1.4 Energetics Flashcards
Define Enthalpy Change
Enthalpy change is the amount of heat energy (change) taken in or given out during any change in a system provided the pressure is constant
What happens in an exothermic reaction?
- In an exothermic change energy is transferred from the system (chemicals) to the surroundings
- The products have less energy than the reactants
- The temperature of the environment increases
In an exothermic reaction the ∆H is negative
Give examples of common oxidation exothermic processes:
- combustion of fuels
- oxidation of carbohydrates such as glucose in respiration
- Neutralisation
What happens in an endothermic reaction?
- In an endothermic change, energy is transferred from the surroundings to the system (chemicals)
- They require an input of heat energy e.g. thermal decomposition of calcium carbonate
- The products have more energy than the reactants
The temperature of the environment decreases
Is bonds breaking endothermic or exothermic?
endothermic
Is bonds breaking endothermic or exothermic?
endothermic
List the standard conditions of enthalpy changes:
- A pressure of 100 kPa
- A temperature of 298 K
- Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)
- Solutions at 1mol dm-3
- all substances should have their normal state at 298K
How do we show that a reaction has been carried out under standard conditions?
- the symbol Ꝋ is used
- ΔHꝊ = the standard enthalpy change
Define the standard enthalpy change of reaction:
- The enthalpy change when the reactants in the stoichiometric equation under standard conditions
- Denoted by the symbol ΔHꝊr
- And includes both exothermic and endothermic reactions
Define the standard enthalpy change of formation:
- The enthalpy change when one mole of a compound is formed from its elements under standard conditions
- all reactants and products being in their standard states
- Denoted by the symbol ΔHꝊf
- And includes both exothermic and endothermic reactions
Define the standard enthalpy change of combustion:
- The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions
- With all substances in their standard states
- Denoted by the symbol ΔHꝊc
- And includes only exothermic reactions
Define the standard enthalpy change of neutralisation:
- The enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions
- Denoted by the symbol ΔHꝊneut
- And includes only exothermic reactions
What is the specific heat capacity of water?
4.18 J g-1 C-1
What is the equation for calculating energy transferred in a calorimeter?
Aqueous solutions of acid, alkalis and salts are assumed to be largely water so you can just use the m and c values of water when calculating the energy transferred.
State the meaning of the term enthalpy change as applied to a chemical reaction (1)
Heat energy change at constant pressure
Define the term enthalpy change (1)
Heat (energy) change at constant pressure
What it meant by enthalpy?
All the heat energy that is stored in a chemical system
Define activation energy
- The minimum amount of energy required for a reaction to take place
- Denoted by the symbol Ea
How do we calculate energy change (q) ?
How do we calculate enthalpy change per mole (ΔH) ?
What is Hess’ Law?
- Enthalpy change
- Independent of route
- ∆H (1) = ∆H (2) + ∆H (3)
Why are Enthalpy cycles important?
- Some reactions do not take place under normal conditions, so we cannot measure the Enthalpy for these reactions
- But we can measure the enthalpy changes of these reactions (different route to the reaction) and then apply Hess law to work out the enthalpy change of that reaction
- Also can be used to calculate enthalpy changes for reactions that you don’t want to do the experiment for
- By using tabulated values of known enthalpy changes
Enthalpies of formation (diagram)
define mean bond enthalpy
the average of the bond dissociation enthalpy for a type of bond, averaged over a range of compounds
define bond dissociation enthalpy
the enthalpy in kj/mol-1 to break a specific bond in the gaseous phase
define standard conditions
- 100kPa
- 298K temperature
- 1mol/dm3
State the meaning of the term standard enthalpy of combustion (2)
- Enthalpy change when one mole of a substance burns completely in oxygen
- With all substances in their standard states (at stated temperature and 100kPa)
The enthalpy of combustion determined experimentally is less exothermic than that calculated using enthalpies of formation.
Give one possible reason for this, other than heat loss (1)
- Incomplete combustion
- Not all substances in their standard states
Explain how the experimental method and use of apparatus can be improved to provide more accurate data.
Describe how this data from the improved method can be used to determine an accurate value for the temperature change (6)
State Hess’s Law (2)
- Enthalpy change
- Independent of route
A data book value for the enthalpy of combustion of cyclohexane is –3920 kJ mol–1. The student concluded that the temperature rise recorded in the experiment was smaller than it should have been.
Suggest a practical reason for this (1)
- thermal energy / heat loss
- incomplete combustion
- not under standard conditions
- evaporation
Write an equation, including state symbols, for the reaction with an enthalpy change equal to the enthalpy of formation for iron(III) oxide (1)
- Has to be one molar
What are the assumptions made when doing simple calorimetry?
- The liquid in the reaction mixture is mostly water
- The liquid containing the fuel has a specific heat capacity of 4.18JK-1g-1
- The reaction mixture has a density of 1gcm-3
- All heat energy is absorbed by the water
Heat energy change equation:
q=mc𝚫t
- q =heat/energy change in J
- m = is the mass of the liquid in the mixture in g
- c = specific heat capacity of water 4.18
- 𝚫t = is the change in temperature in oc/K
How do we calculate heat energy change per mole? (𝚫H)
𝚫H= q/n
𝚫H = is the enthalpy change in kJ mol-1
q = energy change from equation above in kJ
n = number of moles of the reacting substance
RP (2) Measuring Enthalpy change
What are the potential uncertainties?
- Heat loss will be the same uncertainty, this can be reduced will insulation from the polystyrene and lid.
- If incomplete combustion occurs there will be too little energy given out in the reaction.
- If the solution is volatile it may evaporate, this would make the value used for mass too high, increasing the calculated enthalpy change.
- Density and specific heat capacity are assumed to be that of water
- Not completed under standard conditions
