9.1 Enthalpy changes

Question 1

Enthalpy, H

Answer 1

is a measure of the heat energy in a chemical system.

Question 2

enthalpy

Answer 2

is sometimes thought as the energy stored within bonds.

Question 3

enthalpy change, ΔH

Answer 3

products - reactants

Question 4

law of conservation of energy

Answer 4

states that energy cannot be created or destroyed

Question 5

energy transfer can be in either of two directions:

Answer 5

• from the system to the surrounding = exothermic change
• from the surroundings to the system = endothermic change

Question 6

exothermic ΔH

Answer 6

negative ΔH

Question 7

endothermic ΔH

Answer 7

positive ΔH

Question 8

activation energy, Ea

Answer 8

is the minimum energy required for a reaction to take place

Question 9

reactions with small Ea take place very rapidly

Answer 9

because the energy needed to break bonds is readily available from the surroundings

Question 10

standard conditions, ⦵

Answer 10

standard pressure = 100 kPa (1 atm = 101kPa)
standard temperature = 298 K (25°C)
standard concentration = 1 mol dm-3
standard state = is the physical state of a substance under standard conditions (100 kPa and 298 K)

Question 11

standard enthalpy change of reaction, ΔrH ⦵

Answer 11

is the ΔH that accompanies a reaction in the molar quantities shown in a chemical equation
under standard conditions, with all reactants and products in their standard states.

Question 12

standard enthalpy change of formation, ΔfH ⦵

Answer 12

is the ΔH that takes place when one mole of a compound is formed from its elements
under standard conditions, with all reactants and products in their standard states

Question 13

which is correct for ΔfH ⦵:
a) Mg(s) + 0.5O2(g) –> MgO(s)
b) 2Mg(s) + O2(g) –> 2MgO(s)

Answer 13

A. because ΔfH ⦵ is when one mole of a compound is formed

Question 14

which is correct for ΔrH ⦵:
a) Mg(s) + 0.5O2(g) –> MgO(s)
ΔrH ⦵ = -602 kJ mol-1
b) 2Mg(s) + O2(g) –> 2MgO(s)
ΔrH ⦵ = -1204 kJ mol-1

Answer 14

Both, but only when respect to their own equations

Question 15

standard enthalpy change of combustion, ΔcH ⦵

Answer 15

is the ΔH that takes place when one mole of a substance reacts completely with oxygen
under standard conditions, with all reactants and products in their standard states

Question 16

the equation for the combustion of one mole of butane is shown below

Answer 16

C4H10(g) + 6.5O2(g) –> 4CO2(g) + 5H2O(l)
ΔcH ⦵ = -2877 kJ mol-1
the butane (C4H10) has to be one mole in a combustion reaction, so O2 has to be 6.5 moles

Question 17

standard enthalpy change of neutralisation, ΔneutH ⦵

Answer 17

is the ΔH that accompanies the reaction of an acid by a base to form one mole of H2O(l)
under standard conditions, with all reactants and products in their standard states

Question 18

using the ionic equation below, workout the neutralisation of HCl by NaOH to form one mole of H2O
H+ (aq) + OH-(aq) –> H2O(l)
acid + base –> 1 mol
ΔneutH ⦵ = -57 kJ mol-1

Answer 18

HCl(aq) + NaOH(aq) –> H2O(l) + NaCl(aq)
acid + base –> 1mol water + salt
the value of a ΔneutH ⦵ Is the same for all neutralisation reactions