Unit 5
eactivity 1.1. Measuring enthalpy changes
R
.1.1 energy in reactions
1
- Chemical reactions involve a transfer of energy between the system and surroundings, while total energy is conserved.
- Heat:a form of energy, the unit used is joules
- Temperature:a measure of the average kinetic energyof the particles in substance. Units for temperature are °C or K
1.1.2 - 1.1.3: exothermic and endothermic reactions
- Reactions are described as endothermic or exothermic depending on the direction of energy transfer between the system and the
surroundings.
- Exothermic: system releases energy to the surroundings. Surroundings feel warm to touch. Temperature increases.
- Endothermic: system absorbs energy from the surroundings. Surroundings feel cool to touch. Temperature decreases.
- The relative stability of reactants and products determines whether reactions are endothermic or exothermic
- Exothermic: reactants are relatively unstable, products are relatively stable
- Endothermic: reactants are relatively stable, products are relatively unstable.
-
- Energy profiles show the potential energy of the reactants and products in a reaction
- Reactants and products are more energetically stable at lower energy
- Exothermic reactions
- Neutralisation
- Combustion
- Endothermic reactions
- Thermal decomposition
1.1.4: enthalpy changes
- The standard enthalpy change for a chemical reaction ΔH refers tothe heat transferredat constant pressure under standard
conditions and states. It can be determined from the change in temperature of apure substance
● Apply the equations q=mcΔT and ΔH= -q/n in the calculation of the enthalpy change of a reaction
- 𝑞 = 𝑚𝑐∆𝑇
- q=
energy transferred (J)
- m=mass of solution (g)
- c=specific heat capacity (Jg-1K-1) → the value c for water is given in the data booklet
- ΔT=change in temperature (oC
/K)
𝑞
- ∆𝐻 = −
𝑛
- ΔH=enthalpy change (kJmol )
-1
- q=
energy transferred (
J)
- n=moles (mol)
- Calorimetry experiments are used to investigate enthalpy changes for combustion reactions experimentally
, - Calorimetry experiments typically measure smaller changes in temperature than is expected from theoretical values as much of the
heat is lost to the surroundings instead of being transferred to the solution
- The main error in an enthalpy change in solution is animpure metal/substance
- NOT a heat lost to surroundings as this is taken into account using the graph method
- There is the assumption that a solution behaves as water in an enthalpy change in a solution experiment.
- Heat capacity = 4.18 Jg-1K-1
- Density 1g=1ml
1.2: Energy cycles in reactions
R
1.2.1: Bond enthalpies
- Bond breakingabsorbsenergy, it is anendothermicreaction
- Bond formingreleasesenergy, it is anexothermicreaction
● Calculate the enthalpy change in a reaction from given average bond enthalpy data
- Bond enthalpy data are average values because there are many different molecules which contain the same individual bond and the
energy needed to break that bond is influenced by the rest of the molecule.
- Bond enthalpy is only covalent bonds
- This data might differ from the values measured experimentally as it might not be the same energy to break the bonds in different
compounds (see above).
- Average bond enthalpy is the enthalpy change when one mole of the gaseous bond is broken, averaged for the same bond in a number of
similar compounds.
1.2.2: Hess’s Law
- Hess’s law states that the enthalpy change for a reaction is independent of the pathway between the initial and final states
● Apply hess’s law to calculate enthalpy changes in multistep reactions
1.2.3: Standard enthalpy of combustion and formation
- Standard enthalpy changes of combustion, ΔHc⦵, and formation, ΔHf⦵ , data are used in thermodynamic calculations.
- The standard enthalpy of formation for a substance is defined as the energy change that occurs whenone mole of a substance is
formed from its constituent elementsin their standardstate.
- The standard enthalpy of combustion for a substance is defined as the enthalpy change whenone mole ofa substance is burned
completely in oxygenunder standard state conditions.
