Energetics I Flashcards
What is enthalpy, ΔE?
A measure of energy content of the substance
What does lower enthalpy indicate?
Greater energetic stability
What is enthalpy change, ΔE
Change in energy content at constant pressure
Is ΔH positive or negative for exothermic reaction and do products or reactants have higher energy?
- products have a lower energy than reactants
- ΔH = H(products) - H(reactants), ΔH < 0
What happens to the temperature of the surroundings in an exothermic reaction?
- heat is released to the surroundings
- temperature of surroundings rises
Is ΔH positive or negative for endothermic reaction and do products or reactants have higher energy?
- products have higher energy than reactants
- ΔH = H(products) - H(reactants), ΔH > 0
What happens to the temperature of the surroundings in an endothermic reaction?
- heat is absorbed from the surroundings
- temperature of the surrounding drop
What is the system and what is the surroundings for calorimeter?
System: reaction
Surroundings: aqueous medium and calorimeter
What happens in calorimeter in exothermic reaction?
- heat evolved by reaction = heat gained by calorimeter + heat gained by aqueous medium
- thus temperature of reaction mixture increases
What happens in calorimeter in endothermic reaction?
- heat absorbed by the reaction = heat lost by calorimeter + heat loss by aqueous medium
- temperature of reaction mixture decreases
What is the formula for heat evolved/absorbed?
Q = mcΔT
For dilute solutions, what can the density and the specific heat capacity of the solution be assumed as?
- density = density of pure water = 1.00g cm⁻³
- specific heat capacity = 4.18 J g⁻¹ K⁻¹
What are the assumptions made in calorimetry experiments?
- heat capacity of the calorimeter vessel is negligible, thus there is no heat gained/loss to the calorimeter
- no heat exchange with the surroundings
What is specific heat capacity, c?
amount of heat required to raise the temperature of 1g of the substance by 1K
What is heat capacity, C?
heat required to raise the temperature of a given quantity of the substance by 1K
What is standard conditions°?
- temperature: 298K or 25 degree celsius
- 1 bar or 1.00 × 10⁵ Pa
- most stable physical state (s, l or g) at 298K and 1 bar
- 1 mol dm⁻³
What is standard enthalpy change of reaction, ΔHᵣ°?
Standard enthalpy change of reaction is the enthalpy change when molar quantities of reactants, as specified by the balanced chemical equation, react to form products under standard conditions
What does ‘per mole’ in the unit of ΔH° imply?
per mole of equation
What does ΔHᵣ° depend on?
- magnitude of ΔHᵣ is directly proportional to number of reactants consumed (eg. if 2a + 2b → c, ΔHᵣ×2)
- enthalpy change is equal in magnitude but opposite in sign for the reverse reaction
- enthalpy change for reaction depends on physical state of reactants and products
What is a thermochemical equation?
A balanced stoichiometric equation, including state symbols for the reaction with ΔH values
What is standard enthalphy change of atomisation, ΔHₐₜ°?
The standard enthalpy change of atomisation of an element is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
What is the sign of ΔHₐₜ° values?
Always positive because atomisation involves bond breaking which takes in energy
What does greater ΔHₐₜ° indicate?
The higher the ΔHₐₜ° value, the greater the strength of the bond broken
What does ΔHₐₜ include for liquids?
enthalpy change of vapourisation, ΔHᵥₐₚ