Enthalpy and Entropy

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poppyekoronka
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OCR AS level Salter's course

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Enthalpy and Entropy
1 An exothermic reaction gives out energy from the system to the surroundings.
1.1 The temp of surroundings increases
1.2 ^H is negative
2 An endothermic reaction takes energy into the system from the surroundings.
2.1 The temp of surroundings decreases
2.2 ^H is positive
3 The standard enthalpy of combustion is H(theta) C, is the enthalpy change when 1 mol of substance burns completely in oxygen under standard conditions
4 The standard enthalpy change of formation ^H(theta)f, is the enthalpy change when 1 mol of a substance is formed from its constituent elements. Both the reactants and the products are in their standard state
5 Any other reaction is can be given the symbol ^h(theta)r, if the are done under standard conditions
6 Measuring enthalpy in the lab
6.1 Measure the temperature rise when a known volume of water is heated by the complete combustion of a measured mass of fuel
6.2 energy transferred= mass of water x heat capacity of water x temp. change
6.2.1 This works out the enthalpy change for 1 mole of fuel used
7 Entropy is a measure of the number of ways in which particles can be arranged- it is a measure of disorder
7.1 Gases> liquids > solids - in order of greatest entropy
7.1.1 Mixtures have a greater entropy than the unmixed constituents
7.1.2 If the number of particles INCREASES during a the course of a reaction then entropy usually increases
8 Energy Density
8.1 energy density = enthalpy change of combustion x amount of fuel
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