Ib Chemistry – Energetics (Hl)

6. 1. 1 If the chemical chemical response produces heat (increases the temperature of the surroundings) consequently its exoergic. If it decreases the temp (i. e. absorbs heat) then its endothermic. Also, the yield of an equilibrium reaction which is exothermic lead be increased if it get alongs at low temps, and so for endothermic reactions at high temperatures. 6. 1. 2 Exothermic A reaction which produces heat. endothermic A reaction which absorbs heat. Enthalpy of reaction The change in home(a) energy (H) through a reaction is ? H. 6. 1. 3 H exit be ostracize for exothermic reactions (because inborn heat is being lost) and dictatorial for endothermic reactions (because internal energy is being gained). 6. 1. 4 The most motion slight country is where all energy has been released. Therefore when going to a to a great extent stable state, energy will be released, and when going to a less stable state, energy will be gained. On an heat content aim diagram, high pos itions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, ? H is negative) but if it is higher, heat is gained (less stable, ?H is official). 6. 1. 5 Formation of confederations Release of energy. Breaking of bonds Gain / absorption of energy. 6. 2 tally of enthalpy changes 6. 2. 1 Change in energy = intensity x specific heat capacity x change in temperature ? (E = m x C x ? T) 6. 2. 2 Enthalpy changes (? H) argon link to the human body of mols in the reaction. If all the coefficients argon doubled, then the value of ? H will be doubled. Attention must be paid to hold reagents though, because enthalpy changes depend on the amount of reactants reacted (extensive property of enthalpy). . 2. 3 When a reaction is carried out in water, the water will gain or lose heat from (or to) the reaction, usually with little escaping the water. Therefore, the change in energy, and so the ? H value, atomic number 50 be countd with E = m x c x ? T where E is equal to ? H, m is the mass of water present, and c = 4. 18 kJ Kg-1 K-1. This ? H value can then be careful back to find the enthalpy change for for each one mol of reactants. 6. 2. 4 The solution should be perplexd in a container as insulated as possible, to keep as much heat as possible from escaping.The temperature should be measured continuously , and the value used in the comparability is the maximum change in temp from the initial position. 6. 2. 5 The results will be a change in temperature. This can be converted into a change in heat (or energy) by utilize the above equation and a known mass of water. This can be used to calculate the ? H for the amount of reactants present, which can then be used to calculate for a effrontery number of mols. 6. 3 Hess equity 6. 3. 1 Hess Law states that the total enthalpy change between given reactants and products is the same regardless of any intermediate steps (or the reaction pathway).To calculate ?Reverse any reactions which are going the wrong way and countermand the sign of their ? H determine. ?Divide or multiply the reactions until the intermediate products will cancel out when the reactions are vertically added (always multiply/divide the ? H value by the same number). ?Vertically add them. ?Divide or multiply the resulting reaction to the correct coefficients. 6. 4 shackle enthalpies 6. 4. 1 Bond enthalpy (aka dissociation enthalpy) The enthalpy change when one mol of bonds are broken homolitically in the gas phase. i. e. X-Y(g) -> X(g) + Y(g) ? H(dissociation).Molecules such as CH4 have multiplex C-H bonds to be broken, and so the bond enthalpy for C-H is actually an average value. These values can be used to calculate unknown enthalpy changes in reactions where only a few bonds are being formed/broken. 6. 4. 2 If the reaction can be expressed in terms of the interruption and ecesis of bonds in a gaseous state, then by adding (or subtracting when bonds are formed) the ? H values the total enthalpy of reaction can be found. 16. 1 Standard enthalpy changes of reaction 16. 1. 1 Standard state 101 kPa, 298 K (or 1 atm, 25 degrees celcuis).Standard enthalpy change of formation The enthalpy change when 1 mol of a substance is make from its elements in their streamer states. For example C(graphite) + 2H2(g) -> CH4(g). Molecules, like H2 are considered to be standard state. Fractions of mols (i. e. fractions in coefficients), may also be used if necessary as 1 mol must be produced). 16. 1. 2 If a reaction can be expressed in terms of changes of formation (and bond enthalpies as in SL) then add up all the ? H values to get the ? H for the reaction. 16. 2 Lattice enthalpy 16. 2. 1Lattice enthalpy The enthalpy change when 1 mol of crystals (i. e. an ionic lattice) is formed from its fate particles at an infinite distance apart. M+(g) + X-(g) -> MX(s) The value of lattice enthalpy is assumed to be positive for the separation of the latti ce, and negative for the formation of the lattice. 16. 2. 2 As above, lattice enthalpies just add another type of reaction to those which can be shown on the Born-Haber cycle. 16. 2. 3 Lattice enthalpy increases with higher ionic charge and with smaller ionic radius (due to increased attraction). 6. 3 Entropy 16. 3. 1 Factors which increase disorder in a system of rules ?Mixing of particles. ?Change of state to greater distance between particles (solid -> liquifiable or liquid -> gas). ?Increased particle movement (temperature). ?Increased number of particles (when more gas particles are produced, this generally outweighs all other factors). 16. 3. 2 Predict the sign of ? S (the change in entropy) for a reaction based on the above factors. ?S is positive when entropy increases (more disorder) and negative when entropy decreases (less disorder). 16. 3. 3The standard entropy change can be calculated by subtracting the absolute entropy of the reactants from that of the products. 16. 4 Spontaneity of a reaction 16. 4. 1 Reactions which release heat (and so increase stability) tend to occur as do reactions which increase entropy (? S is positive). Neither of these can be used to accurately predict spontaneity alone however. 16. 4. 2 When ? G is negative, the reaction is spontaneous, when its positive, the reaction is not. 16. 4. 3 ?G = ? H Temperature(in kelvin) x ? S Spontaneity depends on ? H, ? S and the temperature at which the reaction takes place (or doesnt as the case may be). 6. 4. 4 Substitute values into the equation above. Hopefully thats not too tricky. 16. 4. 5 There are quadruple possibilities 1.? G is always negative when ? H is negative and ? S is positive. 2.? G is negative at high temperatures if ? H is positive and ? S is positive (i. e. an endothermic reaction is spontaneous when T x ? S is greater than ? H). 3.? G is negative at lower temperatures if ? H is negative and ? S is negative (exothermic reactions are spontaneous if ? H is big ger than T x ? S). ?G is never negative if ? H is positive and ? S negative.