Energetics

Question 1

ΔH =

Answer 1

ΔH= energy to break bonds (+ve) + energy to make bonds (-ve)

Question 2

Enthalpy change

Answer 2

The change in heat energy of a substance at constant pressure.

Question 3

Endothermic

Answer 3

Describes a reaction in which heat is taken in as the reactants change to products; the temperature of the surroundings thus drops

Question 4

Exothermic

Answer 4

Describes a reaction in which heat is given out as the reactants change to products; the temperature of the surroundings rises

Question 5

Describe the enthalpy diagram for an endothermic reaction

Answer 5

Reactants are at a lower enthalpy (y axis) than the products, which are at a higher enthalpy. The difference is a +ve ΔΗ

Question 6

Describe the enthalpy diagram for an exothermic reaction

Answer 6

The enthalpy of the reactants is higher than that of the products. The difference is -ve ΔΗ

Question 7

Standard conditions

Answer 7

100kPa
298K

Question 8

Enthalpy of Formation

Answer 8

The standard molar enthalpy of formation, ΔfH, is the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states

Question 9

Enthalpy of combustion

Answer 9

The standard molar enthalpy of combustion ΔcH, is the enthalpy change when one mole of substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states

Question 10

q=

Answer 10

q=mcΔT

Question 11

Calorimeter

Answer 11

An instrument for measuring the heat changes that accompany chemical reactions

Question 12

What does q=mcΔT stand for and what are the units

Answer 12

q : Energy change (J)
m : mass (g)
ΔΤ : change in temperature (K)

Question 13

Equation to calculate enthalpy change per mole of a substance

Answer 13

ΔH = q/mol

Question 14

Give the units of ΔH = q/mol

Answer 14

ΔΗ (kJ/mol)
q (kJ)
moles

Question 15

Specific heat capacity

Answer 15

Is the amount of energy required to raise 1g of a substance by 1K without a change in states. (J g-1 K-1)

Question 16

Thermochemical cycle

Answer 16

A sequence of chemical reactions (with the enthalpy changes) that convert a reactant into a product.
The total enthalpy change of the sequence of the reactions will be the same as that for the conversion of the reactant to the product directly (or by any other route).

Question 17

Hess’s Law

Answer 17

Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 18

Mean bond Enthalpy

Answer 18

The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds.

Question 19

Bond disassociation enthalpy

Answer 19

The enthalpy change required to break a covalent bond with all species in standard states