Chemical energetics

Question 1

what is the first law of thermodynamics?

Answer 1

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

Question 2

what is an exothermic reaction and what effect does this have on the system and surrounding ?

Answer 2

A reaction where heat energy is released to the surroundings. This causes the systems temperature to reduce and the surroundings temp to increase

Question 3

what is an endothermic reaction and what effect does this have on the system and surrounding?

Answer 3

A reaction where heat energy is transferred from the surroundings to the system, this results in the surroundings temperature reducing and the systems temperature increasing

Question 4

What are the conditions for standard condition?

Answer 4

100kpa

298K

Question 5

What is chemical energy made up of, give definitions of them

Answer 5

kinetic energy: measure of motion of particles in a substance
potential energy: measure of how strongly the particles interact with one another

Question 6

define enthalpy

Answer 6

measure of the total energy of a system

Question 7

How can the enthalpy of a system be detemined

Answer 7

by measuring the enthalpy change that occurs during a physical or chemical change

Question 8

What is enthalpy change?

Answer 8

The heat energy transferred between the system and surroundings at a constant pressure

Question 9

What is meant by ‘system’

Answer 9

The substances that are reacting (the reaction mixture)

Question 10

what is meany by ‘surroundings’

Answer 10

Everything outside the system- that aren’t actually a part of the reaction mixture. (For example the air in the room where a reaction is taking place)

Question 11

What things must we include in enthalpy level diagrams?

Answer 11

-Formulae and state symbol for both reactants & products
- Value for enthalpy change including correct sign (+ or -)
- Arrow pointing from reactant to products
-Labelled axis
x= progress of reaction
y= enthalpy

Question 12

How do we calculate the enthalpy change of a reaction

Answer 12

enthalpy of products - enthalpy of reactants

Question 13

What is meant by standard enthalpy change of combustion

Answer 13

The enthalpy change measured at 100kpa and 298K when ONE MOLE of a substance is completely burned in oxygen

Question 14

How do we calculate the enthalpy change of combustion

Answer 14

Use Q=mc^T to calculate the heat energy of the entire system

Then divide Q by the number of moles of reactant to calculate the enthalpy change per mole of reactant.

Question 15

state what the letters represent and the units for the equation: Q=mc^T

Answer 15

Q=heat energy (J or KJ)
m= mass (g)
c= specific heat capacity (J g-1 K-1)
^T= change in temperature (k same value as in C)

Question 16

How to experimentally determine the enthalpy of combustion of a liquid (calorimetry)

Answer 16

• weigh liquid in a spirit burner
• add known volume of water to copper can
• measure original temp of water
• burn the burner
• stir mixture with thermometer until changes are no longer seen
• measure final temp
• use q=mc^T to get enthalpy change

Question 17

states a few sources of errors and assumptions made in calorimetry experiment

Answer 17

• some of heat energy may be transferred to air or copper can and not water
  -The conditions were not standard resulting in production of water vapour instead of liquid water
  -Experiment takes a long time, not all of heat energy transferred from water to surroundings is compensated for
  -

Question 18

What is meant by standard enthalpy change of neutralisation

Answer 18

The enthalpy change measured at 100kpa and 298k when ONE mole of water is produced by the neutralisation of an acid with an alkali

Question 19

Why are some standard enthalpy change of neutralisation values constant for some acids and alkalis

Answer 19

As strong acids and alkali’s fully disociate in aqueous solutions, so the reaction is essentially the same in each of them, so makes sense that enthalpy changes are similar.
(H+ + OH- –> H2O

Question 20

How to experimentally determine the enthalpy change of neutralisation

Answer 20

• pipette acid into polysterene cup
• measure temp of acid
• pipette alkali with a slighty stronger concentration into beaker(this is to ensure all of acid is neutralised
• measure temp of alkali
• add alkali to acid and stir with thermometer, record maximum temperature reached

Question 21

How do we calculate the enthalpy change of neutralisation

Answer 21

Use Q=mc^T to calculate the heat energy of the entire solution
Then divide Q by the number of moles of reactant (acid or alkali) to calculate the enthalpy change per mole of reactant.

Question 22

state any sources of error and assumptions made in the experiment (enthalpy change of neutralisation)

Answer 22

• measurement uncertainties with pipette and thermometer

- some of heat energy will be transferred to cup and thermometer

Question 23

what is meant by standard enthalpy change of formation

Answer 23

The enthalpy change measured at 100kpa and 298K when ONE mole of a substance is formed from its elements in their standard states

Question 24

What does Hess law state?

Answer 24

The enthalpy change of a reaction is independent of the path taken in converting reactants to products as well as conditions are the same in each case

Question 25

What is meant by bond enthalpy

Answer 25

The enthalpy change when ONE mole of a bond in gaseous state is broken