energetics Flashcards
what is the definition of an enthalpy change
the heat energy it absorbs or releases per mole
it has the symbol ∆h and it always has a sign, a value and units
what does a positive sign mean on an enthalpic reaction
heat energy - absorbed
- endothermic
what does a negative sign mean on an enthalpic reaction
heat energy- released
- exothermic
what does breaking bonds mean
absorbs energy
what does making bonds mean
releasing energy
how can you calculate the enthalpy change for a reaction ?
∆h = bonds broken- bonds made
what is calorimetry
experimental method used to measure enthalpy change
when drawing an energy level diagram for an endothermic reaction what do you see ?
- heat energy is absorbed
- ∆h is positive
- this means the products end up with more energy than the reactants had
when drawing an energy level diagram for an exothermic reaction what do you see ?
- heat energy is released
- ∆h is negative
- this means the products end up with less energy than the reactants had
what do strong bonds require ?
- more energy to break
- they also release more energy when made
what is a reaction profile diagram
- like an energy level diagram
- but it shows the path the reaction actually takes from reactants to products
what is the energy needed to break the bonds called and what is it represented by in a diagram
activation energy and its represented by an arrow from the reactants up to the top of the hump
on a reaction profile diagram, what does a hump going from reactants to the products represent
new bonds being made
what happens with energy in an exothermic reaction
energy absorbed < energy released
what happens with energy in an endothermic reaction
energy absorbed > energy released
describe the 3 steps of an exothermic reaction
1) the reaction releases heat energy
2) that heat energy is absorbed by the nearby water
3) the temperature of the nearby water increases
what are the 2 formulae used to convert your measurements into ∆h
1) Q = mc ∆T
2) ∆h = -Q/moles
what is combustion calorimetry
combusting a fuel to heat up water
what is the method to measure the ∆h for the combustion of the liquid fuel methanol
1) weigh out 50g of water and place in a can above a burner
2) measure the initial mass of the burner and the initial temperature of the water
3) ignite the burner, stir water for 30 seconds, the extinguish the burner
4) measure the final mass of the burner and the final (maximum) temp of the water
how do you calculate your results from a combustion calorimetry reaction?
- first use Q=mc∆T to calculate how much heat energy has been absorbed by the water
-then use ∆H=-Q/moles, for this we need to convert Q into KJ and calculate the moles of methanol using moles=mass/RFM
what are the 2 sources of error in a combustion calorimetry reaction
- heat loss, most of the heat energy dissipates into the air instead of the water
- incomplete combustion, this releases less heat energy than expected
what is solution calorimetry
when the reaction you are measuring dissolves in water
what is the method for solution calorimetry ( measure the ∆h for the displacement reaction between mg and cuso4)
1) pipette 25cm3 of cuso4 into a polystyrene cup, 1cm3 of solution has a mass of 1 g
2) measure the initial temp of the reaction
3) add 1g of mg and stir continuously
4) measure the maximum temp of the solution
how to calculate your results from a solution calorimetry reaction
- first use Q=mc∆T to calculate how much heat energy has been absorbed by the water
- then use ∆h = -Q/moles, for this we need to convert Q into KJ, and calculate the moles of magnesium using moles = mass/RFM
what are the sources of error of a solution calorimetry reaction
- heat loss
- incomplete combustion
- water being heated means its no longer pure as it has things dissolve in it, that means the specific heat capacity changes
can you do calorimetry experiments for endothermic reactions in solution as well ?
yes
the reaction absorbs heat energy from the water, so the water cools down
this results in a negative temp change
what happens to the minus sign as it gets passed through the equation for a calorimetry experiment for an endothermic reaction and why is it a good thing
it comes out the other side as a plus because minus times a minus is a plus
- this is good because reactions that absorb heat energy are endothermic and endothermic reactions are supposed to have positive enthalpy changes
