Energetics

• enthalpy change


  - the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.
• what is the standard conditions for enthalpy changes?

  - 100 KPa pressure

  -298 K (25 degrees celsius) temperature
• standard enthalpy of combustion ∆H°c


  - when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants being in their standard states
• how do you calculate enthalpy change of reaction using enthalpies of combustion?

  (sum of enthalpies of combustion of reactants) - (sum of enthalpies of combustion of products)
• standard enthalpy of formation ∆H°f?

  -when one mole of a compound is formed from its constituent elements in their standard states and under standard conditions
• standard enthalpy change of a reaction

  -the enthalpy change when the number of moles of a reaction react with one another under standard conditions
• what is the equation to calculate the enthalpy change of a reaction (Delta H)?


  enthalpy change per mole (Delta H) =

  -q/number of moles reacting

  
  

  Delta H= J mol^-1
• energy required per mole

  q = m c ∆T

  where:

  q= heat loss/gained

  in J

  m = mass of water/ other solution in g

  c = specific heat capacity= 4.18j g-1k-1

  ∆T = change in temperature of water or solution = k or c
• how do you calculate out ΔH from heat energy change, q?

  divide q by the number of moles (and 1000) of reactants (or products?) to find KJmol⁻¹ for the reaction.
• what does hess's law state?

  -the total enthalpy change for a reaction is independent of the route by which the chemical change takes place
• using Hess's Law to find enthalpy of combustion

  1) write out the products of complete combustion

  
  

  2) add in arrows pointing downwards

  
  

  3) reverse the arrows necessary to make a circuit and change the corresponding enthalpy signs

  
  

  4) substitute the values from the question

  
• using hess' law to find enthalpy change of formation

  1) write out reactants and products

  
  

  2) add arrows pointing upwards from from elements

  
• what is an exothermic reaction?

  -heat energy is released into surroundings

  -enthalpy change (delta H) is negative

  -temperature of substance goes up
• What happens in an exothermic enthalpy change?

  overall, energy is transferred from the reaction mixture to the surroundings
• In an exothermic reaction, do the products have less or more energy than the reactants?

  lessSee an expert-written answer!We have an expert-written solution to this problem!
• what is an endothermic reaction?

  -heat energy is absorbed from surroundings

  -enthalpy change (delta H) is positive

  -temperature of substance goes down
• what is the specific heat capacity of a substance?

  heat energy (J) required to raise the temperature of 1g of a substance by 1°C.
• what is the mean bond energy?

  -the enthalpy needed to break one mole of a covalent bond in a gaseous molecule into gaseous atoms. If bond occurs in different molecules then values are averaged to give a mean.
• what is bond breaking?

  -endothermic process -energy is supplied to break existing bonds

  -delta H is positive
• what is bond making?

  -exothermic process

  -energy is released to make new bonds

  -delta H is negative
• reaction profile for an exothermic reaction

  
• reaction profile for an endothermic reaction

  
• what is the enthalpy change of formation equation for NH3(g) including all state symbols?

  1/2 N2 (g) + 3/2 H2 (g) > NH3 (g)
• what is the enthalpy change of formation equation for C2H5OH(l) including all state symbols?

  2 C (s) + 3 H2 (g) + 1/2 O2 (g) > C2H5OH (l)
• what is the enthalpy change of formation equation for CH3Br(l) including all state symbols?

  C (s) + 3/2 H2 (g) + 1/2 Br2 (l) > CH3Br (l)
• what is the enthalpy change of combustion equation for C2H6 (g) including all state symbols?

  C2H6 (g) + 7/2 O2 (g) > 2 CO2 (g) + 3 H2O (l)
• what is the enthalpy change of combustion equation for C2H5OH (l) including all state symbols?

  C2H5OH (l) + 3 O2 (g) > 2 CO2 (g) + 3 H2O (l)
• what is the enthalpy change of combustion equation for C6H14 (l) including all state symbols?

  C6H14 (l) + 19/2 O2 (g) > 6 CO2 (g) + 7 H2O (l)
• what is the enthalpy change of neutralisation equation between HCl (aq) and NaOH (aq) including all state symbols?

  HCl (aq) + NaOH (aq) > NaCl (s) + H2O (l)
• what is the enthalpy change of neutralisation equation between H2SO4 (aq) and Ba(OH)2 (aq) including all state symbols?

  1/2 H2SO4 (aq) + 1/2 Ba(OH)2 (aq) > 1/2 BaSO4 (s) + H2O (l)
• what is calorimetry?

  -how much heat is given out by a reaction via measuring a temperature change
• what is an experiment used to find the standard enthalpy of a combustion reaction?

  -weigh fuel before + after combustion-place water inside a calorimeter

  
  

  -water heats as fuel burns-find amount of heat energy absorbed by water using mass of water, temperature change and specific heat capacity

  
  

  -all heat given out by fuel is absorbed by water-find enthalpy change of combustion of liquid using heat energy absorbed and moles of fuel
• Why may it be hard to get an accurate result in an experiment to find the standard enthalpy change of combustion?

  -heat always lost to surroundings-some combustion may be incomplete (less energy given out)

  
  

  -liquid may be volatile so could lose some fuel to evaporation
• What is an experiment used to find the standard enthalpy change of a neutralisation reaction?

  add known volume of acid to insulated container (polystyrene cup)

  
  

  -clamp thermometer to clamp stand loosely

  
  

  -measure the temperature every minute for 3 minutes

  
  

  -add known volume of alkali at 4 minutes

  
  

  -don't record temperature change

  
  

  -record temperature change of mixture every minute for 10 minutes

  
  

  -stir solution using stirring rod simultaneously gently

  
  

  -find temperature change of solution, mass of solutions assuming solutions have same density as water (1 g dm^-3) including both masses of solutions and specific heat capacity of water -find enthalpy change