Topic 8- Chemical Energetics Flashcards
Definition for the standard enthalpy change of a reaction (r)
The enthalpy change measured at 100kPa and a stated temperature (usually 298k) when the number of moles of substances in the equation as written react
What does system mean in a chemical reaction?
the atoms and bonds involved in a chemical reaction
Explain the law of conservation
the amount of energy in an isolated system remains the same. energy cannot be destroyed or created, it can only be transferred from one to the another
What energy change is breaking bonds associated with?
endothermic reaction –> energy is taken in to break bonds
What energy change is making bonds associated with?
exothermic reaction –> energy is released to make bonds
What is an endothermic reaction?
a reaction with an overall positive energy change (enthalpy of products is greater than enthalpy of reactants)
What is an exothermic reaction?
a reaction with an overall negative enthalpy change (enthalpy of products is lower than the reactants)
What does activation energy mean?
the minimum energy required for a reaction to take place
Which way does the activation energy arrow point on an enthalpy profile diagram?
always points upwards
What are the standard conditions?
100kpa
298k
What does ‘in standard state’ mean?
the state an element/compound exists in standard conditions
Define enthalpy change of formation (f)
the energy change that takes place when one mole of a compound is formed from its constituent elements in their standard state under standard conditions
Define enthalpy change of combustion (c)
the energy change that takes place when one mole of a substance is completely burned in oxygen
Define enthalpy change of neutralisation
the energy change that takes place when one mole of water is formed from a neutralisation reaction
Give examples of enthalpy change of formation and combustion
formation: H2 (g) + 0.5O2 (g) —> H2O (l)
combustion: C(s) + O2(g) —> CO2 (g)
How can you calculate enthalpy change from experimental data?
Q=mc/\T
m=mass of substance heated (usually water)
c=specific heat capacity of water
/=temperature change
What does a simple calorimeter include?
- thermometer
- beaker with known mass of water
- gauze
- tripod
- spirit burner containing substance to be tested
- heatproof mat
Why might experimental methods for enthalpy determination not be accurate?
- heat lost to surroundings
- not in standard conditions
- reaction not gone to completion
What does average bond enthalpy mean?
mean energy required to break 1 mole of bonds in gaseous molecules
Why will using bond enthalpies not be as accurate as using standard enthalpy of formation/combustion?
bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule- therefore more accurate
How do you calculate enthalpy change using just bond enthalpies?
sum of (bond enthalpies of reactants)- sum of (bond enthalpy of products)
Give examples of exothermic processes
- freezing water
- condensing water vapour
- dissolving sodium hydroxide in water
- dilute HCL + NaOH (aq)
- combustion of petrol
Give examples of endothermic processes
- melting ice
- evaporating water
- dissolving ammonium nitrate in water
- dilute ethanoic acid + solid sodium hydrogencarbonate
- photosynthesis
Give the method for the experimental way to determine the enthalpy change of combustion of a substance
1) a spirit burner containing the liquid under test is weighed
2) a known volume of water is added to a copper can
3) the temperature of the water is measured
4) the burner is lit
5) the mixture is constantly stirred with the thermometer
6) when the temperature of the water has reached approximately 20 degrees above its initial temperature, the flame is extinguished and the burner is immediately reweighed
7) the final temperature is measured
