Energetics

Question 1

exothermic reaction

Answer 1

When heat energy released forming bonds in the products is greater than the energy required to break bonds in the reactant, energy is transferred to the surroundings so enthalpy change is negative

Question 2

example of exothermic reaction (2)

Answer 2

combustion and displacement

Question 3

use of exothermic reaction example

Answer 3

combustion of wood/ fossil fuels to heat homes

Question 4

endothermic reaction

Answer 4

When the energy released when forming bonds in the products is less than the energy required to break the bonds in the reactants, energy is transferred from surroundings

Question 5

example of endothermic reaction (2)

Answer 5

thermal decomposition and photosynthesis

Question 6

use of endothermic reaction example

Answer 6

cold pack for sports injuries

Question 7

catalyst definition

Answer 7

speeds up the rate of reaction without being chemically changed after the reaction so can be reused

Question 8

how do catalysts work

Answer 8

provides an alternative pathway for the reaction with a lower activation energy so more particles have the activation energy and therefore a higher frequency of collisions are successful

Question 9

activation energy

Answer 9

minimum amount of energy required to break bonds between the atoms of the reactants

Question 10

ΔH

Answer 10

heat energy change measured under conditions of constant pressure

Question 11

Standard enthalpy change of formation (ΔHfƟ)

Answer 11

when one mole of a compound is formed in its elements under standard conditions with all substances in their standard states

Question 12

what is the enthalpy of formation of an element

Answer 12

0kj/mol

Question 13

compare incomplete and complete combustion

Answer 13

Incomplete combustion is less exothermic than complete combustion

Question 14

Standard enthalpy change of combustion (ΔHcƟ)

Answer 14

when one mole of an element or compound reacts completely with oxygen under standard conditions with all elements in their standard states

Question 15

Calculating ΔH per mole in a reaction

Answer 15

Q=mc∆T

• m: mass of solution being heated cm3= g
• c: specific heat capacity 4.18J/g/K
• convert Q into ΔH in kJ by adding negative sign if exothermic and ÷1000

find the number of moles of reactant not in excess

• moles =cv
• moles = mass/Mr

ΔH for 1 mol
- ΔH/ moles (kJ/mol) 3sf

Question 16

calorimetry practical errors (4)

Answer 16

heat loss to surroundings
assuming all solutions have heat capacity of water
ignoring heat absorbing apparatus
incomplete/ slow reaction

Question 17

Hess’s law

Answer 17

the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided the initial and final conditions are the same

Question 18

why is hess’s law used

Answer 18

Used due to difficulties in measuring temperature change (solid)

Question 19

enthalpy of formation equation

Answer 19

ΔH reaction = ΣfH products - ΣfH reactants

Question 20

enthalpy of combustion equation

Answer 20

ΔH reaction = ΣΔcH reactants - ΣΔcH products

Question 21

mean bond energy

Answer 21

enthalpy needed to break the covalent bonds into gaseous atoms averaged over different molecules

Question 22

enthalpy using bond enthalpy data

Answer 22

ΔH = Σ bond energies broken - Σ bond energies formed

Question 23

which is more accurate and why: using mean bond enthalpy values or Hess’s law

Answer 23

Average bond enthalpy values are averaged values from various compounds so less accurate than value using Hess’s law

Question 24

relationship between number of carbon atoms and size of enthalpies of combustion in a homologous series

Why tho?

Answer 24

as number of carbon atoms increases, size of enthalpies of combustion increases (directly proportional) so more exothermic

Able to form more CO2 and H2O molecules which energy released is greater than the energy required to break extra bonds of the reactants from increased size