4. 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.

Answer 2

If an enthalpy change occurs then energy is transferred between system and surroundings. The system is the chemicals and the surroundings is everything outside the chemicals.

Answer 3

• In an exothermic change energy is transferred from the system (chemicals) to the surroundings.
  The products have less energy than the reactants.

Answer 4

• In an exothermic reaction the deltaH is negative

Answer 5

In an endothermic change, energy is transferred from the surroundings to the system (chemicals). They require an input of heat energy e.g. thermal decomposition of calcium carbonate.
The products have more energy than the reactants.

Answer 6

In an endothermic reaction the delta H is positive

Question 7

Name common

Answer 7

Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration

Question 8

Define the standard molar enthalpy change of formation

Answer 8

• Enthalpy change when ONE mole of a compound is formed from its elements under standard conditions with all reactants and products being in their standard states.

Question 9

Define the standard molar enthalpy change of combustion

Answer 9

• Enthalpy change when ONE mole of a substance is completely burnt in oxygen under standard conditions with all reactansts and products being in their standard states.

Question 10

Define the standard molar enthalpy change of reaction

Answer 10

*

Question 11

formation

Answer 11

Mg(s)+Cl2 (g)MgCl2 (s) 2Fe (s) + 1.5 O2 (g)Fe2O3 (s)
The enthalpy of formation of an element = 0 kJ mol-1

Question 12

combustion.

Answer 12

CH4 (g) + 2O2 (g)CO2 (g) + 2 H2O (
Incomplete combustion will lead to soot (carbon), carbon monoxide and water. It will be less exothermic than complete combustion.

Answer 13

When an enthalpy change is measured at standard conditions the symbol is used
Eg H

Answer 14

Enthalpy changes are normally quoted at standard conditions. Standard conditions are:
* 100 kPa pressure
* 298 K (room temperature or 25oC)
* Solutions at 1mol dm-3
* all substances should have their normal state at 298K

Question 15

State the word equation for energy change

Answer 15

energy change = mass os solution x heat capacity x temp change

Question 16

State the symbol equation for energy change

Answer 16

q = mc delta T

Question 17

Calculating the enthalpy change of reaction, H from experimental data

Answer 17

General method
1. Using q = m x cp 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 not in excess to give H
4. Add a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-1