Energetics

Question 1

Enthaply change

Answer 1

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

Question 2

Exothermic change

Answer 2

energy is transferred from the system to the surroundings

(products have less energy than surroundings

Question 3

Endothermic change

Answer 3

energy is transferred from the surroundings to the system

require heat energy(thermal decomposition of calcium carbonate)

Question 4

Standard enthalpy change of formation

Answer 4

enthalpy change when 1 mole of a substance is formed from its elements under standard conditions,all reactant and products being in their standered states

Question 5

Standard enthaply change of combustion

Answer 5

enthalpy change that occurs when 1 mole of a substance is combusted completely in oxygen under its standard conditions all reactants and products being in their standard state

Question 6

Standard conditions

Answer 6

• 100kPa Pressure
• 298 k (room temp/25)
• Solutions at 1 mol dm^-3
• all substances should have mornal state at 298 k

Question 7

Energy change

Answer 7

mass of solution x heat capacity x temperature change

Q (J) = m (g) x c x delta T (K)

Question 8

Calorimetry

Answer 8

wash the equipment & dry
-put polystyrene cup in a beaker for insulation and support
- Measure out desired volumes of solutions and transfer to insulated cup
- clamp thermometer into place making sure the thermometer is immersed in solution
- measure the initial temperatures of the solution or both solutions if 2 are used.
- 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 & record temp

Question 9

Errors in this method

Answer 9

-energy transfer from surroundings(loss)
-reaction or dissolving may be incomplete/slow
-density of solution is taken to be the same as water
Incomplete combustion of fuel
• Incomplete transfer of energy

Question 10

Calculating enthalpy change of reaction

Answer 10

1. Using q = m x c x T calculate energy change for quantities used
2. Work out the moles of the reactants used
3. Divide q by the number of moles of the reactant
4. Add a sign and unit (divide by a thousand to convert Jmol-1
   to kJmol-1

Question 11

Hesses law

Answer 11

total enthalpy change fro a reaction is independent of the route by which the chemical change takes place

Question 12

Mean bond energies

Answer 12

enthalpy needed to break the covelant bonds into gaseous atoms,averaged over differenet moleculed