Unit 1: Section 4- Energetics + RP2 Flashcards
What is the definition of enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What are the units for enthalpy change?
KJ/mol-1
What does ΔH^∅ mean?
The substances were in their standard states and the measurement was made under standard conditions.
What are standard conditions of temperature and pressure?
298K
100KPa
Is ΔH for an exothermic reaction positive or negative?
negative
Is ΔH for an endothermic reaction positive or negative?
positive
What is the definition of bond enthalpy?
The energy needed to break a bond.
Why is using Hess’s law more exact for calculating bond enthalpies than using the enthalpy change equation?
Because when using this equation, you are using average values from the mean bond enthalpies, so they are less accurate.
What are the conditions needed for standard enthalpy of formation?
- 1 mole of compound formed
- formed from ‘its elements’
- in standard conditions
What are the conditions needed for standard enthalpy of combustion?
- 1 mole of substance
- complete combustion
- in standard conditions (even tho not accurate as burning is not at room temp)
What is the enthalpy change equation?
q=mcΔT
q- enthalpy change in joules
m- mass of substance being heated in grams
c- specific heat capacity
T- change in temp in K
How do you convert from °C to K?
+273
Which way do the arrows point in a Hess’s cycle for enthalpy of formation?
away from the elements, pointing up
Which way do the arrows point in a Hess’s cycle for enthalpy of combustion?
towards the products of combustion, pointing down
What does Hess’s Law state?
Hess’s Law states that overall enthalpy change for a reaction is independent of the route it takes.
Give an example of a exothermic reaction
combustion of fuels
oxidation of carbohydrates e.g. glucose in respiration
Give an example of an endothermic reaction
thermal decomposition of calcium carbonate
What is the enthalpy change of formation of an element?
0
Which is more exothermic- complete or incomplete combustion?
complete combustion
How can we know if enthalpy change is measured at standard conditions?
the symbol is used
RP2- What is Hess’s law?
the enthalpy change for a chemical reaction is always the same, regardless of the route from reactants to products
RP2- How can you reduce the uncertainty in the mass measurement?
Use a balance with a greater resolution
Use a larger mass
RP2- How do you calculate percentage uncertainty?
100x absolute uncertainty/calculated value
RP2- How can you calculate enthalpy change of reaction experimentally?
q=mcΔT
q- enthalpy change in joules
m- mass of substance being heated in grams
c- specific heat capacity (4.18)
T- change in temp in K
divide this number (q) by the number of moles of the reactant used
make sure to add a sign to show whether enthalpy change is exo or endo thermic
RP2- How do you convert from degrees Celsius to Kelvin?
add 273
RP2- Why may an experimental value for enthalpy change be different to the theoretical value?
- heat loss to apparatus/surrounding
- incomplete combustion
- non-standard conditions
- evaporation of alcohol/ water
RP2- How do you prevent heat loss to surroundings/apparatus?
- insulate the beaker by placing it in a polystyrene cup with a lid
- avoid large temperature differences between surroundings and calorimeter
RP2- Other than preventing heat loss, how can the accuracy of this experiment be improved?
- read the thermometer at eye level to avoid parallax errors
- stir the solution so the temperature is evenly distributed
- use a digital thermometer for more accurate and faster readings
- use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty
RP2- How can you calculate the number of moles of something?
conc x volume
How do you calculate enthalpy change for a reaction?
ΔH reaction = Σ ΔH products - Σ ΔH reactants
Define mean bond energies
the mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
this only applies when the substances start AND end in gaseous state
Why do we use mean values of bond energies?
because every single bond in a compound has a slightly different bond energy
How do you calculate ΔH if all the substances are gases?
ΔH = Σ bond energies broken - Σ bond energies made
What happens to the enthalpies of combustion for successive members of a homologous series?
there is a constant rise in the size of the enthalpies of combustion as the number of
carbon atoms increases.
