3.2.1 Enthalpy Changes

Question 1

What’s enthalpy

Answer 1

The heat change in a reaction (hotter/colder)

Question 2

Define enthalpy change (🔺H)

Answer 2

Heat energy change at constant pressure when a chemical process occurs in molar amounts

Question 3

What does the triangle symbol mean

Answer 3

Change in

Question 4

What’s the standard enthalpy change

Answer 4

Enthalpy change occurring under standard conditions

298K (25*c room temperature)
100kPa (1 atm)

Question 5

What’s the units of standard enthalpy change

Answer 5

Jmol-1/KJmol-1

Question 6

When is an element/compound in its standard state

E.g

Answer 6

When it’s under standard conditions

E.g H2O(l), CO2(g), H2(g)

Question 7

What do exothermic reactions do as they proceed

E.g

Answer 7

Give out heat

E.g neutralisations, combustions

Question 8

What symbol shows reaction taken place under standard conditions

Answer 8

Circle with line through it

Question 9

What symbol shows reactions is exothermic

Why

Answer 9

A minus (-)
As heat energy is lost to the surroundings (negative absorption)

Question 10

What’s an exothermic enthalpy diagram always like

Answer 10

Reactants always higher than products

Question 11

What do endothermic reactions do

Answer 11

Absorb heat

Question 12

What type of reactions are most common

Answer 12

Exothermic

Question 13

What symbol shows the reaction is endothermic

Why

Answer 13

Plus (+)

As heat energy is being gained (absorbed) by chemicals

Question 14

What’s an endothermic diagrams always like

Answer 14

Reactants always lower enthalpy than products

Question 15

What is the method used to find the enthalpy change in a chemical reaction

Answer 15

Calorimetry

Question 16

Why can we use water to measure heat changes

Answer 16

Due to knowing specific heat capacity

Question 17

Define specific heat capacity

Answer 17

Amount of energy needed to raise temperature of 1kg of substance by 1*c

Question 18

What’s the specific heat capacity of water

Answer 18

4.18Jg-1K-1

Question 19

What equation can we use to work out the amount of heat released/absorbed by the reaction if we find out the temperature change

Answer 19

q = mc🔺t

q - heat energy (J)
m - mass of water (g)
c - SHC of water
🔺t - change in temp (K)

Question 20

2 ways water can absorbed heat from a reaction

Answer 20

Directly

Indirectly

Question 21

What’s a direct method of water absorbing heat

Answer 21

Chemicals are dissolved in water + transfer their energy directly to water causing temperature changes

Question 22

What’s an indirect method of water absorbing heat

Answer 22

Chemicals aren’t in direct contact with water, but heat is transferred through the surrounding air to the water, causing a temperature chance

Question 23

What’s a problem with an indirect method of heating water

Answer 23

It’s inefficient

Question 24

What are the 2 direct reactions

Answer 24

1. Calculating standard enthalpy change of a MASH reaction

2. Calculating standard enthalpy neutralisation

Question 25

What’s a MASH reaction

Answer 25

Metal + Acid -> Salt + Hydrogen

Question 26

What can we measured in a MASH reaction (direct)

Answer 26

The water temperature rising as heat energy is generated

Question 27

Equipment for direct MASH reaction

Answer 27

``` Polystyrene cup Lid HCl (aq) Magnesium metal Thermometer ```

Question 28

Method of direct MASH reaction (4)

Answer 28

1. Measure + record the mass of magnesium ribbon 2. Measure + record the temperature of the HCl(aq) in the cup (with the Mg added) 3. Add the metal to the acid + stir gently 4. Measure the maximum temperature rise on the thermometer

Question 29

What type of reaction is the direct MASH reaction

Answer 29

Exothermic

Question 30

Define 🔺H* neutralisation

Answer 30

The enthalpy change when an acid reacts with a base to form 1 mole of water, under standard conditions and all reactants and products are in standard state

Question 31

Equipment for 🔺H* neutralisation

Answer 31

2 polystyrene cups and lids Thermometers 50cm3 HCl 50cm3 NaOH

Question 32

Method for calculating 🔺H* neutralisation (4)

Answer 32

1. Ensure temperatures of each solutions are the same (put in same room overnight) 2. Record HCl temperature for 3 minutes every minute 3. Add NaOH on 4th minute (don’t take temp reading for this) 4. Continue to record temperature every minute

Question 33

Why should a polystyrene cup be used | Why should a lid be used

Answer 33

Insulating material | Lid prevents heat loss

Question 34

Ionic equation for 🔺H* neutralisation

Answer 34

H+(aq) + OH-(aq) -> H2O(l)

Question 35

What’s the indirect method

Answer 35

Calculating 🔺H* combustion of methanol

Question 36

What can methanol be used as

Answer 36

A fuel

Question 37

Equipment for indirect combustion of methanol

Answer 37

Clamp + stand Alcohol burner with methanol 100cm3 water Beaker (often copper)

Question 38

Method of indirect combustion of methanol (6)

Answer 38

1. Measure + record mass of the spirit burner before heating 2. Place 100cm3 water in the copper beaker 3. Measure + records water’s temperature 4. Light burner and stir the water gently 5. Extinguish the burner once temperature has risen by 20*c 6. Measure mass of burner after heating

Question 39

What is more common in the indirect method | Why

Answer 39

Errors like heat loss | as the thermometer is in direct contact with the reaction mixture

Question 40

Define standard enthalpy of combustion (🔺H*c)

Answer 40

The enthalpy change when 1 mole of an element/compound is completely burned in oxygen under standard conditions, with all reactants and products in their standard states

Question 41

Define standard enthalpy of formation (🔺H*f)

Answer 41

The enthalpy change at constant pressure when 1 mole of a substance is formed from its elements with all reactants + products in their standard states. All reactants and products are under standard conditions (100kPa, 298K)

Question 42

Which standard enthalpy is backwards as we know products

Answer 42

Formation

Question 43

What is Hess’s law

Answer 43

In a reaction, the total enthalpy change is independent of the route taken

Question 44

4 steps to calculating enthalpy changes using Hess’s law

Answer 44

1. Write desired chemical equation 2. Write down the data chemical equations 3. Construct a cycle with desired equation on top line 4. Identify where all arrows start and end + then use Hess’s law

Question 45

When do arrows point up/down

Answer 45

Down - combustion | Up - formation

Question 46

When does the sign become positive using Hess’s law

Answer 46

If going in the opposite direction of the arrow

Question 47

What’s the short cut equation for Hess’s law combustion Acronym

Answer 47

total🔺H*c reactants - total🔺H*c products CR-P Combustion = reactants-products

Question 48

What’s the short cut equation for Hess’s law formation Acronym

Answer 48

Total🔺H*f products - total🔺H*f reactants FP-R

Question 49

Example of an endothermic reaction

Answer 49

Thermal decomposition

Question 50

What’s bond dissociation energy

Answer 50

Energy needed to break a particular covalent bond

Question 51

Define average bond enthalpy

Answer 51

Enthalpy needed to break 1 mole of bonds in gaseous molecules averaged over different molecules

Question 52

What’s the value of both bond dissociation energy and average bond enthalpy always Why

Answer 52

Always positive in value | As we need to put energy in to a chemical to break a bond (endothermic)

Question 53

Bond enthalpy shortcut

Answer 53

Broken - formed