Drivers Of Reactions
Energy Changes in Chemical Reactions
What energy Changes Occur in chemical reactions
1 conduct practical investigations to measure temperature changes in examples of endothermic and
exothermic reactions, including:
- combustion
- dissociation of ionic substances in aqueous solution
Standard Enthalpy Change of Reactions:
- The enthalpy change of any reaction- in the equation quantities of moles
⊖
- ∆𝐻 𝑟
- E.g. 2𝐻2(𝑔) + 𝑂2(𝑔)→2𝐻2𝑂(𝑙)
- This enthalpy change can be positive or negative
- Physical states that the molecules exist in under standard conditions
Standard Enthalpy Change of Combustion:
- Occurs when one mole of a substance is completely burned in oxygen
⊖
- ∆𝐻 𝑐
-
- Important to identify fuel- one mol
Dissociation Of Ions:
- When an ionic substance dissolves in water the strong electrostatic attractions
within the ionic lattice and the hydrogen bonds between water molecules are both
broken
- They are overcome by ion dipole attractions between the individual ions and water
molecules (cation attracted to oxygen in water and anion attracted to hydrogen)
- Thus the ionic lattice is pulled apart and hydrated ions are formed
2 investigate enthalpy changes in reactions using calorimetry and 𝑞 = 𝑚𝑐∆𝑇 (heat capacity formula)
to calculate, analyse and compare experimental results with reliable secondary-sourced data, and
to explain any differences
Enthalpy: heat content of a system
- Not able to measure the total heat energy of the system
- ∆𝐻 = 𝐻(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − 𝐻(𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)
- The gain or loss of heat is impacted by the amount of substance, the temperature
change of a system
- Specific heat capacity of the substance
Standard Heat Capacity:
Definition: Amount of heat required to raise the temperature of 1 gram of a substance by
1 Kelvin
- Unit for heat capacity is J/g/K
- Dependent on the substance and mass
➔ Mass: larger mass can absorb heat without getting hot
➔ Substance: water can absorb more heat than metal
Enthalpy Change Equation:
, Whatever happens to the system, the equal and opposite happens to the suroundings
Calorimetry experiments:
Purpose: Measuring the heat changes using a thermometer
1. Reactants are solids, liquids or solutions
2. At least one reactant is a gas
- Enthalpy change of the combustion of a gas
- Using a calorimeter
- Let flame heat water then measure the heat change of the water- insert into
equation
Limitations:
- Heat absorbed and released will be lost or gained- use a heat shield or flask
- Incomplete combustion can produce soot instead of CO2 not releasing as much
energy
3 construct energy profile diagrams to represent and analyse the enthalpy changes and activation
energy associated with a chemical reaction
- Lower enthalpy chemicals are more stable- chemicals like to be at lower energy
state
- Higher enthalpy means easier to react
Endothermic: reaction absorbing heat
Enthalpy Level Diagram
Reaction Profiles:
Energy Changes in Chemical Reactions
What energy Changes Occur in chemical reactions
1 conduct practical investigations to measure temperature changes in examples of endothermic and
exothermic reactions, including:
- combustion
- dissociation of ionic substances in aqueous solution
Standard Enthalpy Change of Reactions:
- The enthalpy change of any reaction- in the equation quantities of moles
⊖
- ∆𝐻 𝑟
- E.g. 2𝐻2(𝑔) + 𝑂2(𝑔)→2𝐻2𝑂(𝑙)
- This enthalpy change can be positive or negative
- Physical states that the molecules exist in under standard conditions
Standard Enthalpy Change of Combustion:
- Occurs when one mole of a substance is completely burned in oxygen
⊖
- ∆𝐻 𝑐
-
- Important to identify fuel- one mol
Dissociation Of Ions:
- When an ionic substance dissolves in water the strong electrostatic attractions
within the ionic lattice and the hydrogen bonds between water molecules are both
broken
- They are overcome by ion dipole attractions between the individual ions and water
molecules (cation attracted to oxygen in water and anion attracted to hydrogen)
- Thus the ionic lattice is pulled apart and hydrated ions are formed
2 investigate enthalpy changes in reactions using calorimetry and 𝑞 = 𝑚𝑐∆𝑇 (heat capacity formula)
to calculate, analyse and compare experimental results with reliable secondary-sourced data, and
to explain any differences
Enthalpy: heat content of a system
- Not able to measure the total heat energy of the system
- ∆𝐻 = 𝐻(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − 𝐻(𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)
- The gain or loss of heat is impacted by the amount of substance, the temperature
change of a system
- Specific heat capacity of the substance
Standard Heat Capacity:
Definition: Amount of heat required to raise the temperature of 1 gram of a substance by
1 Kelvin
- Unit for heat capacity is J/g/K
- Dependent on the substance and mass
➔ Mass: larger mass can absorb heat without getting hot
➔ Substance: water can absorb more heat than metal
Enthalpy Change Equation:
, Whatever happens to the system, the equal and opposite happens to the suroundings
Calorimetry experiments:
Purpose: Measuring the heat changes using a thermometer
1. Reactants are solids, liquids or solutions
2. At least one reactant is a gas
- Enthalpy change of the combustion of a gas
- Using a calorimeter
- Let flame heat water then measure the heat change of the water- insert into
equation
Limitations:
- Heat absorbed and released will be lost or gained- use a heat shield or flask
- Incomplete combustion can produce soot instead of CO2 not releasing as much
energy
3 construct energy profile diagrams to represent and analyse the enthalpy changes and activation
energy associated with a chemical reaction
- Lower enthalpy chemicals are more stable- chemicals like to be at lower energy
state
- Higher enthalpy means easier to react
Endothermic: reaction absorbing heat
Enthalpy Level Diagram
Reaction Profiles:
