Ib Chemistry – Energetics (Hl)

6. 1. 1 If the reply produces rage ( appends the temperature of the surroundings) then(prenominal)(prenominal) its ex oppositemal. If it decreases the temp (i. e. absorbs warmheartedness) then its endothermal. Also, the yield of an equilibrium reception which is exothermic give be increase if it sinks at low temps, and so for endothermic answers at broad(prenominal) temperatures. 6. 1. 2 Exothermic A response which produces oestrus. Endothermic A reply which absorbs heat. Enthalpy of reaction The tilt in sexual energy (H) through a reaction is ? H. 6. 1. 3 H exit be forbid for exothermic reactions (because internal heat is being lost) and verifying for endothermic reactions (because internal energy is being viewed). 6. 1. 4 The most inactive state is where every(prenominal) energy has been released. at that placefore when going to a more stable state, energy will be released, and when going to a less(prenominal) stable state, energy will be gained. On an atomic number 1 level diagram, high positions 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 demonstrable). 6. 1. 5 Formation of bonds Release of energy. intermission of bonds Gain / absorption of energy. 6. 2 reckoning of hydrogen commutes 6. 2. 1 Change in energy = mass x proper(postnominal) heat capacity x inter re invest in temperature ? (E = m x C x ? T) 6. 2. 2 Enthalpy diversitys (? H) atomic number 18 related to the number of mols in the reaction. If every(prenominal) the coefficients ar doubled, then the value of ? H will be doubled. Attention must be paid to limiting reagents though, because heat content throws depend on the amount of reactants reacted (extensive holding of enthalpy). . 2. 3 When a reaction is carried forth in water, the water will gain or lose heat from (or to) the reaction, normally with little es caping the water. Therefore, the change in energy, and so the ? H value, ordure be work out with E = m x c x ? T where E is follow to ? H, m is the mass of water present, and c = 4. 18 kJ Kg-1 K-1. This ? H value back end then be calculated foul to find the enthalpy change for for each one mol of reactants. 6. 2. 4 The solution should be placed in a container as insulated as possible, to keep as ofttimes heat as possible from escaping.The temperature should be measured continuously , and the value apply 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 fag be converted into a change in heat (or energy) by using the above equation and a known mass of water. This locoweed be employ to calculate the ? H for the amount of reactants present, which can then be used to calculate for a assumption number of mols. 6. 3 Hess Law 6. 3. 1 Hess Law states that the jibe enthalpy change between given reac tants and products is the same regardless of any intermediate steps (or the reaction pathway).To calculate ?Reverse any reactions which argon going the wrong way and lift the sign of their ? H values. ? separate or regurgitate the reactions until the intermediate products will scrub up out when the reactions atomic number 18 vertically enlargeed ( ever so multiply/divide the ? H value by the same number). ?Vertically add them. ?Divide or multiply the resulting reaction to the do coefficients. 6. 4 hamper enthalpies 6. 4. 1 Bond enthalpy (aka dissociation enthalpy) The enthalpy change when one mol of bonds are broken homolitically in the gunman 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 actually an honest 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 exp ressed in terms of the breaking and defining 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 ci kPa, 298 K (or 1 atm, 25 degrees celcuis).Standard enthalpy change of formation The enthalpy change when 1 mol of a nucleus is made from its elements in their measure states. For manakin 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 grillwork enthalpy 16. 2. 1Lattice enthalpy The enthalpy change when 1 mol of crystals (i. e. an loft grill) is formed from its component particles at an unnumberable d istance apart. M+(g) + X-(g) - MX(s) The value of lattice enthalpy 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 type of reaction to those which can be shown on the Born-Haber cycle. 16. 2. 3 Lattice enthalpy increases with higher ionic bear down and with smaller ionic radius (due to increased attraction). 6. 3 Entropy 16. 3. 1 Factors which increase disorder in a scheme ?Mixing of particles. ?Change of state to greater distance between particles (solid - liquid or liquid - gas). ?Increased particle travail (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 sulfur) for a reaction based on the above factors. ?S is positive when noise increases (more disorder) and negative when entropy decreases (less disorder). 16. 3. 3The standard entropy change can be calculated by subtracting the arrogant 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 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 t emperatures if ? H is negative and ? S is negative (exothermic reactions are spontaneous if ? H is bigger than T x ? S). ?G is never negative if ? H is positive and ? S negative.