Energetics

Question 1

Define Enthalpy change

Answer 1

The amount of heat energy taken in or given out during any change in a system provided the pressure is constant

Question 2

Describe the enthalpy change in an exothermic reaction (2)

Answer 2

Energy is transferred from the system(chemicals) to the surroundings
The products also have less energy than the reactants

Question 3

Is the Change in energy positive or negative in an Exothermic reaction

Answer 3

Negative

Question 4

Give 2 examples of common oxidation exothermic processes

Answer 4

Combustion of fules
Oxidation of carbohydrates such as glucose in respiration

Question 5

What happens if there is an enthalpy change (2)

Answer 5

Energy is transferred between the system and the surroundings
The system is the chemicals and the surroundings is everything outside the chemicals

Question 6

Describe the enthalpy change in an endothermic reaction (2)

Answer 6

Energy is transferred from the surroundings to the system
Requires an input of heat energy

Question 7

What is an example of an endothermic reaction (3)

Answer 7

The thermal decomposition of calcium carbonate
The products have more energy than the reactants
Enthalpy change is positive

Question 8

What is the standard enthalpy change of formation

Answer 8

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

Question 9

What is the standard enthalpy change of combustion

Answer 9

The enthalpy change that occurs when one mole of a substance is completely combusted in oxygen under standard conditions all reactants and products being in their standard stages

Question 10

What are the 4 standard conditions

Answer 10

100kPa
298K
Solutions at 1moldm^3
All substances should have their normal state at 298

Question 11

What does incomplete combustion lead to

Answer 11

Soot( carbon)
Carbon monoxide
Water
It will be less exothermic than complete combustion

Question 12

What is the equation used for a reaction in a solution?

Answer 12

Energy change (J)=Mass of solution (g) x Heat capacity (Jg^-1 K^1) x Temperature change (K)

Q= Mx Cp x Delta T

Question 13

What is the general method of the Calorimetric method (10)

Answer 13

• Wash 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
• Stir mixture (ensures that all of the solution is at the same temperature)
• Record temperature every minute after addition for several minutes

Question 14

What are the errors in the calorimetric method?

Answer 14

• Energy transfer from surroundings (usually loss)
• 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.