Energetics

Question 1

Define enthalpy change

Answer 1

Amount of heat energy taken in or given out during any change in a system providing the pressure is constant

Question 2

Describe what happens during an exothermic reaction

Answer 2

Energy is transferred from the system to the surroundings
Products have less energy than the reactants

Question 3

What is the enthalpy change always in an exothermic reaction?

Answer 3

Negative

Question 4

Give examples of exothermic reactions

Answer 4

Combustion of fuels
Oxidation of carbohydrates such as glucose

Question 5

Describe what happens during an endothermic reaction

Answer 5

-Energy is transferred from the surroundings to the system
- Requires an input of heat energy
- Products have a higher energy than reactants

Question 6

Give examples of endothermic reactions

Answer 6

Thermal decomposition of calcium carbonate

Question 7

What is always the enthalpy change of endothermic reactions?

Answer 7

positive

Question 8

Define standard enthalpy change of formation

Answer 8

• The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions (298K and 100kpa) with all reactants and products being in their standard states
  Triangle FH

Question 9

Define standard enthalpy change of combustion

Answer 9

The enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions ( 298K and 100kpa) with all reactants and products in their standard states
Triangle CH

Question 10

Describe the standard conditions used in combustion and formation

Answer 10

100kpa
298K or 25 degrees celcius
Solutions at 1 mol dm-3

Question 11

Define standard enthalpy change of neutralisation

Answer 11

Enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions

Question 12

Give the equation for heat energy change of a reaction

Answer 12

q=m X cp X triangleT
q represents the heat energy change of water (J)
m represents mass of each element given(g)
c represents specific heat capacity of water J g-1K-1

Question 13

Give the general working out used to determine enthalpy change from experimental data

Answer 13

• Use q= m X c X triangleT to work out the heat energy change of reactants in the caliometer
• Calculate moles of the reactant not in excess
• Calculate enthalpy change per mole ( enthalpy change of reaction)
• Divide ans by 1000 to make it KJ per mol to 3sf
• Add the sign e.g exothermic results in a negative sign value (-)

Question 14

Name errors in the method of calculating enthalpy change from experimental data

Answer 14

• Energy losses from the caliometer
• Incomplete combustion of the fuel
  -Incomplete transfer of energy
• Evaporation of fuel after weighing
• Heat capacity of caliometer not included
  -Measurements not carried out under standard conditions as H2O is a gas not a liquid in this experiment

Question 15

Why might the published value of enthalpy change differ to the value you work out yourself?

Answer 15

If an unlit spirit burner is left uncapped the fuel will evaporate, making it appear that we burned more fuel than we actually did
- This means our results will be shown to be less exothermic than they actually are
- Alot of the heat energy released by the fuel does not pass into the water, some of it is passed into the metal calimoeter
- Not all of the fuel might have gone under complete combustion which may result in less thermal energy being released
- Experiment may not have been carried out under standard conditions

Question 16

State Hess law

Answer 16

-Total enthalpy change of a reaction is independent of the route by which the chemical reaction takes place

Question 17

How can the enthalpy change of combustion pattern of hydrocarbons be explained by mean bond enthaply?

Answer 17

-We have to break the same number of additional bonds, C-C and 2 C-H bonds
- We also have to make the same number of additional bonds, for each ch2 added we make 2 O-H bonds and 2=O bonds

Question 18

Why might the value using bond enthalpy data be different to the experimental value of the enthalpy change?

Answer 18

Bond enthalpys use mean values, dont take into account the certain molecule the bond is actually in

Question 19

Define mean bond enthalpy

Answer 19

The energy required to break a covalent bond
Avg values

Question 20

Give equation of enthalpy change in terms of bond enthalpys

Answer 20

Enthalpy change= Sum of bond energies of reactants- the sum of bond energies of products

Question 21

Give one reason why the bond enthalpy that is calculated is different to the mean bond enthalpy in a data book

Answer 21

Data book value is derived from a number of different compounds, not just different NH3 molecules

Question 22

Describe the general calorimetric method

Answer 22

-washes the equipment (cup and pipettes etc) with the solutions to be used
-dry the cup after washing
-put polystyrene cup in a beaker for insulation and support
-Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
-clamp thermometer into place making sure the thermometer bulb is immersed in solution
- measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
-At minute 3 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.
- If using a solid reagent then use ‘before and after’ weighing method
-stirs mixture (ensures that all of the solution is at the same temperature) Record temperature every minute after addition for several minutes

Question 23

How should someone adjust if reaction is too slow and what is the problem with this

Answer 23

If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the
reaction.
To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.

Question 24

What are the errors in the caliometry method

Answer 24

• energy transfer from surroundings (usually loss)
• approximation in specific heat capacity of solution. The method assumes all
  solutions have the heat capacity of water.
• neglecting the specific heat capacity of the calorimeter- we ignore any
  energy absorbed by the apparatus.
• reaction or dissolving may be incomplete or slow.
• density of solution is taken to be the same as water.