Topic 8 - Energetics I Flashcards
What is enthalpy change
the heat energy change in a reaction at constant pressure
What are standard conditions
100 kPa
298 K
What is enthalpy change if a reaction is exothermic
negative
(temp goes up)
What is enthalpy change if a reaction is enfothermic
positive
(temp goes down)
How are energy level diagrams different to reaction profile diagrams
energy level = overall change
reaction profile = how enthalpy changes throughout reaction
What does an enthalpy level diagram look like
one line drawn for reactants
one line drawn for products
vertical arrow in between
What does a reaction profile diagram look like
one line drawn for reactants
one line drawn for products
curved line connects the two
vertical arrow to top of curve = activation energy
vertical arrow between react/prod = energy change
What is standard enthalpy change of reaction
enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions
What is standard enthalpy change of formation
enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is enthalpy change of combustion
enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
What is standard enthalpy change of neutralisation
enthalpy change when an acid and an alkali react together to form 1 mole of water under standard conditions
How can you find enthalpy changes using an experiment
directly measuring temperature change of some reaction mixture
What is the problem with using an experiment to find enthalpy change
some heat lost to surroundings = temp change measured less than actual temp change
How can you solve the problem of energy loss when finding enthalpy change in an experiment
extrapolate the data
What is calorimetry used for
hard to measure temp change of gases = calorimeter to measure temp change of some water
to find enthalpy change of combustion of flammable liquids
How does calirometry work (include the set up)
- spirit burner containing fuel in combustion chamber with airflow
- closed lid, stirring rod and thermometer
- as fuel burns, water heats up
- work out heat absorbed by water if known mass of water, temp change of water and specific heat capacity
What is the reality of using a calorimeter
some heat is always lost (to apparatus and surroundings)
What is the formula to find heat lost or gained
q = m x c x ∆T
q = joules
enthalpy change = kJ (divide by 1000)
Formula to find enthalpy change of combustion from finding heat loss (q)
∆H = q / moles
What is Hess’s Law
totaly enthalpy change of a reaction is always the same , no matter which route is taken
In enthalpy change of formation (using Hess’s cycle) what direction do the arrows point
away from elements to the reactants and products
In enthalpy change of formation (using Hess’s cycle) what is the ‘elements’
what makes up each of the reactants/products
What is the enthalpy change of formation for elements
zero
In enthalpy change of combustion (using Hess’s cycle) what direction do the arrows point
towards combustion products (CO₂ + H₂O) and away from reactants/products
What happens when as bond is broken
energy taken in
endothermic
What happens when a bond is made
energy released
exothermic
What is bond enthalpy
the amount of energy needed to break 1 mole of a type of bond in a molecule in the gas phase
Why are mean bond enthalpies not exact
energy needed to break one mole of bonds in the gas phase, averaged over different compounds
How can enthalpy changes be calculated from mean bond enthalpies
enthalpy change of reaction = (sum of bond enthalpies of reactants) - (sum of bond enthalpies of products)
