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Enthalpy Changes

Chemical reactions often have enthalpy changes. When chemical reactions happen some bonds are broken and some bonds are made. This usually causes a change in energy. Enthalpy change (delta H), is the heat energy transferred in a reaction at constant pressure. The units of delta H are kJ per mol. Writing delta H shows that the elements are in their standard states and that the measurements are under standard conditions (298K and 101.3 KPa). Reactions can be either exothermic or endothermic. Exothermic reactions give out energy (delta H is negative). In exothermic reactions the temperature of the surroundings often goes up. Oxidation is exothermic, such as combustion. Endothermic reactions absorb energy (delta H is positive). In these reactions the temperature of the surroundings often falls. Thermal decomposition is endothermic. Enthalpy profile diagrams show energy change in reactions. The activation energy is the minimum amount of energy needed to begin breaking bonds and start a chemical reaction. Delta H arrows point downwards for exothermic changes and upwards for endothermic changes on an enthaply profile diagram. You need to specify the conditions for enthalpy changes. Enthalpy itself can't be measured, but only enthalpy change matters. Enthalpy changes can be found by experiment or in textbooks. Textbook ones are often standard enthalpy changes. There are different types of delta H depending on the reaction. Standard enthaply change of reaction is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions in their standard states. Standard enthaply change of formation 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 is the enthaply change when 1 mole of a substance is completely burned in oxygen under standard conditions. Standard enthaply change of neutralisation is the enthaply change when 1 mole of water is formed from the neutralisation of hyrdogen ions by hydroxide ions under standard conditions. Standard enthalpy change of atomisation is the enthalpy change when 1 mole of an element in its standard state is converted to gaseous atoms.

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Finding Enthalpy Changes

You can find out enthalpy changes using calorimetry. To find the enthalpy of combustion of a flammable liquid, you burn it, using a specific apparatus where the fuel you're burning is below some water and you use a thermometer to measure tempearture change. As the fuel burns, it heats the water. You can work out the heat absorbed by the water if you know the mass of water, the temperature change and the specific heat capacity of water. Some heat will be lost to the surroundings, however. Calorimetry can also be used to work out enthalpy change of neutralisation. Add a known volume of acid to an insulated container and measure the temperature. Then add a known volume of alkali, and record the temperature rise. Calulate enthalpy changes using the equation q=mc delta T. q= heat lost or gained in joules. this is the same as enthalpy change if pressure is constant. m=mass of water in the calorimeter or solution in the polystyrene beaker in grams. This is NOT the mass of fuel. c= specific heat capacity of water (4.18J per gram per Kelvin) delta T=the change in temperature of the water or solution. Experimental results always include errors. Systematic errors are repeated every time you carry out the experiment, and always affect it in the same way (they make you answer bigger of smaller than it should be). They're due to experimental set-up or limitations of the equipment. In calorimetry for example, some heat is absorbed by the container, or lost to the surroundings. With flammable-liqiud calorimetry, the combustion may be incomplete or some liqiud may escape by evaporation. Random errors affect your results randomly, and they always happen. Accuracy and reliability are NOT the same. Accuracy means 'how close to the true value' your results are. Reliability means 'how reproducible' your results are. Repeating the experiment makes your results more reliable, and cancels out the effect of random errors (they cancel each other out.) The more repeats, the more reliable the results, but not more accurate, as repeating doesn't get rid of systematic errors. 

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Using Hess's Law

Hess's Law - the total enthalpy change is independent of the route taken. Hess's Law says that the total enthalpy change of a reaction is always the same, no matter which route is taken. Enthalpy changes can be worked out from enthalpies of formation. Enthalpy changes of formation are useful for calculating enthalpy changes you can't find directly. You need to know delta Hf for all the reactants and products that are compounds. Delta Hf for elements is zero - the elements being formed from the element, so there's no change. Working out delta Hf always works, even for complicated reactions. You can also work out enthalpy changes from enthalpy changes of combustion. You use a similar method to work out enthalpy changes of formation from those of combustion. You use the enthalpy changes (exothermic is negative, endothermic is positive) for both routes, put them equal to each other and work out the missing change. Hess's Law lets you find enthalpy changes indirectly from experiments. You can't find the enthalpy change of the thermal decomposition of calcium carbonate by measuring a temperature change (you need to put energy in). But it can be found in a more indirect way. The aim is to make a Hess's cycle (A Hess's Law triangle diagram thingy.) Start by drawing the top of the triangle, including your reactants and products. Then carry out two neutralisation reactions involving hydrochloric acid and use the results to complete your Hess's cycle. You can find the enthalpy changes of the reactions using calorimetry, called delta H1 and delta H2. Now you can build the other two sides of your Hess's cycle, having reactants at the top and products at the bottom. The enthalpy change you want is just delta H1- delta H2.

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Bond Enthalpy

Reactions are all about breaking and making bonds. When reactions happen, reactant bonds are broken and product bonds are formed. Breaking bonds is endothermic (you put energy in), making bonds is exothermic (energy is released). The enthalpy change for a reaction is the overall effect of these two changes. If you need more energy to break bonds than make them, delta H is positive. if it's less, delta H is negative. You need energy to break the attraction between atoms or ions. In ionic bonding positive and negative ions are attracted to each other. In covalent molecules, the positive nuclei are attracted to the negative charge of the shared electrons. You need energy to break this attraction - stronger bonds need more energy to break. The amount of energy you need per mole is called the bond dissociation enthalpy, or just bond enthalpy. Bond enthalpies involve bond breaking in gaseous compounds. Bond enthalpies influence how quickly a reaction will occur. The smaller the bond enthalpy, the faster the reaction (because less energy has to be taken in to start it.) You can use bond enthalpy values to predict rates of reactions. Average bond enthalpies are not exact. There isn''t just one bond enthalpy value between two particular types of atom. For example it is easier to break the second O-H bond in water than the first because of extra electron repulsion. To work out the mean bond enthalpy you take an average. For water this would be 492+428 divided by 2=+460 kJ per mol. When you look up mean bond enthalpy, what you get is the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds. Bond dissociation enthalpies are always positive. You can use bond enthalpies in Hess's Law cycles, by following the normal calculation procedures covered in Using Hess's Law, but with bond enthalpies.

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