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Chem 3 (3.2 Physical Chemistry) > 3.2.1 Enthalpy Changes > Flashcards

3.2.1 Enthalpy Changes Flashcards

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1
Q

What’s enthalpy

A

The heat change in a reaction (hotter/colder)

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2
Q

Define enthalpy change (🔺H)

A

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

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3
Q

What does the triangle symbol mean

A

Change in

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4
Q

What’s the standard enthalpy change

A

Enthalpy change occurring under standard conditions

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

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5
Q

What’s the units of standard enthalpy change

A

Jmol-1/KJmol-1

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6
Q

When is an element/compound in its standard state

E.g

A

When it’s under standard conditions

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

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7
Q

What do exothermic reactions do as they proceed

E.g

A

Give out heat

E.g neutralisations, combustions

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8
Q

What symbol shows reaction taken place under standard conditions

A

Circle with line through it

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9
Q

What symbol shows reactions is exothermic

Why

A 
A minus (-)
As heat energy is lost to the surroundings (negative absorption)
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10
Q

What’s an exothermic enthalpy diagram always like

A

Reactants always higher than products

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11
Q

What do endothermic reactions do

A

Absorb heat

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12
Q

What type of reactions are most common

A

Exothermic

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13
Q

What symbol shows the reaction is endothermic

Why

A

Plus (+)

As heat energy is being gained (absorbed) by chemicals

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14
Q

What’s an endothermic diagrams always like

A

Reactants always lower enthalpy than products

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15
Q

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

A

Calorimetry

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16
Q

Why can we use water to measure heat changes

A

Due to knowing specific heat capacity

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17
Q

Define specific heat capacity

A

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

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18
Q

What’s the specific heat capacity of water

A

4.18Jg-1K-1

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19
Q

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

A

q = mc🔺t

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

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20
Q

2 ways water can absorbed heat from a reaction

A

Directly

Indirectly

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21
Q

What’s a direct method of water absorbing heat

A

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

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22
Q

What’s an indirect method of water absorbing heat

A

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

23
Q

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

A

It’s inefficient

24
Q

What are the 2 direct reactions

A
  1. Calculating standard enthalpy change of a MASH reaction

2. Calculating standard enthalpy neutralisation

25
Q

What’s a MASH reaction

A

Metal + Acid -> Salt + Hydrogen

26
Q

What can we measured in a MASH reaction (direct)

A

The water temperature rising as heat energy is generated

27
Q

Equipment for direct MASH reaction

A 
Polystyrene cup
Lid
HCl (aq)
Magnesium metal
Thermometer
28
Q

Method of direct MASH reaction (4)

A
  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
29
Q

What type of reaction is the direct MASH reaction

A

Exothermic

30
Q

Define 🔺H* neutralisation

A

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

31
Q

Equipment for 🔺H* neutralisation

A

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

32
Q

Method for calculating 🔺H* neutralisation (4)

A
  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
33
Q

Why should a polystyrene cup be used

Why should a lid be used

A

Insulating material

Lid prevents heat loss

34
Q

Ionic equation for 🔺H* neutralisation

A

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

35
Q

What’s the indirect method

A

Calculating 🔺H* combustion of methanol

36
Q

What can methanol be used as

A

A fuel

37
Q

Equipment for indirect combustion of methanol

A

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

38
Q

Method of indirect combustion of methanol (6)

A
  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
39
Q

What is more common in the indirect method

Why

A

Errors like heat loss

as the thermometer is in direct contact with the reaction mixture

40
Q

Define standard enthalpy of combustion (🔺H*c)

A

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

41
Q

Define standard enthalpy of formation (🔺H*f)

A

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)

42
Q

Which standard enthalpy is backwards as we know products

A

Formation

43
Q

What is Hess’s law

A

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

44
Q

4 steps to calculating enthalpy changes using Hess’s law

A
  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
45
Q

When do arrows point up/down

A

Down - combustion

Up - formation

46
Q

When does the sign become positive using Hess’s law

A

If going in the opposite direction of the arrow

47
Q

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

Acronym

A

total🔺Hc reactants - total🔺Hc products

CR-P

Combustion = reactants-products

48
Q

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

Acronym

A

Total🔺Hf products - total🔺Hf reactants

FP-R

49
Q

Example of an endothermic reaction

A

Thermal decomposition

50
Q

What’s bond dissociation energy

A

Energy needed to break a particular covalent bond

51
Q

Define average bond enthalpy

A

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

52
Q

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

A

Always positive in value

As we need to put energy in to a chemical to break a bond (endothermic)

53
Q

Bond enthalpy shortcut

A

Broken - formed

Chem 3 (3.2 Physical Chemistry) (3 decks)

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