Standard Enthalpy Change of Neutralisation

Question 1

what is the standard enthalpy change of neutralisation

Answer 1

• the enthalpy change measured at 100kPa at a specified temp (298K)
• when one mole of water is produced by the neutralisation of an acid and an alkali

Question 2

what is the symbol for the standard enthalpy change of neutralisation

Answer 2

delta neut H- (with a circle around the -)

Question 3

if the balanced equation for a reaction is H2SO4 + 2NaOH = Na2SO4 + 2H2O, what is the equation for the enthalpy change of neutralisation for it

Answer 3

1/2H2SO4 + NaOH = 1/2N2SO4 + H2O

Question 4

what is something you notice about the enthalpy changes of neutralisation for certain combinations of acid and alkali

Answer 4

they are exceptionally constant

Question 5

what would the simplified ionic equation of a reaction between any strong acid and alkali be

Answer 5

H+ + OH- = H2O

Question 6

what does ionic equation now clarify about the differences between the enthalpy changes of neutralisation for strong acids and alkalis

Answer 6

• it is now not surprising that have very constant values for their delta neut H-
• as the ionic equation, aka net change in energy for their reactions, is more or less the same

Question 7

what is the practical to find the enthalpy change of neutralisation

Answer 7

• using a pipette with a safety filter, place 25cm^3 of 1 moldm-3 acid into an expanded polystyrene cup
• measure the temperature of the acid
• using a pipette with a safety filter, place 25cm^3 of the alkali into a beaker
• measure the temperature of the alkali
• add the alkali to the acid
• stir with the thermometer and measure the maximum temp reached

Question 8

what would you do to make sure all of the acid is neutralised in this reaction

Answer 8

• use a dilute alkali (usually sodium hydroxide) with a concentration slightly greater than 1 moldm-3
• to make sure all of the acid is neutralised

Question 9

what are the apparatus used in this practical

Answer 9

• beaker
• polystyrene cup
• beaker to place polystyrene cup in
• thermometer

Question 10

for this practical, the volumes of HCl and NaOH are 25cm^3. the initial temp of the acid was 18.6C and for the alkali it was 18.8C, while the maximum temp reached was 298.5K. what is the first thing you should do to calculate the enthalpy change of neutralisation

Answer 10

calculate the average initial temperature

Question 11

how would you calculate the initial temperature

Answer 11

• (18.6 + 18.8) / 2 = 18.7C

- 18.7 + 273 = 291.7K

Question 12

how would you then calculate the temperature change

Answer 12

298.5 - 291.7 = +6.7K

Question 13

how would you then calculate the mass of the solution heated

Answer 13

• 25cm^3 + 25cm^3 = 50cm^3
• assuming the density is 1gcm-3
• 50cm^3 x 1gcm-3 = 50g

Question 14

what is the assumed specific heat capacity of the solution

Answer 14

4.18JK-1g-1

Question 15

what equation would you then use to calculate the heat energy

Answer 15

Q = mc delta T

Question 16

what is Q

Answer 16

• Q = 50 x 4.18 x 6.7

- Q = 1400.5J or 1.4003kJ

Question 17

what do you then need to calculate in order to find delta H if you recall the formula of it

Answer 17

• delta H = -Q / n

- so you need to calculate the amount of mols

Question 18

how would you calculate the amount of mols of water

Answer 18

• the amount of acid neutralised = the amount of water formed
• meaning amount of water = 25cm^3
• moles = concentration x volume
• 1moldm-3 x 0.025dm3 = 0.025 mols

Question 19

what is then the value of delta neut H-

Answer 19

• delta H = -(Q / n)

- -(1.4003kJ / 0.025 mol) = -56kJmol-1

Question 20

why should your answer in this case only be given to two significant figures

Answer 20

because your change in temperature (6.7K) is only given to two s.f.

Question 21

what are some reasons that errors or inaccuracies could arise

Answer 21

• there are always uncertainties with using instruments like pipettes ans thermometers
• some heat energy will be transferred to the thermometer and polystyrene cup