Enthalpy Changes

Question 1

Enthalpy

Answer 1

Measure of heat energy in a chemical system
Can be thought of as the energy stored within bonds
Only changes can be measured

Question 2

Enthalpy change

Answer 2

Delta H, difference in enthalpies

Can be positive or negative

Question 3

Law of conservation of energy

Answer 3

Energy cannot be made or destroyed, it can only be transferred from 1 to another
Chemical reactions occur between systems and surroundings
System, chemicals
Surroundings, apparatus

Question 4

Chemical system

Answer 4

Atoms, molecules, ions making up chemicals

Question 5

exothermic reactions

Answer 5

Energy from system to surroundings
Surroundings increase in energy
-ve delta H

Question 6

Endothermic reactions

Answer 6

Energy from surroundings to system
Surroundings decrease in energy
+ve delta H

Question 7

Activation energy

Answer 7

Energy input required to break bonds which act as a barrier to reactions
Minimum energy required for a reaction to take place

Question 8

Standard conditions for standard enthalpy changes

Answer 8

Standard pressure, 101kPa, 1 atm
Standard temperature, 298K, 25C
Standard concentration, 1 mol dm-3 (solutions only)
Standard state, physical state under standard conditions
kJ mol-1

Question 9

Standard enthalpy change of reaction

Answer 9

Standard enthalpy change of reaction that accompanies a reaction in molar quantities in a chemical reaction
Depends on balancing numbers

Question 10

Standard enthalpy change of formation

Answer 10

Enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions

Question 11

Standard enthalpy change of combustion

Answer 11

Enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions

Question 12

Standard enthalpy change of neutralisation

Answer 12

Enthalpy change that accompanies the reaction of an acid by a base to form 1 mole of water under standard conditions

Same for all neutralisation reactions

Question 13

Measuring energy changes

Answer 13

Q=mc delta t

Calculated from mass, specific heat capacity and temperature changes.
C=4.18J g-1 K-1

Question 14

Determination of delta c H

Answer 14

Weigh spirit burner
Place under colorimeter filled with 150 cm3 water
Measure initial temperature
Burn fuel
Measure temperature of water, weigh spirit burner

Find the moles burnt
Use the mass of water and use q=mc delta t
Q/moles=enthalpy change

Question 15

How accurate is the experiment for delta c H

Answer 15

Heat loss to surroundings other than water
Incomplete combustion of methanol
Evaporation of methanol from wick
Non standard conditions

Use fraught screens and input of oxygen gas, minimise errors from heat loss, incomplete combustion

Question 16

Determination of delta r H

Answer 16

Reactions between 2 solutions, solid and solution
Polystyrene cups, offer insulation against heat loss to surroudings
When carrying out reactions between aqueous solutions, the solution itself is the immediate surrounding
Chemical particles within solution react when they collide, energy transferred between chemical particles and water molecules

Same method as before, use mass of solution only in first calculation

Question 17

Determination of delta neut H

Answer 17

2 solutions for water and and a salt

Same method as before, the total volume is the mass for the first calculation