Wednesday, March 6, 2019
Ib Chemistry – Energetics (Hl)
6. 1. 1 If the chemical reaction produces conflagrate ( enlarges the temporaryerature of the surroundings) thus its exothermic. If it decreases the temp (i. e. absorbs foment) then its endothermic. Also, the yield of an offset reaction which is exothermic leave behind be increased if it occurs at humble temps, and so for endothermic reactions at high temperatures. 6. 1. 2 Exothermic A reaction which produces oestrus. Endothermic A reaction which absorbs incite. heat content of reaction The modify in internal slide fastener (H) through a reaction is ? H. 6. 1. 3 H will be prohibit for exothermic reactions (because internal heat is universe lost) and substantiating for endothermic reactions (because internal energy is being gained). 6. 1. 4 The most shelter state is where every(prenominal) energy has been released. in that respectfore when deviation to a more stable state, energy will be released, and when departure to a less stable state, energy will be gained. On an hydrogen level diagram, higher positions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, ? H is banish) but if it is higher, heat is gained (less stable, ?H is corroborative). 6. 1. 5 Formation of bonds Release of energy. Breaking of bonds Gain / tightness of energy. 6. 2 Calculation of hydrogen motleys 6. 2. 1 Change in energy = sess x specific heat capacity x win over in temperature ? (E = m x C x ? T) 6. 2. 2 Enthalpy transports (? H) are related to the look of mols in the reaction. If all the coefficients are doubled, then the time mensurate of ? H will be doubled. Attention must be paid to limiting reagents though, because enthalpy changes depend on the amount of reactants reacted (extensive belongings 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 ene rgy, and so the ? H appraise, give notice be calculated with E = m x c x ? T where E is get even to ? H, m is the mass of water present, and c = 4. 18 kJ Kg-1 K-1. This ? H value sens then be calculated back to find the enthalpy change for each mol of reactants. 6. 2. 4 The solution should be dictated in a container as insulated as possible, to keep as a good deal heat as possible from escaping.The temperature should be measured continuously , and the value utilise in the equation 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 using 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 given number of mols. 6. 3 Hess Law 6. 3. 1 Hess Law states that the total enthalpy change between given reactants and products is the same regardless of any intermed iate go (or the reaction pathway).To calculate ?Reverse any reactions which are going the awry(p) way and invert the sign of their ? H values. ?Divide or regurgitate the reactions until the intermediate products will cancel out when the reactions are vertically kick ined (always breed/divide the ? H value by the same number). ?Vertically add them. ?Divide or multiply the resulting reaction to the correct coefficients. 6. 4 vex enthalpies 6. 4. 1 Bond enthalpy (aka dissociation enthalpy) The enthalpy change when unitary 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 multiple C-H bonds to be broken, and so the bond enthalpy for C-H is in reality an average value. These values can be used to calculate enigmatical 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 breaking and formation of bonds in a gaseous stat e, 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 made from its elements in their standard 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 radiator grille enthalpy 16. 2. 1 wicket door enthalpy The enthalpy change when 1 mol of crystals (i. e. an garret lattice) is formed from its component particles at an infinite outdistance apart. M+(g) + X-(g) - MX(s) The value of lattice e nthalpy is assumed to be positive for the separation of the lattice, and negative for the formation of the lattice. 16. 2. 2 As above, lattice enthalpies just add another(prenominal) 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 spoke (due to increased attraction). 6. 3 Entropy 16. 3. 1 Factors which increase disorder in a system ?Mixing of particles. ?Change of state to greater distance between particles (solid - liquid or liquid - gas). ?Increased particle vogue (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 spontaneousness 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 support values into the equation above. Hopefully thats not too tricky. 16. 4. 5 There are four 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 larger than T x ? S). ?G is never negative if ? H is positive and ? S negative.