Energetics

Question 1

define endothermic reaction

Answer 1

a reaction that absorbs thermal energy from the surroundings, causing the temperature of the surroundings to decrease

Question 2

By considering bond energies, what makes a reaction endothermic?

Answer 2

More energy is required to break bonds than is released when forming bonds

Question 3

define exothermic reaction

Answer 3

a reaction that releases thermal energy into the surroundings, causing the temperature of the surroundings to increase

Question 4

By considering bond energies, what makes a reaction exothermic?

Answer 4

More energy is released forming bonds than is required to break bonds

Question 5

what type of process is bond-breaking?

Answer 5

bond-breaking is an endothermic process

Question 6

what type of process is bond-making?

Answer 6

bond-making is an exothermic process

Question 7

draw the energy level diagrams and enthalpy profile diagrams of endothermic and exothermic reactions

Answer 7

REFER TO ONENOTE

Question 8

define molar enthalpy change

Answer 8

• the amount of heat energy taken in or given out in a chemical reaction
• it is the difference in energy from reactants to products

Question 9

what is the symbol for enthalpy change?

Answer 9

ΔH

Question 10

what are the units for enthalpy change?

Answer 10

kJ/mol

Question 11

is enthalpy change positive or negative for exothermic reactions? why?

Answer 11

• ΔH is negative for exothermic reactions
• ΔH = (bonds broken) − (bonds made)
• more energy is released during bond breaking than is required during bond making, so the system loses chemical energy from its bonds as it is transferred as thermal energy into the surroundings (therefore increasing their temperature)
• the products end up with less energy than the reactants, so ΔH is negative

Question 12

is enthalpy change positive or negative for endothermic reactions? why?

Answer 12

• ΔH is positive for endothermic reactions
• ΔH = (bonds broken) − (bonds made)
• in endothermic reactions, more energy is required during bond breaking than is released during bond making, so the system absorbs chemical energy into its bonds by absorbing thermal energy from the surroundings (therefore decreasing their temperature)
• as a result, the products end up with more energy than the reactants, so ΔH is positive

Question 13

define specific heat capacity

Answer 13

the amount of energy/heat needed to raise the temperature of 1g of substance by 1 degree celsius

Question 14

what is the SHC of water?

Answer 14

4.2J/g/°C

Question 15

Heat energy change formula

Answer 15

Q = m x c x ΔT
(energy heat change = mass x SHC x change in temp)

Question 16

why is water used in calorimetry experiments?

Answer 16

• its very good at absorbing and releasing heat so when reactions happen in or near it, the heat is transferred to the water and we can measure ΔT to find how much energy was absorbed/released by reaction
• water has a well-known SHC of 4.18 J/g/℃ so can be used in Q = m x c x ΔT calcs
• many reactions often happen in water to form an aqueous solution as water is a good solvent

Question 17

method for practical - enthalpy change of combustion reaction of ethanol

Answer 17

• measure 100cm³ of water into a copper calorimeter using a measuring cylinder
• measure its initial temperature using a thermometer
• weigh a spirit burner containing ethanol with its lid on (prevents evaporation)
• clamp calorimeter with thermometer exactly 1cm above spirit burner and cover apparatus using a draught shield and light spirit burner
• stop heating when temperature has risen by 40 degrees and then extinguish flame by placing lid on burner
• stir water well and measure max final temperature
• immediately record final mass of spirit burner w/lid on
• using Q = m x c x change in T, find the energy change of the combustion reaction
• calculate the moles of ethanol used using moles = mass/M_r (as this is the limiting reagent)
• using enthalpy change = -Q/1000n, find the enthalpy change of the reaction

Question 18

method for practical - enthalpy change for displacement reaction

Answer 18

• place a polystyrene cup in a 250cm³ beaker to keep it stable
• transfer 25cm³ of hydrochloric acid solution into the polystyrene cup using a measuring cylinder
• place thermometer into acid + record initial temperature of solution
• carefully weigh out 0.30g of magnesium powder into a weighing boat using a mass balance
• add magnesium to acid quickly and stir solution gently with a thermometer
• regularly check temperature until it stops rising and note change in temperature
• using Q = m x c x change in T, find the energy change of the reaction
• calculate the moles of both reactants and determine limiting reagent using mole ratio
• using enthalpy change = -Q/1000n, find the enthalpy change of the reaction

