1.4 Energetics

• What is enthalpy change (∆H) in a reaction? What are the units?

  The heat energy change measured under constant pressure. The units are kJmol^-1
• How is it show that a standard enthalpy change (∆H) is measured under standard conditions?

  indicated by the symbol: ⦵.
• What is an exothermic reaction and what would the ∆H be?

  An exothermic reaction is one where heat energy is given out from a system to the surroundings; this reaction will feel warm.

  The ∆H is negative.
• What is an endothermic reaction and what would the ∆H be?

  An endothermic reaction is one where heat energy is taken into a system from the surroundings; this reaction wil feel cold.

  The ∆H is positive.
• In a reversable reaction if the foward reaction is endothermic what is the reverse reaction?

  The reverse reaction will be exothermic. It will always be the opposite of the foward reaction.
• What are the state symbols under standard conditions for Cl₂, Br₂ and I₂?

  Cl₂ is gaseous, Br₂ is liquid and I₂ is solid.
• What is the name of this compound: HCl in gaseous form

  Hydrogen chloride
• What is the standard enthalpy of formation?

  The enthalpy change when one mole of a substance is formed from its elements under standard conditions.
• What is the ∆H⦵f of an element in its standard state

  0 as there is no energy change.
• Give the standard enthalpy of formation for CO2

  C (s) + O2 (g) --> CO2 (g)
• Give the standard enthalpy of formation for MgO

  Mg (s) + 1/2 O2 (g) --> MgO (s)
• Why is the standard enthalpy of formation of Al (s) = 0?

  Because this is Al's natural state so there is no energy change.
• What is the standard enthalpy of combustion?

  The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions with all reactants and products in their standard states.
• What is the standard enthalpy of combustion for carbon and what else could it be called?

  It is C (s) + O2 (g) --> CO2 (g) and it could be called the standard enthalpy of formation of CO2
• What is the ∆Hr for the reaction to combust 3 moles of propan-1-ol if ∆H⦵c is -2018 kJ mol⁻ ¹?

  ∆Hr = -2018 x 3 = -6054kJmol⁻¹
• What does measuring the enthalpy change by calorimetry involve?

  It involves measuring the heat change of a reaction by transferring the heat into a mass of a substance, usually water or an aqueous solution. This change in temprature is then used in an equation to calculate the enthalpy change of a reaction.
• What equipment is used to determine enthalpy changes during calorimetry when the reaction occurs in an aqueous solution? [3]

  Styrofoam cup, thermometer and the reactants dissolved in water.
• What equipment is used to determine enthalpy changes during calorimetry when a combustion reaction occurs and the heat energy is transferred into a mass of water? [4]

  Thermometer, calorimeter, water and a spirit burner.
• What's the equasion for the measurement of heat energy change, q? Include units.

  q(J) = m(g) x c(Jg⁻ ¹K⁻ ¹) + ∆T(K)
• In the equasion for heat energy change how do you know what mass to use?

  You always use the mass of the solution thats temprature is being measured.
• What is the question for calculating enthalpy change (∆H)? Include units.

  ∆H = q(kJ) / n
• Why may the results be inaccurate for the temprature change of an aqueous solution when using calorimetry? [1]

  Heat will be lost from the sides and top of the cup.
• When measuring the temprature change of an aqueous solution during calorimetry heat is lost from the top and sides of the cup, how can this error be rectified? [2]

  By plotting a cooling curve we can extend the lines and find a value for the fourth minute where the reactants were added giving us an accurate temperature rise.
• When measuring the temprature change of an aqueous solution during calorimetry a student continuously stired the solution, why?

  to ensure that all reactants reacted.
• What does Hess' law state that the enthalpy change for a chemical reaction is dependant on?

  It's dependant on the route taken. Whichever route taken, indirect or direct, by which the reaction preceeds, the overall enthalpy change for the reaction will be the same.
• Hess' Law can be used to calculate many enthalpy changes which cannot be measured directly. Why may this be the case?

  The reacion doesn't occur under standard conditions, the activation energy is too high or the rate of reaction is too slow.
• What is the equasion you can use to work out the ∆H if you are given enthalpy of combustion data?

  ∆H = Σ[∆Hc(reactants)] - Σ[∆Hc(products)]
• What is the equasion you can use to work out the ∆H if you are given enthalpy of formation data?

  ∆H = Σ[∆H⦵f (products)] - Σ[∆H⦵f (reactants)]
• What is a mean bond enthalpy?

  The average energy across a range of compounds required to separate completely the atoms on one mole of covalent bonds in the gas phase.
• During a chemical reaction the bonds in the reactants are broken. Is this an exothermic or endothermic reaction? Why?

  Endothermic as energy input is required for this to happen.
• What's the equasion for the enthalpy change for a chemical reaction where covalent bonds are broken and formed.

  ∆Hr = sum of bonds broken - sum of bonds formed
• Why is it important to remember that bond enthalpies are mean values? Use methane and propane as an example.

  The exact strength of a bond depends on its enviroment. Methane and propane both contain C-H bonds but the strength of these bonds will be different between the molecules.
• What are the two ways of calculating ∆H values?

  • using Hess's Law with ΔHc⦵ and ΔHf⦵

  • using mean bond enthalpies
• Which of the two ways of calculatig ∆H values is more accurate? Why?

  Mean bond enthalpies is as
• An experiment is using a styofoam cup, a thermometer and has dissolved the reactants in water. What is the aim of this experiment?

  To determine the enthalpy changes during calorimetry when the reaction occurs in an aqueous solution.
• An experiment is using a thermometer, calorimeter, water and a spirit burner. What is the aim of this experiment?

  To determine the enthalpy changes during calorimetry when a combustion reaction occurs and the heat energy is transferred into a mass of water.