Enthalpy changes

Question 1

Exothermic reactions

Answer 1

A reaction in which the enthalpy of the product is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings; dH is -ve

Question 2

Endothermic reactions

Answer 2

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken from the surroundings; dH is +ve

Question 3

Oxidation and enthalpy

Answer 3

Most exploited exothermic reaction - in combustion of fuels and and in respiration. We rely on this for living and for travel by airplane, car, train etc. Respiration is really important

Question 4

Requirements for endothermic processes

Answer 4

Need and input of heat energy from surroundings, as products have more energy than reactants. E.g, to decompose calcium carbonate, the calcium carbonate needs to be heated.

Question 5

Basic enthalpy profile diagram

Answer 5

• x axis is progress of reaction
• y axis is enthalpy
• begin with a line representing reactants
• end with a products line
• dH is the gap between them
• exothermic reactions, dH goes down
• endothermic reactions, dH goes up

Question 6

Activation energy

Answer 6

The minimum energy required to start a reaction by the breaking of bonds. Hump on an enthalpy profile diagram

Question 7

Standard conditions

Answer 7

Pressure of 100kPa (1 atm) temperature of 25degC (298degK) and a concentration of 1.0mol dm^-3 for reactions in aqueous solution

Question 8

Standard enthalpy change of reaction

Answer 8

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

Question 9

Standard enthalpy change of formation of a compound

Answer 9

Enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 10

Standard enthalpy change of combustion

Answer 10

Enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions. all reactants and products being in their standard states

Question 11

Q=

Answer 11

mc(deltaT)

Question 12

Calculating from reactants, dH=

Answer 12

-(Q/mol)

Question 13

Endothermic reactions, in terms of bond enthalpies

Answer 13

The bonds that are formed are weaker than the bonds that are broken: the bonds that are broken absorb more energy than the the bonds that are made release

Question 14

Exothermic reactions, in terms of bond enthalpies

Answer 14

The bonds that are formed are stronger than the bonds that are broken: the bonds that are broken absorb less energy than the bonds that are made release

Question 15

Average bond enthalpy

Answer 15

The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species

Question 16

Bond enthalpy, dH=

Answer 16

(sum of bond enthalpies of bonds broken)-(sum of bond enthalpies of bonds made)