C4 - Energetics

Question 1

What is an exothermic reaction?

Answer 1

Reaction that release energy

Question 2

What is an endothermic reaction?

Answer 2

Reaction that absorbs energy

Question 3

Is breaking bonds endothermic or exothermic?

Answer 3

Endothermic

Question 4

Is forming bonds endothermic or exothermic?

Answer 4

Exothermic

Question 5

What is enthalpy?

Answer 5

The enthalpy, H, of a system is a measure of the energy stored in (or heat content of) a system. It cannot be measured directly.

Question 6

What is enthalpy change?

Answer 6

Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure

Question 7

What is the units of enthalpy change?

Answer 7

kJ/mol

Question 8

What are standard enthalpy change conditions?

Answer 8

100kPa and 298K

Question 9

What is the standard enthalpy of formation ΔHf?

Answer 9

This is the enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, with all being in their natural states

Question 10

What is the standard enthalpy of combustion ΔHc?

Answer 10

This is the enthalpy change when 1 mole of a substance undergoes complete combustion under standard conditions, with all substances being in their standard states

Question 11

What is the standard enthalpy of atomization ΔHa?

Answer 11

This is the enthalpy change when 1 mole of gaseous atoms of a substance are formed from the substance under standard condition

Question 12

What is the standard enthalpy of neutralization ΔHn?

Answer 12

This is the enthalpy change when 1 mole of water is produced by the reaction of an acid and a base under standard conditions, with all substances being in their standard state

Question 13

What does a negative enthalpy change represent?

Answer 13

Exothermic reaction

Question 14

What does a positive enthalpy change represent?

Answer 14

Endothermic reaction

Question 15

What is the rule when forming a standard enthalpy of formation equation?

Answer 15

The product has to have 1 mole
E.g. H2(g) + ½O2(g) → H2O(l)

Question 16

What is the rule when forming a standard enthalpy of combustion equation?

Answer 16

The substance undergoing combustion has to be 1 mole
E.g. H2 (g) + ½O2 (g) → H2O(l)

Question 17

What is the rule when forming a standard enthalpy of neutralisation equation?

Answer 17

Has to be 1 mole of water
E.g. H+(aq) + OH¯(aq) → H2O(l)

Question 18

What is the specific heat capacity equation?

Answer 18

q = mc∆T
q = enthalpy change in joules
m = mass of substance being heated (often water) in grams
c = specific heat capacity in joules per Kelvin per gram (4.18 JK-1g-1 for water)
∆T = change of temperature in Kelvin

Question 19

What is Hess’s law?

Answer 19

Hess’s law states that the overall enthalpy change for a reaction is independent of the route the reaction takes

Question 20

When using Hess’s law to work out enthalpy of combustion what equation is used?

Answer 20

Sum of reactants - sum of products

Question 21

When using Hess’s law to work out enthalpy of formation what equation is used?

Answer 21

Sum of products - Sum of reactants

Question 22

What is the mean bond enthalpy?

Answer 22

The mean bond enthalpy is a measure of the energy required to break one mole of a covalent bond in the gaseous state averaged (mean) over a range of different compounds containing the bond

Question 23

What do mean bond enthalpy’s relate to?

Answer 23

The strength of a covalent bond. A higher bond enthalpy = stronger covalent bond.
Typically the shorter the covalent bond, the stronger the bond. E.g a triple bond is shorter than a double bond

Question 24

How do you use bond energies to calculate enthalpy change?

Answer 24

Write down equation
Work out how many of each bond type are broken
Use mean bond enthalpies
Substitute values into the equation to calculate enthalpy change when bonds are broken
Repeat for bonds formed
Value will be negative as energy is released
Add too values together for overall enthalpy change