Question 19

method for practical - enthalpy change of salt dissolving in water

Answer 19

• place a polystyrene cup in a 250cm³ beaker to keep it stable
• transfer 25cm³ of distilled water into the polystyrene cup using a measuring cylinder
• place thermometer into acid record initial temperature
• carefully weigh out 1.5g of potassium chloride into a weighing boat using a mass balance
• add potassium chloride to water quickly and stir solution gently with a thermometer
• regularly check temperature until it stops rising and note change in temperature
• using Q = m x c x change in T, find the energy change of the reaction
• calculate the moles of both reactants and determine limiting reagent using mole ratio
• using enthalpy change = -Q/1000n, find the enthalpy change of the reaction

Question 20

method for practical - enthalpy change for neutralisation reaction of sodium hydroxide and hydrochloric acid

Answer 20

• place a polystyrene cup in a 250cm³ beaker to keep it stable
• transfer 25cm³ of hydrochloric acid solution into the polystyrene cup using a measuring cylinder
• place thermometer into acid
• carefully measure out 25cm³ of sodium hydroxide solution into the cup using a measuring cylinder
• record initial temperature of solution
• add sodium hydroxide to acid quickly and stir solution gently with a thermometer
• regularly check temperature until it stops rising and note change in temperature
• calculate change in temperature and insert into Q = m x c x change in T
• calculate the moles of both reactants and determine limiting reagent using mole ratio
• insert moles of lining reagent into change in enthalpy = -Q/1000n

Question 21

Molar enthalpy change formula

Answer 21

ΔH = -Q/1000n

Question 22

Moles formulae

Answer 22

moles = mass/molar mass
moles = concentration x volume

Question 23

dm³ to cm³ conversion

Answer 23

1dm³ = 1000cm³

Question 24

steps to calculating the ΔH from calorimetry experiment data

Answer 24

• use Q = m x c x ΔT to find the energy heat change of the reaction
• calculate moles of substance causing the chemical change (e.g limiting reagent, alcohol being combusted) using moles = mass/M_r or moles = conc x vol
• using the values for Q and moles, calculate ΔH using ΔH = -Q/1000n

Question 25

four sources of error in calorimetry experiments

Answer 25

- heat loss to the surroundings = some heat escapes, so measured heat is less than actual - incomplete reaction = the reaction doesn’t fully finish, releasing less heat than expected - calorimeter placed too far above spirit burner = not all thermal energy released from combustion reaction is transferred to water in calorimeter - incomplete combustion = not enough oxygen means less energy is released from combustion reaction so water not heated properly

Question 26

Name and describe 4 ways to increase accuracy in the calorimetry experiments

Answer 26

INCREASE insulation: - use a polystyrene cup = a better insulator so less heat energy is transferred to surroundings - place a lid on calorimeter (same as above) - add a draught excluder around apparatus (same as above) AVOID incomplete combustion by providing a plentiful supply of oxygen

Question 27

name the two assumptions made during calorimetry experiments

Answer 27

- the density of the reaction mixture is the same as water, so 1g = 1cm³ - the SHC of the solution is the same as that of water - reasonable assumption as most solutions undergoing calorimetry experiments contain lots of water

Question 28

define bond energy

Answer 28

the amount of energy needed to break 1 mole of covalent bonds of gaseous molecules

Question 29

enthalpy change (ΔH) calculation using bond energies

Answer 29

ΔH = (bonds broken) - (bonds made)

Question 30

method for calculating enthalpy change (ΔH) using bond energies

Answer 30

- draw the displayed formulas for each of the reactants and products - carefully counting the bonds, use the reference table to multiply and find bond energies for each reactant - add total bond energies for reactants to find total energy absorbed by reactants - repeat for products - use this formula to calculate enthalpy change: ΔH = (bonds broken) - (bonds made)