ENERGETICS

Question 1

Define enthalpy change

Answer 1

energy change per mole at constant pressure

Question 2

What are the standards conditions

Answer 2

298K and 100kPa pressure

Question 3

Define enthalpy of formation

Answer 3

enthalpy change when one mole of a substance is formed from its constituent elements in their standard states

Question 4

Define enthalpy of combustion

Answer 4

enthalpy change where one mole of substance is completely burned in oxygen

Question 5

Rules

Answer 5

-sulfur = S8 + phosphorus = P4
-halogens, hydrogen, oxygen, nitrogen = diatomic
-gaseous elements = H2, F2, CL2, O2, N2
-liquid elements = Hg and Br2

Question 6

Define exothermic reaction

Answer 6

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

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

Question 7

Define endothermic reaction

Answer 7

In an endothermic change, energy is transferred from the surroundings to the system (chemicals).

require an input of heat energy e.g. thermal decomposition of calcium carbonate. The products have more energy than the reactants.

Question 8

Equation that links energy change, temp change, shc and mass of water

Answer 8

Q=MC∆t

Question 9

Write and equation that links enthalpy change, energy change and moles reacted

Answer 9

∆H = q/moles

Question 10

Give 3 reasons why enthalpy changes of combustion calculated by calorimetry may not be accurate

Answer 10

-heat lost to surroundings
-evaporation of fuels
-incomplete combustion of fuels

Question 11

State Hess’ Law

Answer 11

total enthalpy change for a process is independent of the route taken

Question 12

equation to calculate enthalpy change of reaction from enthalpy changes of formation

Answer 12

E∆Hf products -E∆Hf reactants

Question 13

equation to calculate enthalpy change of reaction from enthalpy changes of combustion

Answer 13

E∆Hc reactants -E∆Hc products

Question 14

Define mean bond (disassociation) enthalpy

Answer 14

enthalpy change when one mole of covalent bonda in the gaseous state are completely broken

Question 15

why are calculations from mean bond enthalpies often different from those calculated from Hess’ law (theoretical values)

Answer 15

bond enthalpies are a mean average taken over a range of compounds

Question 16

equation to calculate enthalpy change of reaction from mean bond enthalpy data

Answer 16

E bond enthalpy (reactant) - E bond enthalpy (products)