4.1 - 4.7 Energetics

Question 1

Define exothermic

Answer 1

Reaction gives out energy to surroundings (Temp increases)

Question 2

Define endothermic

Answer 2

Reaction takes in energy from the surroundings (Temp decreases)

Question 3

Define energetics/thermochemistry/thermodynamics

Answer 3

The study of heat changes during chemical reactions

Question 4

Define temperature

Answer 4

• A measure of the average kinetic energy of the particles in a system
• Faster particles, greater kinetic energy, higher temp

Question 5

Define heat

Answer 5

• A measure of the total energy of all the particles in a given amount of substance

Question 6

Define enthalpy change (∆H)

Answer 6

A heat change at constant pressure
-∆H = Exothermic reaction
+∆H = Endothermic reaction

Question 7

Define standard enthalpy change (ΔH°)

Answer 7

The heat change measured under standard conditions (100kPa and a stated temp, usually 298K) with all reactants and products in their standard states

Question 8

Define standard enthalpy change of combustion (∆Hc°)

Answer 8

The enthalpy change when one mole of an element or compound is completely burned in oxygen, under standard condition, with all reactants and products in their standard states

Question 9

Define standard enthalpy change of formation (∆Hf°)

Answer 9

The enthalpy change when one mole of an element or compound is formed from its element, under standard condition, with all reactants and products in their standard states

Question 10

∆Hf°(any element) =

Answer 10

0

Question 11

What is the equation for calculating heat energy transferred?

Answer 11

q = m x c x ∆T

q = Heat energy change (J)
m = Mass of water/solution
c = Specific heat capacity of water (4.18 J K⁻¹ g⁻¹ )
∆T = Temp change

Question 12

What is the equation for converting heat energy transferred into enthalpy?

Answer 12

∆H = (q/1000)/moles

∆H = Enthalpy change (kJ mol⁻¹)
q = Heat energy change (J)

MUST MAKE IT NEGATIVE IF THE TEMP INCREASED

Question 13

What is the name of the technique used to measure the enthalpy change of a reaction?

Answer 13

Calorimetry

Question 14

Name the biggest source of error in calormietry and how to reduce it

Answer 14

• Loss of heat to the surroundings
• Reduce by = Insulation, bung callorimeter

Question 15

State the 1st Law of Thermodynamics

Answer 15

Energy cannot be created or destroyed, only converted from one form to another

Question 16

State Hess’s Law

Answer 16

The enthalpy change for a reaction is the same, whatever route is taken from reactants to products
∆H1 = ∆H2 + ∆H3

Question 17

Which way do the arrows in an energy cycle point when formation is at the bottom?

Answer 17

Up

Question 18

Which way do the arrows in an energy cycle point when combustion is at the bottom?

Answer 18

Down

Question 19

Define bond dissociation enthalpy

Answer 19

The enthalpy (energy) required to break a particular covalent bond in a molecule when all species are in the gaseous state

Question 20

Define mean bond enthalpy

Answer 20

The average value of the bond dissociation enthalpy (enthalpy to break a bond) in a particular type of covalent bond in a range of different compounds

Question 21

Describe how to improve temperature recording for an enthalpy of reaction measurement

Answer 21

• Record the temp for a suitable time before adding the metal
• To establish an accurate initial temp
• Record temp at regular intervals of time
• Plot temp against time on a graph

Question 22

Describe how to improve analysis of results for an enthalpy of reaction measurement

Answer 22

• Extrapolate the cooling back to the point of addition
• To establish a theoretical temp change

Question 23

How to work out enthalpy change of a reaction when the bonds are given?

Answer 23

Bonds broken - Bonds formed

Question 24

How do you calculate K1 from a graph of concentration against time?

Answer 24

Gradient
mol dm-3 s-1