental set-up or...
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