C HEMISTRY AS R EVISION Chains, Energy and Resources: Module 3

K EY W ORDS Enthalpy; H, is the heat content that is stored in a chemical system Exothermic; refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (ΔH= -ve) Endothermic; refers to a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (ΔH = +ve) Enthalpy Change Diagram; diagram of reaction to compare the enthalpy of the reactants with the enthalpy of the products Activation Energy; the minimum energy required to start a reaction by the breaking of bonds Standard Conditions; pressure 100kPa, 25 C, conc. 1.o mol dm -3 Standard State; the physical state of a substance under the standard conditions Standard Enthalpy Change of Reaction; Δ r H ⊖, Enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equations under standard conditions, all reactants and products being in their standard states. Standard Enthalpy Change of Combustion; ΔH f O, enthalpy change when one mole of substance reacts completely with oxygen under standard conditions. Standard Enthalpy Change of Formation; ΔH c O, enthalpy change when one mole of compound is formed from its constituent elements under standard conditions Specific Heat Capacity; c, energy required to raise the temperature of 1 g of substance by 1C Bond Enthalpy; enthalpy change that takes place when breaking, by homolytic fission, 1 mol of a given bond in the molecules of a gaseous species. Average Bond Enthalpy; the average enthalpy change that takes place when breaking, by homolytic fission, 1 mol of a given type of bond in the molecules of a gaseous compound Hess’ Law; if a reaction can take place by more than one router and the initial and final conditions are the same, the total enthalpy change is the same for each route. Enthalpy Cycle; diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy change using Hess’ law. Heterogeneous Catalysis; catalysis of a reaction in which the catalyse has a different physical state from the reactants Homogenous Catalysis; catalysis of a reaction in which the catalyst and reactants are the same physical state Boltzman Distribution; distribution of energies of molecules at particular temperature Le Chatelier’s Principle; when a system in dynamic equilibrium is subjected to change, the position of the equilibrium will shift to counteract that change

E NTHALPY C HANGES ΔH=H products – H reactants Exothermic, ΔH= -ve Endothermic, ΔH = +ve

E NTHALPY C HANGE D IAGRAMS Exothermic Reactions Endothermic Reactions

A CTIVATION E NERGY Has to be an input of energy to break the first bond in a reaction NEED THE ENTHALPY CHANGE DIAGRAMS FROM P189

C ALCULATING E NTHALPY C HANGES AND Q= MC ΔT M is the mass of the surroundings (Wherever the thermometer is) C is the specific heat capacity of the surroundings ΔT is the change in temperature To calculate Enthalpy Change Find the energy change, using the formula. Find the amount in mol that reacted Scale the quantities to match the molar quantities in the equation Write down the equation with the enthalpy change in kJ mol -1

E NTHALPY C HANGE OF C OMBUSTION Find the heat change during the experiment using Q=mcΔT Inverse that number, to find the heat lost by the chemicals Find the amount of fuel, in mol, that reacted Divide the heat lost by chemicals by the amount of fuel, in mol. Remember that if the reaction is exothermic, the end number must be negative There may be a difference between standard enthalpy change of combustion values in data books and what we obtain experimentally due to: Incomplete combustion Heat lost to surroundings

B OND E NTHALPIES Bonds are when energy is stored Bond enthalpy tells you how much energy is required to break a particular bond Average bond enthalpies are used when some bonds occur in different molecules; the strength of these bonds changes on the molecule. Exothermic reaction; bonds that are formed are stronger than the ones that were broken Endothermic reaction; bonds that are formed are weaker than the ones that were broken ΔH= Σ(Bond enthalpies of bonds broken) – Σ(Bond enthalpies of bonds made) Every species must be in the gaseous state When doing calculating enthalpy change using bond enthalpies, it is best to draw out all of the molecules and count the different types of bonds up.

E NTHALPY C HANGE OF C OMBUSTION E NTHALPY C YCLE Not always possible to measure the enthalpy change of reaction directly High activation energy Slow reaction rate More than one reaction taking place We can use enthalpy cycles to work out enthalpy change

E NTHALPY C HANGE OF F ORMATION E NTHALPY C YCLE Not always possible to measure the enthalpy change of reaction directly High activation energy Slow reaction rate More than one reaction taking place We can use enthalpy cycles to work out enthalpy change

C OLLISION T HEORY Rate of reaction is change in concentration of reactant or product over time Temperature  Particles have more kinetic energy, more collisions, increased chance of effective collisions Pressure (g)  Molecules will be closer together, greater chance of collision, collisions more frequent Concentration  Same number of molecules occupy smaller volume, molecules closer together, greater chance of collision, collision more frequent Surface Area  Present of Catalyst  Lowers the activation energy required by offering an alternative, lower energy, route for the reaction to follow. Catalyst not used up

B OLTZMANN D ISTRIBUTION Shows the distribution of molecular energies in a gas at constant temperature. Area under the curve is equal to the total number of molecules in the sample No molecules in the system with no energy Curve never touches the x axis after leaving (0,0) Only molecules with energy greater than the activation energy are able to react The presence of a catalyst will lower the activation energy

L E C HATELIER ’ S P RINCIPLE When a system is in dynamic equilibrium is subjected to a change, the position of the equilibrium will move to counteract that change

C HEMICAL E QUILIBRIUM A dynamic equilibrium exists when, in a closed system, the rate of the forward reaction is equal to the rate of the reverse reaction And when the concentrations of the reactants and the products remain the same

E QUILIBRIUM AND I NDUSTRY Ammonia from nitrogen and hydrogen; Haber process Sulphur trioxide from sulphur dioxide and oxygen; contact process Compromise between safety, running costs and the conditions favoured by the equilibrium to bring about the best yield.