4 - Energetics

Question 1

Define ‘enthalpy change’

Answer 1

The heat energy change measured under conditions of constant pressure

Question 2

What units is enthalpy change (ΔH) measured in?

Answer 2

kJ mol^-1

Question 3

What are the standard conditions?

Answer 3

• 298K (25°C)
• 100kPa
• Solutions at 1 moldm^-1

Question 4

Reactions can be ____thermic or _____thermic

Answer 4

Exothermic or endothermic

Question 5

What happens in an exothermic reaction?

Answer 5

Energy is transferred from the chemicals to the surroundings

- Enthalpy change is negative

Question 6

In exothermic reactions, do the products have more or less energy than the reactants?

Answer 6

Less energy

Question 7

What are some examples of exothermic reactions?

Answer 7

• Combustion
• Neutralisation

Question 8

What happens in an endothermic reaction?

Answer 8

Energy is transferred from the surroundings to the chemicals

- enthalpy change is positive

Question 9

In endothermic reactions, do the products have more or less energy than the reactants?

Answer 9

More energy

Question 10

What are some examples of endothermic reactions?

Answer 10

• Thermal decomposition

Question 11

Define standard enthalpy of formation

Answer 11

The enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, all reactants and products being in their standard states

Question 12

Formation example - magnesium chloride

Answer 12

Mg _(s) + Cl_{2 (g)} → MgCl_{2 (s)}

Question 13

Define standard enthalpy of combustion

Answer 13

The enthalpy change when 1 mole of substance is combusted completely in oxygen under standard conditions, all reactants and products being in their standard states

Question 14

Combustion example - methane

Answer 14

CH_{4 (g)} + 2O_{2 (g)} → CO_{2 (g)} + 2H₂O _(l)

Question 15

What does incomplete combustion produce?

Answer 15

• Soot (carbon)
• Carbon monoxide
• Water

Question 16

Calorimetry

What equation do we use to measure the enthalpy change in solution?

Answer 16

Energy change = mass of solution x heat capacity x temperature change

q = m x c x ΔT

Question 17

Calorimetry

What are the units of q = m x c x ΔT?

Answer 17

q = J
m = grams
c = J g^-1K^-1
ΔT = K

Question 18

Calorimetry

What equation do we use to calculate the enthaply change per mole?

Answer 18

ΔH (per mole) = q / number of moles

Question 19

What is Hess’s Law?

Answer 19

That the total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 20

What is meant by ‘mean bond enthalpy’?

Answer 20

The average enthalpy change when 1 mole of bonds in gaseous molecules are broken

Question 21

How do we calculate the enthalpy change for reactions in the gaseous phase?

Answer 21

ΔH = ∑ bond energies broken - ∑ bond energies made

Question 22

Why do values from mean bond enthalpy calculations differ from those determined by Hess’s law?

Answer 22

Mean bond enthalpy calculations use mean values while Hess’s law uses enthalpy data from individual compounds so therefore values calculated from Hess’s law will be more accurate

Question 23

Bond breaking __________ energy

Answer 23

requires

Question 24

Bond making ___________ energy

Answer 24

Releases