energetics

Question 1

what is the definition of an enthalpy change

Answer 1

the heat energy it absorbs or releases per mole
it has the symbol ∆h and it always has a sign, a value and units

Question 2

what does a positive sign mean on an enthalpic reaction

Answer 2

heat energy - absorbed
- endothermic

Question 3

what does a negative sign mean on an enthalpic reaction

Answer 3

heat energy- released
- exothermic

Question 4

what does breaking bonds mean

Answer 4

absorbs energy

Question 5

what does making bonds mean

Answer 5

releasing energy

Question 6

how can you calculate the enthalpy change for a reaction ?

Answer 6

∆h = bonds broken- bonds made

Question 7

what is calorimetry

Answer 7

experimental method used to measure enthalpy change

Question 8

when drawing an energy level diagram for an endothermic reaction what do you see ?

Answer 8

• heat energy is absorbed
• ∆h is positive
• this means the products end up with more energy than the reactants had

Question 9

when drawing an energy level diagram for an exothermic reaction what do you see ?

Answer 9

• heat energy is released
• ∆h is negative
• this means the products end up with less energy than the reactants had

Question 10

what do strong bonds require ?

Answer 10

• more energy to break
• they also release more energy when made

Question 11

what is a reaction profile diagram

Answer 11

• like an energy level diagram
• but it shows the path the reaction actually takes from reactants to products

Question 12

what is the energy needed to break the bonds called and what is it represented by in a diagram

Answer 12

activation energy and its represented by an arrow from the reactants up to the top of the hump

Question 13

on a reaction profile diagram, what does a hump going from reactants to the products represent

Answer 13

new bonds being made

Question 14

what happens with energy in an exothermic reaction

Answer 14

energy absorbed < energy released

Question 15

what happens with energy in an endothermic reaction

Answer 15

energy absorbed > energy released

Question 16

describe the 3 steps of an exothermic reaction

Answer 16

1) the reaction releases heat energy
2) that heat energy is absorbed by the nearby water
3) the temperature of the nearby water increases

Question 17

what are the 2 formulae used to convert your measurements into ∆h

Answer 17

1) Q = mc ∆T

2) ∆h = -Q/moles

Question 17

what is combustion calorimetry

Answer 17

combusting a fuel to heat up water

Question 18

what is the method to measure the ∆h for the combustion of the liquid fuel methanol

Answer 18

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

Question 19

how do you calculate your results from a combustion calorimetry reaction?

Answer 19

• 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

Question 20

what are the 2 sources of error in a combustion calorimetry reaction

Answer 20

• heat loss, most of the heat energy dissipates into the air instead of the water
• incomplete combustion, this releases less heat energy than expected

Question 21

what is solution calorimetry

Answer 21

when the reaction you are measuring dissolves in water

Question 22

what is the method for solution calorimetry ( measure the ∆h for the displacement reaction between mg and cuso4)

Answer 22

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

Question 23

how to calculate your results from a solution calorimetry reaction

Answer 23

• 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

Question 24

what are the sources of error of a solution calorimetry reaction

Answer 24

• 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

Question 25

can you do calorimetry experiments for endothermic reactions in solution as well ?

Answer 25

yes
the reaction absorbs heat energy from the water, so the water cools down
this results in a negative temp change

Question 26

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

Answer 26

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