Enthalpy

Question 1

What is enthalpy

Answer 1

The heat energy stored in chemical bonds. It’s a measure of energy content of a substance at a constant pressure

Question 2

What happens if the enthalpy increases

Answer 2

• reaction is endothermic (heat taken in fro the surroundings)
• change is +ve

Question 3

What happens in the enthalpy decreases

Answer 3

• Reaction is exothermic ( heat lost to the surroundings)
• change is -ve

Question 4

Enthalpy change equation

Answer 4

Enthalpy of products - enthalpy of reactants

Question 5

Standard enthalpy change
Pressure
Temperature

Answer 5

100kPa (1 atmosphere)
298k (25 degrees)

Question 6

Common exothermic reactions

Answer 6

• combustion
• respiration
• self heating cans

Question 7

Common endothermic reactions

Answer 7

• photosynthesis
• cooling energy packs
• thermal decomposition

Question 8

How to work out enthalpy change on a graph

Answer 8

Bond enthalpies of bonds broken - bond enthalpies of bonds made

Question 9

What is the standard enthalpy of formation

Answer 9

The enthalpy change when one mole of a compound is formed in its standard state from its element in their standard states

Question 10

Features of standard enthalpy of formation

Answer 10

• usually, but not alway exothermic
• only one mole of product on the right hand side of the equation
• elements in their standard states have zero enthalpy of formation

Question 11

Standard enthalpy of combustion

Answer 11

The enthalpy change of one mole of a substance undergoes complete combustion under standard conditions. All reactants and products are in their standard states

Question 12

Features of standard enthalpy of combustion

Answer 12

• always one mole of what you are burning on the left hand side of the equation
• always exothermic

Question 13

Standard enthalpy of reaction

Answer 13

The enthalpy change when molar quantities, as shown in an equation reacts in their standard states

Question 14

Features of standard enthalpy of reaction

Answer 14

• enthalpy change is given
• equations must be balanced

Question 15

Standard enthalpy of neutralisation

Answer 15

Th enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

Question 16

what is calorimetry?

Answer 16

practical determination of enthalpy changes. this usually involves heating/cooling known amounts of water

Question 17

calculation for heat energy

Answer 17

q = m c T

q = heat energy joules
m = mass of object releasing or absorbing q (grams) usually of the surrounding water
c = specific heat capacity, water is 4.18 Jg-1K-1
t = change in temp (t final - t initial) K or degrees

Question 18

calculation for enthalpy changes

Answer 18

change in H = -q/ n

q = heat energy
n = number of moles

Question 19

mass and moles in a neutralisation reaction

Answer 19

m = mass of acid and alkali that react
n = moles of water that are formed in a reaction, from the balanced equation

Question 20

what is Hess’s law?

Answer 20

enthalpy change is independent of the path taken providing that the starting and final conditions are the same for both pathways

Question 21

what is an exothermic reaction?

Answer 21

enthalpy of the products are smaller than the enthalpy of reactions resulting in the heat loss to the surroundings

Question 22

what is an endothermic reaction?

Answer 22

enthalpy of the products are greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings

Question 23

definition of standard conditions

Answer 23

conditions required for all enthalpy calculations. all substances must be in their normal/ most stable physical state

Question 24

why may calculated enthalpy values be different from experimental values?

Answer 24

• value of enthalpy change calculated may not be standard value .e.g. solids or liquids produced
• an average form several different compounds

Question 25

what are potential sources of enthalpy practical errors

Answer 25

• fuel may not have burned completely (incomplete combustion)
• conditions aren’t standard
• evaporation of alcohol
• heat capacity of the calorimeter is not included
• heat transferred to the air not water
• experiment takes time, not all of the heat energy transferred from the water too surroundings were compensated for

Question 26

how would you improve accuracy in an enthalpy practical?

Answer 26

• use a lid
• use a bomb calorimeter, its designed to ensure that the sample burns completely and energy losses are minimised

Question 27

What is enthalpy change

Answer 27

Heat change at constant pressure

Question 28

What has a higher enthalpy a liquid or a solid

Answer 28

Liquid

Question 29

What are the standard condition

Answer 29

Pressure 100kPa
Temp 298k

Question 30

Why is graphite more thermodynamically stable than diamond

Answer 30

The change of graphite into diamond is endothermic/ change tends to go in the direction of lower energy level

Question 31

Disadvantages of using a glass beaker on a tripod and gauze

Answer 31

• glass beaker is a poorer conductors than copper
• Tripod and gauze would reduce heat transfer

Question 32

How could you reduce percentage uncertainty in the temperature change during a calorimetry experiment?

Answer 32

Increase the concentration of a solutionS

Question 33

Why is the heat change Calculated from the bomb colorimeter experiment Not an Enthalpy change

Answer 33

The pressure is not constant in a bomb calorimeter

Question 34

How can you determine an accurate value for the enthalpy of reaction in a calorimetry experiment?

Answer 34

• place a lid on a beaker to reduce heat loss
• Use a calorimeter instead of a beaker
• Extrapolate back to the addition, so you can find a temperature change
• Record temperature in set intervals, so you can plot a graph
• Measure the initial temperature of a solution before addition
• Measure the temperature after addition at regular intervals
• We have a glass after adding a solid