Enthalpy changes

Question 1

What is the definition of enthalpy change?

Answer 1

Heat energy change measured at constant pressure

Question 2

What happens when bonds are formed?

Answer 2

Energy is released
Process is exothermic
Δ H is negative

Question 3

What happens when bonds are broken?

Answer 3

Energy is absorbed
Process is endothermic
ΔH is positive

Question 4

What is the definition of an exothermic reaction?

Answer 4

More energy is released in bond forming than is absorbed in bond breaking, resulting in an overall negative ΔH

Question 5

What is the definition of an endothermic reaction?

Answer 5

More energy is absorbed in bond breaking than is released in bond forming, resulting in an overall positive ΔH

Question 6

What is the definition of activation energy of a reaction?

Answer 6

Minimum energy required for a reaction to occur

Question 7

What is the symbol for enthalpy change?

Answer 7

ΔH

Question 8

What is the symbol for activation energy?

Answer 8

Ea

Question 9

What are some examples of exothermic reactions?

Answer 9

Combustion
Test tube heating up after adding a metal to an acid

Question 10

What are some examples of endothermic reactions?

Answer 10

Photosynthesis
Decomposition of carbonates
Cool packs

Question 11

What is the standard condition for temperature?

Answer 11

298k

Question 12

What is the standard condition for pressure?

Answer 12

100kPa

Question 13

What is the standard condition for solution concentration?

Answer 13

1 moldm-3

Question 14

What is the definition for the standard enthalpy of formation?

Answer 14

The formation of 1 mole of a compound from its elements

Question 15

What is the definition for the standard enthalpy of combustion?

Answer 15

The complete combustion of 1 mole of a substance

Question 16

What is the definition for the standard enthalpy of neutralisation?

Answer 16

The formation of 1 mole of water from neutralisation

Question 17

What is the definition for the standard enthalpy of reaction?

Answer 17

The enthalpy change associated with a stated equation

Question 18

What is the definition for the average bond enthalpy?

Answer 18

The breaking of 1 mole of bonds in gaseous molecules

Question 19

Give of an example of the standard enthalpy of formation

Answer 19

C (s, graphite) +2H2 (g) —> CH4 (g)

Question 20

Give of an example of the standard enthalpy of combustion

Answer 20

CH4 (g) + 2O2 (g) —> CO2 (g) + 2H2O (l)

Question 21

Give of an example of the standard enthalpy of neutralisation

Answer 21

HCl (aq) + NaOH (aq) —> NaCl (aq) + H2O (l)

Question 22

Give of an example of the standard enthalpy of reaction

Answer 22

N2 (g) + 3H2 (g) —> 2NH3 (g)
C2H6O (l) —> C2H4 (g) + H2O (l)

Question 23

Give of an example of the average bond enthalpy

Answer 23

1/3 NH3 (g) —> 1/3 N (g) + H (g)

Question 24

What equation is used to calculate the standard enthalpies of reactions?

Answer 24

q = mcΔT

Question 25

Define the enthalpy of neutralisation

Answer 25

The energy change when 1 mol of water is created by the neutralisation of an acid with an alkali under standard conditions

Question 26

Define the enthalpy of combustion

Answer 26

The enthalpy change when 1 mole of a substance in its standard state
reacts completely with oxygen under standard conditions

Question 27

Why might the ΔcH value be less exothermic than the expected?

Answer 27

Heat loss to surroundings
Incomplete combustion
None standard conditions used

Question 28

What would happen if a student repeated the enthalpy of combustion experiment with a lid on the beaker and with the same mass of fuel burned?

Answer 28

ΔT would be greater; because less heat is lost to surroundings
q would increase
ΔH would become more exothermic/negative

Question 29

What would happen if the student repeated the enthalpy of combustion experiment with a supply of gaseous oxygen feeding the flame?

Answer 29

ΔT would increase; because the fuel burns with more complete combustion
q would increase
ΔH would become more exothermic/negative

Question 30

What would happen if the student repeated the enthalpy of combustion experiment with a fresh beaker of water that had an initial temperature of 15° C?

Answer 30

ΔT would remain similar because n(fuel) and v(H2O) remain the same
q would stay the same
ΔH would stay the same

Question 31

What would happen if the student reads the measuring cylinder incorrectly and added more than X grams of water, without realising their mistake and then heated using the same mass of fuel?

Answer 31

ΔT would decrease; more water is being heated by the same mass of
fuel
q would decrease
ΔH would become less exothermic/negative

Question 32

What would happen if the student repeated the enthalpy of combustion with a beaker which had a layer of soot on the bottom surface of the glass?

Answer 32

ΔT would decrease because heat is absorbed by the soot
q would decrease
ΔH would become less exothermic/negative

Question 33

Explain the method to calculating ΔrH for a solid and a solution using a cooling curve

Answer 33

Pipette 100cm3 of 0.5moldm-3 CuSO4 (aq) into a polystyrene cup
Weigh out an excess of magnesium
Start a stop-clock and take the temperature of the solution every 30s until the solutions temperature stays constant
Add the magnesium to the solution and stir the mixture
Record the temperature every 30 sec from 30 second after the addition of magnesium until the temperature has fallen for several minutes
Plot a graph of temperature against time