Unit 1: Section 4 - Energetics MDY *

Question 1

What is an enthalpy change?

Answer 1

ΔH - heat energy transferred in a reaction at constant pressure

Question 2

What does ΔH⦵ mean?

Answer 2

The substances were in their standard states and the measurement was made under standard conditions.

Question 3

What are standard conditions?

Answer 3

100kPa pressure

A stated temperature (e.g. 298 K/ 25*C)

Question 4

What does exothermic mean?

Answer 4

Exothermic reactions give out energy 
ΔH is negative 
Oxidation reactions are usually exothermic 
e.g. combustion of fuel like methane
Oxidation of carbohydrates like glucose

Question 5

What does endothermic mean?

Answer 5

Endothermic reactions absorb energy
ΔH is positive
e.g. Thermal decomposition of calcium carbonate

Question 6

What happens to bonds of reactants when they react?

Answer 6

Energy is needed to break bonds, endothermic

Stronger bonds need more energy

Question 7

What happens to bonds of products when they react?

Answer 7

Energy is released when bonds are formed, exothermic

Stronger bonds release more energy when they form

Question 8

How to find whether a reaction is endothermic or exothermic?

Answer 8

The enthalpy change for a reaction is the overall effect if the breaking and making of bonds
If more energy is needed to break bonds then released when made ΔH is positive and vice versa

Question 9

What is bond enthalpy?

Answer 9

The energy required to break bonds
Mean bond enthalpy = the average energy needed to break a certain type of bond over a range of compounds
Bond enthalpy are always positive

Question 10

What is the overall enthalpy change of a reaction?

Answer 10

Enthalpy change of reaction = total energy absorbed- total energy released
OR
energy to break all bonds - energy to make all bonds

Question 11

Why aren’t enthalpy changes calculated from bond enthalpy exact?

Answer 11

Average values for bond enthalpy’s are used, so enthalpy changes calculated from them aren’t exact

Question 12

What is the standard enthalpy of formation ΔfH⦵?

Answer 12

The enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions

Question 13

What is the standard enthalpy of combustion ΔcH⦵?

Answer 13

The enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions

Question 14

What is calorimetry?

Answer 14

the process of measuring the amount of heat released or absorbed during a chemical reaction

Question 15

How can you find the enthalpy of combustion of a flammable liquid?

Answer 15

Burn it inside a flame calorimeter
As the fuel burns in a bottle with a wick, it heats the water. You can work out the heat energy that has been absorbed by the water if you know the mass of the water, the temperature change and the specific heat capacity of the water
Ideally heat given out by fuel would be absorbed by the water so you can work out enthalpy change exactly

Question 16

What is the specific heat capacity of water?

Answer 16

4.18 J g^-1 K^-1

Question 17

What downsides do calorimetry tests have?

Answer 17

In any calorimetry experiment, you always lose heat to the surroundings. This makes it hard to get an accurate result
Some of the combustion may be incomplete so less energy is given out
Flammable liquids are often quite volatile too, so you may lose some fuel to evaporation

Question 18

How can calorimetry be used to measure enthalpy changes in solution?

Answer 18

Neutralisation
Dissolution (dissolving)
Displacement

Question 19

How to find the enthalpy change for a neutralisation reaction?

Answer 19

Add a known volume of acid to an insulated container and measure the temperature
Add known volume of alkali and record the temperature of the mixture at regular intervals over a period of time
Find the temperature change for the experiment. Use it to calculate enthalpy change

Question 20

What do you need to know to calculate the enthalpy change of a neutralisation reaction?

Answer 20

The mass of the solutions that you’ve used in order to calculate the enthalpy change of the reaction. Assume all solutions have have same density as water
Find number of moles concentration x volume

Question 21

How to find temperature change in calorimetry experiment?

Answer 21

Subtract start temp from highest temp recorded - not very accurate because heat lost to surroundings
Graphs can be used for an accurate value

Question 22

How can you use a graph to find the temperature change in a calorimetry experiment?

