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Chem
Physical chemistry
| Question | Answer |
|---|---|
| Enthalpy | Measure of heat energy in a chemical system |
| Enthalpy changes | From system to surroundings - exothermic From surroundings to system - endothermic |
| Activation energy | Minimum energy required for a reaction to take place |
| Standard conditions | 100 kPa, 298K, 1 moldm^-3 (solutions only) |
| Enthalpy change of formation | Enthalpy change when 1 mole of compound is formed from its elements under standard conditions |
| Enthalpy change of combustion | Enthalpy change when 1 mole of substance reacts completely with oxygen under standard conditions |
| Enthalpy change of neutralisation | Enthalpy change when an acid reacts with a base to form 1 mole of water under standard conditions |
| Calculating enthalpy changes (combustion + reaction) | Combustion Spirit burner weighed before+after water heated, temp recorded before+after Energy change of water, mol of fuel burnt, enthalpy Reaction Temp of solution recorded before+after reaction Energy change of solution, mol of reactant, enthalpy |
| Cooling curves | 1. Temperature of solution recorded every X intervals before and after reactant is added 2. Graph plotted, extrapolated back in time to when the reactant first added to find temp change (accounts for heat loss) |
| Calculating enthalpy changes (neutralisation) | Temp recorded before+after reaction Energy change of solution, mol of acid/base in order to find mol of water, enthalpy |
| Average bond enthalpy | Breaking bonds - endothermic, energy required Making bonds - exothermic, energy released A reaction is exothermic is the energy released when making bonds is greater than energy required when breaking them - vice versa |
| Effect of concentration and pressure on rate of reaction | Concentration: Increases number of particles in the same volume, more frequent collisions, increase rate of reaction Pressure: Same number of particles in a smaller volume, more frequent collisions |
| Catalyst definition | Provide an alternate pathway for a reaction with lower activation energy to increase the rate of reaction - they are not used up but instead regenerated |
| Homogeneous and heterogeneous | Homogeneous - same physical state as reactants Heterogeneous - different physical state as reactants, typically solid catalyst with gas reactants, gas molecules are adsorbed (weakly bonded), reacted with, then desorbed |
| Sustainability and economic importance of catalysts | Lowering activation energy means less temperature and thus less energy required - reduces costs as less electricity/fossil fuel is needed - also means fewer carbon emissions Some catalysts may be toxic and harmful to the environment |
| Boltzmann distribution | The spread of energy in gaseous molecules - typically more molecules have lower energy Temperature: more molecules have higher energy, peak is shorter but moved to the right Catalyst: lower activation energy, Ea moved to the left |
| Dynamic equilibrium | Rate of the forward reaction equals the rate of the reverse reaction The concentration of the reactants and products do not change |
| Effect of pressure, temperature, and concentration on equilibrium | Pressure: Increase in pressure means reaction that produces fewer moles is favoured Temperature: Temperature increase means the endothermic reaction is favoured Concentration: Concentration increase in reactants means the forwards reaction is favoured |
| Effect of catalysts on equilibrium | Unchanged position of equilibrium, rate of both forward and reverse reactions increase equally |
| Investigating change in concentration | 2(CrO4)2- + 2H+ <--> (Cr2O7)2- + H2O Reactants are yellow, products are orange When H+ concentration is increased, solution turns orange When OH- concentration is increased, solution turns yellow |
| Investigating change in temperature | [Co(H2O)6]2+ + 4Cl- <--> (CoCl4)2- + 6H2O Forwards is endothermic, reverse in exothermic Reactants are pink, products are blue When cooled in ice water, solution turns pink When heated in boiling water, solution turns blue |
| Haber process | 200 atm - higher pressure is too expensive and requires lots of energy 400C - while exothermic reaction is favoured, too low temp would cause a very slow rate of reaction Iron catalyst - used to allow for lower temps and reduce energy costs |
| The equilibrium constant Kc | aA + bB <--> cC + dD Kc = ([C]^c * [D]^d) / ([A]^a * [B]^b) [] - concentration of |
| Interpreting Kc | Kc = 1: equilibrium halfway between reactants and products Kc > 1: equilibrium towards the products, towards the right Kc < 1: equilibrium towards the reactants, towards the left |
Created by:
silver54331