5.2 - Energetics

Question 1

What is an enthalpy change

Answer 1

The enthalpy change of a reaction is the heat energy it releases or absorbs per mole

Question 2

What is the symbol used for enthalpy change

Answer 2

ΔH

Question 3

What signs can enthalpy changes have

Answer 3

+ or -

Question 4

What is the value in an enthalpy change

Answer 4

the number

Question 5

What is the unit used for enthalpy change

Answer 5

kJ/mol

Question 6

what tells us whether the reaction absorbs or releases energy

Answer 6

the sign
+ or -

Question 7

If the sign is positive (+) has the reaction absorbed or released heat energy

Answer 7

absorbed

Question 8

If the sign is negative (-) does the reaction absorbed or released heat energy

Answer 8

released

Question 9

If the sign is positive (+) is the reaction endothermic or exothermic

Answer 9

endothermic

Question 10

If the sign is negative (-) is the reaction endothermic or exothermic

Answer 10

exothermic

Question 11

If the reaction has absorbed heat is the sign positive (+) or negative (-)

Answer 11

positive (+)

Question 12

If the reaction has released heat is the sign positive (+) or negative (-)

Answer 12

negative (-)

Question 13

If the reaction has absorbed heat is it endothermic or exothermic

Answer 13

endothermic

Question 14

If the reaction has released heat is it endothermic or exothermic

Answer 14

exothermic

Question 15

If the reaction is endothermic is its sign positive (+) or negative (-)

Answer 15

positive (+)

Question 16

If the reaction is exothermic is its sign positive (+) or negative (-)

Answer 16

negative (-)

Question 17

If the reaction is endothermic has it absorbed or released heat

Answer 17

absorbed

Question 18

If the reaction is exothermic has it absorbed or released heat

Answer 18

released

Question 19

What does the value of an enthalpy change show

Answer 19

how much heat energy is absorbed or released

Question 20

What do the units of an enthalpy change always the same

Answer 20

they are always kJ/mol so that different reactions can be compared easily

Question 21

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

Answer 21

it depends on the balance between:
- breaking bonds, absorbs energy
- making bonds, releases energy

Question 22

what does breaking bonds do

Answer 22

absorbs energy

Question 23

what does making bonds do

Answer 23

releases energy

Question 24

what does an energy level diagram look like for an endothermic

Answer 24

¦ ____products___
¦ ^
H¦ ¦
¦ ¦ΔH(+)
¦_____reactants_____¦
¦

Question 25

In an endothermic reaction do the products end up with more or less energy than the reactants had

Answer 25

more

Question 26

what does an energy level diagram look like for an exothermic reaction

Answer 26

^ ¦ ¦_____reactants_______ H¦ ¦ ¦ ¦ΔH(-) ¦ ¦ ¦ ˇ___products___ ¦ ¦

Question 27

In an exothermic reaction do the products end up with more or less energy than the reactants had

Answer 27

less

Question 28

in bond breaking is heat absorbed or released

Answer 28

absorbed

Question 29

in bond breaking is the reaction endothermic or exothermic

Answer 29

endothermic

Question 30

in bond making is heat absorbed or released

Answer 30

released

Question 31

in bond breaking is the reaction endothermic or exothermic

Answer 31

exothermic

Question 32

do stronger bonds require more or less energy to break

Answer 32

more

Question 33

do stronger bonds release more or less energy when they are made

Answer 33

more

Question 34

what is the equation for calculating bond energies in an enthalpy change reaction

Answer 34

ΔH = Bonds broken - Bonds made

Question 35

when writing out a bonds energy calculation what should you do

Answer 35

write out the bonds broken and bonds made separately before you subtract them

Question 36

how is a reaction profile diagram different to an energy level diagram

Answer 36

it shows the path the reaction actually takes from reactants to products

Question 37

what is the inequality or energy absorbed and released in an exothermic reaction

Answer 37

energy absorbed < energy released

Question 38

what is the inequality or energy absorbed and released in an endothermic reaction

Answer 38

energy absorbed > energy released

Question 39

what is the energy needed to break the bonds called

Answer 39

activation energy

Question 40

what is calorimetry

Answer 40

an experimental method used to measure an enthalpy change

Question 41

in principle of calorimetry what is the reaction carried out near to

Answer 41

water

Question 42

explain what happens for calorimetry for an exothermic reaction

Answer 42

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

Question 43

what are the two forumlae used in calorimetry to convert measurements into ΔH

Answer 43

Q=mcΔT ΔH= -Q/moles

Question 44

explain each component of Q=mcΔT equation

Answer 44

Q - heat energy (J) m - mass of water (g) c - specific heat capacity (degrees C) ΔT - temperature change

Question 45

explain each component of ΔH= -Q/moles

Answer 45

ΔH - enthalpy change (kJ/mol) - Q - minus heat energy (degrees C) moles - moles

Question 46

what is the method for combustion calorimetry

Answer 46

1. Weigh out 50 g of water and place in a can above the 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, then extinguish the burner. 4. Measure the final mass of the burner and the final (maximum) temperature of the water.

Question 47

what are two sources of error in combustion calorimetry

Answer 47

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 48

what is the method for solution calorimetry

Answer 48

1. Pipette 25 cm3 of CuSO4(aq) into a polystyrene cup. 1 cm3 solution has a mass of 1 g 2. Measure the initial temperature of the solution. 3. Add 1 g of Mg(s) and stir continuously. 4. Measure the maximum temperature of the solution.

Question 49

what are sources of error for solution calorimetry

Answer 49

heat loss - most of the heat energy dissipates into the air instead of the water incomplete combustion - this releases less heat energy than expected water being heated is not longer pure water - it has things dissolved in it so the specific heat capacity is no longer exactly 4.18 J/g/degrees C