Answer 22

Record temperature at regular intervals, beginning a couple of minutes before you start the reaction
Plot a graph, draw 2 lines of best fit: 1 through the points before the reaction started and 1 through the points after it started
Extend both lines so they both pass the time when the reaction started
The distance between the 2 lines at the time started is the accurate temperature change for the reaction

Question 23

equation to calculate enthalpy change from a calorimetry experiment?

Answer 23

q = mcΔT
q = heat lost or gained (enthalpy change) (J)
m = mass of substance heated (g)
c = specific heat capacity (J/g/K)
ΔT = change in temperature of substance (K)

Question 24

what is Hess’s law?

Answer 24

the total enthalpy change for a reaction is independent of the route taken

Question 25

why is Hess’s law useful?

Answer 25

for working out enthalpy changes that you can’t find directly by doing an experiment

Question 26

how to use Hess’s law from enthalpies of formation?

Answer 26

the reactants and products have the same constituent elements
the energy required to form each mole of reactants + the enthalpy change of the reactants is the same as the energy required to form each mole of product
ΔfH⦵(reactants) + ΔrH⦵ = ΔfH⦵(products)

Question 27

what is the enthalpy of formation of an element?

Answer 27

0 - the element is being formed from an element which is itself so no energy is required

Question 28

how to use Hess’s law from enthalpies of combustion?

Answer 28

both reactants and products burn to form the same products
reaction energy + energy to burn products = energy to burn reactants
ΔcH⦵(products) + ΔrH⦵ = ΔcH⦵(reactants)

Question 29

what is thermochemistry?

Answer 29

the study of enthalpy changes during chemical reactions

Question 30

what is enthalpy?

Answer 30

a measure of the heat content of a substance (H)

Question 31

what does isothermic mean?

Answer 31

energy in a reaction stays the same

Question 32

what happens in an endothermic change?

Answer 32

energy is absorbed from the surroundings by the reactants to form the products
enthalpy of products increases so ΔH is positive

Question 33

what happens in an exothermic change?

Answer 33

energy is released into the surrounding by the reactants to form the products
enthalpy of products has decreased so ΔH is negative

Question 34

what is a transition state in a reaction?

Answer 34

a point where some bonds are being broken and some being made. it is high energy and unstable

Question 35

what is the standard enthalpy of reaction?

Answer 35

ΔrH⦵, the enthalpy change for a reaction with the quantities shown in the chemical reaction

Question 36

what is the standard enthalpy change of neutralisation?

Answer 36

ΔneutH⦵, enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali with standard states under standard conditions

Question 37

how to carry out a calorimetry question?

Answer 37

1. check sig figs
2. check units
3. calculate q using q=mcΔT
4. calculate moles of whatever is reacting to raise temp
5. use ΔH=q/moles
6. use - sign if reaction was exothermic
7. answer to correct sig figs

Question 38

equation to find enthalpy change per mole?

Answer 38

ΔH = q/ number of moles reacting

Question 39

how to use calorimetry of reactions taking place in solution?

Answer 39

reaction carried out in insulated beaker and the temp change measured
reaction must be fast so max temp is reached quickly
specific heat capacity of the solution must be taken as being 4.18 J/g/K the mass of water (not solution) is usually used

Question 40

how to find enthalpy of formation from enthalpies of combustion?

Answer 40

the constituent elements of a substance burn to form the same products so the energy required to burn the elements - energy needed to burn product = enthalpy of formation

Question 41

what is bond energy?

Answer 41

the amount of energy needed to break a bond in the gaseous state
according to Hess’s law it is the same amount of energy needed to form a bond

Question 42

what is bond dissociation energy?

Answer 42

bond energy for diatomic molecules is breaking the only bond in the substance. this is called bond dissociation energy

Question 43

what is mean bond enthalpy?

Answer 43

the enthalpy to break 1 mol of named bonds
averaged over a range of compounds/ molecules
in the gaseous state

Question 44

how to find the enthalpy of a reaction using bond energy’s?

Answer 44

if molecules aren’t in gaseous state, include enthalpy of vaporisation then find bond enthalpies for all reactants and products and subtract energy for products from reactants