● Deduce equations and solutions to problems involving these terms.
eactivity 1.1. Measuring enthalpy changes
R
.1.1 energy in reactions
1
- Chemical reactions involve a transfer of energy between the system and surroundings, while total energy is conserved.
- Heat:a form of energy, the unit used is joules
- Temperature:a measure of the average kinetic energyof the particles in substance. Units for temperature are °C or K
1.1.2 - 1.1.3: exothermic and endothermic reactions
- Reactions are described as endothermic or exothermic depending on the direction of energy transfer between the system and the
surroundings.
- Exothermic: system releases energy to the surroundings. Surroundings feel warm to touch. Temperature increases.
- Endothermic: system absorbs energy from the surroundings. Surroundings feel cool to touch. Temperature decreases.
- The relative stability of reactants and products determines whether reactions are endothermic or exothermic
- Exothermic: reactants are relatively unstable, products are relatively stable
- Endothermic: reactants are relatively stable, products are relatively unstable.
-
- Energy profiles show the potential energy of the reactants and products in a reaction
- Reactants and products are more energetically stable at lower energy
- Exothermic reactions
- Neutralisation
- Combustion
- Endothermic reactions
- Thermal decomposition
1.1.4: enthalpy changes
- The standard enthalpy change for a chemical reaction ΔH refers tothe heat transferredat constant pressure under standard
conditions and states. It can be determined from the change in temperature of apure substance
● Apply the equations q=mcΔT and ΔH= -q/n in the calculation of the enthalpy change of a reaction
- 𝑞 = 𝑚𝑐∆𝑇
- q=
energy transferred (J)
- m=mass of solution (g)
- c=specific heat capacity (Jg-1K-1) → the value c for water is given in the data booklet
- ΔT=change in temperature (oC
/K)
𝑞
- ∆𝐻 = −
𝑛
- ΔH=enthalpy change (kJmol )
-1
- q=
energy transferred (
J)
- n=moles (mol)
- Calorimetry experiments are used to investigate enthalpy changes for combustion reactions experimentally
, - Calorimetry experiments typically measure smaller changes in temperature than is expected from theoretical values as much of the
heat is lost to the surroundings instead of being transferred to the solution
- The main error in an enthalpy change in solution is animpure metal/substance
- NOT a heat lost to surroundings as this is taken into account using the graph method
- There is the assumption that a solution behaves as water in an enthalpy change in a solution experiment.
- Heat capacity = 4.18 Jg-1K-1
- Density 1g=1ml
1.2: Energy cycles in reactions
R
1.2.1: Bond enthalpies
- Bond breakingabsorbsenergy, it is anendothermicreaction
- Bond formingreleasesenergy, it is anexothermicreaction
● Calculate the enthalpy change in a reaction from given average bond enthalpy data
- Bond enthalpy data are average values because there are many different molecules which contain the same individual bond and the
energy needed to break that bond is influenced by the rest of the molecule.
- Bond enthalpy is only covalent bonds
- This data might differ from the values measured experimentally as it might not be the same energy to break the bonds in different
compounds (see above).
- Average bond enthalpy is the enthalpy change when one mole of the gaseous bond is broken, averaged for the same bond in a number of
similar compounds.
1.2.2: Hess’s Law
- Hess’s law states that the enthalpy change for a reaction is independent of the pathway between the initial and final states
● Apply hess’s law to calculate enthalpy changes in multistep reactions
1.2.3: Standard enthalpy of combustion and formation
- Standard enthalpy changes of combustion, ΔHc⦵, and formation, ΔHf⦵ , data are used in thermodynamic calculations.
- The standard enthalpy of formation for a substance is defined as the energy change that occurs whenone mole of a substance is
formed from its constituent elementsin their standardstate.
- The standard enthalpy of combustion for a substance is defined as the enthalpy change whenone mole ofa substance is burned
completely in oxygenunder standard state conditions.
● Deduce equations and solutions to problems involving these terms.
