Energetics - 5th

Question 1

what is the enthalpy change of a reaction?

Answer 1

• the heat energy it releases or absorbs per mole

Question 2

what is the symbol of enthalpy change?

Answer 2

• DELTA H
• has a sign, value and units

Question 3

what does this tell us:
N2 + 3H2 -> 2NH3
DELTA H = -92 kJ/mol

Answer 3

• this tells is that the reaction releases 92 kJ of heat energy every time 1 mole of N2 and 3 moles of H2 react to make 2 moles of NH3

Question 4

what sign tells us whether the reaction absorbs or releases energy?

Answer 4

+ = POSITIVE = ABSORBED = ENDOTHERMIC
- = NEGATIVE = RELEASED = EXOTHERMIC

Question 5

what tells us how much heat energy is absorbed or released?

Answer 5

• the value tells us how much heat energy is absorbed or released
• the units are always kJ / mol so that different reactions can be compared easily

Question 6

what is the meaning of this reaction and enthalpy change:
BaCO3 + heat
DELTA H = +245 kJ/mol

Answer 6

• absorbs a lot of heat energy

Question 7

what is the meaning of this reaction and enthalpy change:
NaCl + H2O
DELTA H = +35 kJ/mol

Answer 7

• absorbs a small amount of heat energy

Question 8

what is the meaning of this reaction and enthalpy change:
CH4 + O2
DELTA H = -890 kJ/mol

Answer 8

• releases a lot of heat energy

Question 9

what is the meaning of this reaction and enthalpy change:
C8H18 + O2
DELTA H = -5330 kJ/mol

Answer 9

• releases a huge amount of heat energy

Question 10

what does how much heat energy a reaction absorbs or releases depend on?

Answer 10

• the balance between breaking bonds (which absorbs energy) and making bonds (which releases energy)

Question 11

how are enthalpy changes represented diagramically?

Answer 11

• on an energy level diagram

Question 12

what happens to heat energy and enthalpy change when the reaction is endothermic?

Answer 12

• heat energy is absorbed
• enthalpy change is positive
• products end up with more energy than the reactants had

Question 13

what happens to heat energy and enthalpy change when the reaction is exothermic?

Answer 13

• heat energy is released
• enthalpy change is negative
• products end up with less energy than the reactants had

Question 14

what contributes towards a reaction’s enthalpy change?

Answer 14

• breaking bonds and making bonds

Question 15

is bond breaking exothermic or endothermic and does bond breaking absorb or release heat energy?

Answer 15

• absorbed
• endothermic

Question 16

is bond making exothermic or endothermic and does bond making absorb or release heat energy?

Answer 16

• released
• exothermic

Question 17

how can the enthalpy change for a reaction be calculated?

Answer 17

• bonds broken - bonds made

Question 18

what is a reaction profile diagram?

Answer 18

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

Question 19

in an exothermic reaction, which is greater, energy absorbed or energy released?

Answer 19

• energy released > energy absorbed

Question 20

in an endothermic reaction, which is greater, energy absorbed or energy released?

Answer 20

• energy released < energy absorbed

Question 21

what is activation energy?

Answer 21

• the energy needed to break the bonds

Question 22

what is calorimetry?

Answer 22

• an experimental method used to measure an enthalpy change

Question 23

describe the experiment of calorimetry.

Answer 23

• a reaction is carried out near some water

Question 24

what happens in an exothermic reaction when calorimetry is done?

Answer 24

• reaction releases heat energy
• heat energy is absorbed by the nearby water
• the temperature of the water increases

Question 25

what are the two formulas needed to work out enthalpy change after doing calorimetry?

Answer 25

- Q = mc deltaT - enthalpy change = -Q/moles

Question 26

what is c in Q=mc deltaT?

Answer 26

- constant value - specific heat capacity of water - always has the value 4.18 J/g/degrees celcius

Question 27

describe the simplest type of calorimetry.

Answer 27

- the simplest type of calorimetry involves combusting a fuel to heat up the water - to measure deltaH (enthalpy change) for the combustion of the liquid fuel methanol, you would combust some methanol using a burner under a can of water

Question 28

method for simplest type of calorimetry.

Answer 28

1. weigh out 50g of water and place in a can above the burner 2. measure the initial mass of the burner and the initial temp of the water 3. ignite the burner, stir water for 30s, then extinguish the burner 4. measure the final mass of the burner and the final (maximum) temperature of the water

Question 29

what are the two significant sources of error in this experiment?

Answer 29

- 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 30

what is solution calorimetry?

Answer 30

- sometimes the reaction you are measuring happens dissolved in some water - in this case, the reaction still releases heat energy and the water still absorbs it- but it just happens that the reaction and the water are in the same place

Question 31

describe the method of solution calorimetry.

Answer 31

- e.g. to measure enthalpy change for the displacement reaction between Mg(s) and CuSO4(aq) - pipette 25cm3 of CuSO4(aq) into a polystyrene cup - measure the initial temp of sol - add 1g of Mg(s) and stir continuously - measure the max temp of the solution - use equations to calculate

Question 32

what happens if you do a calorimetry experiment for endothermic reactions?

Answer 32

- reaction absorbs heat energy from water, so the water cools down - this results in a negative temp change for the water - the minus sign gets passed through the equations and comes out at the end as a plus, because a minus times a minus is a plus