1.4 Energetics Flashcards
(17 cards)
Define an enthalpy change
Heat energy change at constant pressure
Symbols for endothermic and exothermic reactions
Delta H +ve = endothermic
Detla H -ve = exothermic
Equipment for calorimetry
thermometer
-INSULATED CUP- to reduce heat loss to surroundings
-glass beaker (for stability)
How to find temp at point where a substance was added from a cooling correction curve (exo) and heating correction curve (endo)
- 2 lines of best fit- one constant at initial temp and one at measured temps
-extrapolate backwards and the change in T = T2 - T1
How to calculate and enthalpy change
Q= m X c X delta T (in joules)
mass is of SOLUTION which is absorbing/giving out heat (if there are 2 add them together)
specific heat capacity of water =4.18
Divide by 1000 to convert into kilojoules
Divide by moles of limiting reactant to get enthalpy change (+ FOR ENDO, - FOR EXO)
Define enthalpy of formation
Enthalpy change when 1 mole of a subtance is made
from its elements
in their standard states under standard conditions
Define enthalpy of combustion
Enthalpy change when 1 mole of a substance is completely burnt in excess oxygen
All substances are in their standard states under standard conditions
What is Hess’s Law?
The enthalpy change for a reaction is independent of the route taken.
Why is enthalpy of formation of an element 0?
By definition
Differences in equations for calculating enthalpy changes from formation data and combustion data
Formation: products- reactants
Combustion: reactants- products
Define bond enthalpy (bond dissociation energy)
The energy required to break 1 mole of a covalent bond
into its gaseous atoms
All substances must be gaseous
Calculating an enthalpy change from bond enthalpy data
sum of bonds broken - sum of bonds made
DRAW DISPLAYED FORMULAE TO SEE ALL BONDS
Why are calculated values different from data book values?
Bond energy data is inaccurate because it is average data taken from a variety of compounds
Why is bond enthalpy of O=O not a mean?
Oxygen is the only substance with an O=O bond
Reasons for results being less exothermic than expected in calorimetry
-heat losses to surroundings - not all heat supplied used to heat water, lower delta T measured
-incomplete combustion
-evaporation of alcohol from the wick
How to reduce percentage uncertainty in calorimetry:
Increase the concentration of the solutions
A student determines the enthalpy change for the reaction between
calcium carbonate and hydrochloric acid.
CaCO3(s) + 2
HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
- measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid using a measuring
cylinder and pour the acid into a 100 cm3 glass beaker - weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into
the acid - stir the mixture with a thermometer
- record the maximum temperature reached.
The student uses the data to determine a value for the enthalpy change.
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.
Stage 1: Apparatus
-Use a burette/pipette (instead of a measuring
cylinder)
-insulate beaker
- Reweigh the watchglass after adding the solid
Stage 2: Temperature Measurements
-Measure and record the initial temperature of
the solution for a few minutes before addition
- Measure and record the temperature after the
addition each minute until a trend is observed
Stage 3:
- Plot a graph of temperature against time
- Extrapolate to the point of addition
- Determine ΔT at the point of addition
