3.2.1 Enthalpy changes Flashcards
1
Q
What is enthalpy?
A
The heat that is stored in a chemical system
2
Q
What is enthalpy change?
A
The difference in enthalpy between the products and reactants in a reaction
3
Q
What contains enthalpy?
A
Elements and compounds
4
Q
What is an exothermic reaction?
A
When heat energy is transferred (from the system) to the surroundings
5
Q
What happens to the temperature in an exothermic reaction?
A
The temperature change of the surroundings increases so the temperature increases
6
Q
Is the enthalpy change negative or positive in an exothermic reaction?
A
Negative
7
Q
Examples of exothermic reactions
A
burning a fuel
oxidation
condensing
neutralising an acid
8
Q
What is an endothermic reaction?
A
A chemical reaction where energy is taken in from the surroundings (into the system)
9
Q
What happens to the temperature in an endothermic reaction?
A
The temperature change of the surroundings decreases, so we see a temperature decrease
10
Q
Is the enthalpy change negative or positive in an endothermic reaction?
A
Positive
11
Q
Examples of endothermic reactions
A
thermal decomposition
photosynthesis
evaporation
12
Q
What should all temperatures always be measured to?
A
One decimal place
13
Q
What is activation energy?
A
The minimum energy required to start a reaction
14
Q
In an exothermic reaction profile what has more energy?
A
The reactants have more energy than the products
15
Q
In an endothermic reaction profile what has more energy?
A
the products have more energy than the reactants
16
Q
Enthalpy profile for an exothermic reaction?
A
17
Q
Enthalpy profile for an endothermic reaction?
A
18
Q
What is the symbol for enthalpy change of reaction?
A
Triangle symbol Hr
19
Q
What is the definition for enthalpy change of reaction?
A
This is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation
E.g. Zn(s) + Cu2+ (aq) —-> Cu(s) + Zn2+ (aq)
20
Q
What is the symbol for enthalpy change of formation?
A
Triangle symbol Hf
21
Q
What is the definition for enthalpy change of formation?
A
This is the enthalpy change when 1 mole of a compound is formed rom its elements under standard conditions
E.g. Na(s) + 1/2 Cl(g) —> NaCl(s)
22
Q
What is the symbol for enthalpy change of combustion?
A
Triangle symbol Hc
23
Q
What is the definition of enthalpy change of combustion?
A
This is the enthalpy change when 1 mole of substance completely combusts, under standard conditions
E.g. CH4(g) + O2(g) —> CO2(g) + 2H2O(g)
24
Q
What is the symbol for enthalpy change of neutralisation?
A
Triangle symbol Hneut
25
What is the definition of enthalpy change of neutralisation?
The enthalpy change when solutions of an acid and alkali react together under standard conditions to produce 1 mole of water
E.g. HCl(aq) + NaOH(aq) ----> NaCl (aq) + H2O(l)
26
What is the symbol for standard conditions?
A zero with a line going across it
27
What is standard pressure?
100KPa
28
What is standard temperature in degrees Celsius?
25
29
What is standard pressure in Kelvin?
298
30
What are standard conditions in solutions only?
1mol dm-3
31
Why may my enthalpy value be different to another experiments?
They may not have used standard conditions
32
What is the equation used for calculating enthalpy change of a reaction?
mass (m) x specific heat capacity (c) x temperature change (triangle symbol T)
33
What are the steps for the calculating enthalpy change calculation?
1 - workout the heat energy (q) using mxcx temp change (IN JOULES)
2 - work out the moles of the limiting reactant (NOT THE ONE IN EXCESS)
3 - workout the enthalpy change by dividing the heat energy by the moles of the limiting reactant
4 - convert any units and add a negative symbol if it is exothermic
ALWAYS EXO IF TEMP HAS INCREASED
34
What is the method for the measuring the enthalpy change of reaction with copper sulphate and zinc practical?
1 - use a pipette and pipette filler to transfer 25.0cm3 of the 1moldm-3 copper (11) sulphate solution into a polystyrene cup which will act as an insulator
2 - weigh out about 6g of zinc powder into a small beaker - this amount means that the zinc powder is in excess
3 - using a thermometer record the temp every 30 seconds of the sopper (11) sulphate solution and stir
4 - after exactly 3 mins add all of the zinc powder and continue to stir and record temp of the solution evert 30 seconds and stop after 10 minutes
35
What is the state of water in standard conditions?
liquid
36
What is the state of methane in standard conditions?
gas
37
What is the state of sulphur dioxide in standard conditions?
gas
38
What is the state of carbon in standard conditions?
solid
39
What is the state of hydrochloric acid in standard conditions?
aqueous
40
What is the state of sodium hydroxide in standard conditions?
aqueous
41
How do you write equations for standard enthalpy of formation?
separate each individual element out
has to be equal to one mole at the end
must include state symbols
42
How do you write equations for standard enthalpy of combustion?
write down the compound with oxygen
most compounds burn to produce carbon dioxide and water
must include state symbols
43
What occurs in an exothermic change?
Energy is transferred from the system (chemicals) to the surroundings.
44
In an exothermic reaction, what is the sign of ΔH?
Negative
45
What are common oxidation exothermic processes?
Combustion of fuels and oxidation of carbohydrates such as glucose in respiration.
46
What happens during an endothermic change?
Energy is transferred from the surroundings to the system (chemicals).
47
What is an example of an endothermic reaction?
Thermal decomposition of calcium carbonate.
48
In an endothermic reaction, what is the sign of ΔH?
Positive
49
What is activation energy?
The minimum energy which particles need to collide to start a reaction.
50
What are the standard conditions for measuring enthalpy changes?
* 100 kPa pressure
* 298 K (room temperature or 25°C)
* Solutions at 1 mol dm-3
* All substances should have their normal state at 298 K
51
What is the definition of enthalpy change of reaction ΔH°?
The enthalpy change when the number of moles of reactants as specified in the balanced equation react together.
52
What is the standard enthalpy change of formation?
The enthalpy change when 1 mole of the compound is formed from its elements under standard conditions.
53
What is the symbol for enthalpy change?
ΔH
54
What does the symbol + indicate when measuring enthalpy change at standard conditions?
It indicates that the enthalpy change is measured under standard conditions.
55
What is the enthalpy of formation of an element?
0 kJ mol-1
56
What is the standard enthalpy change of combustion?
The enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions.
57
What are the standard conditions for measuring standard enthalpy change of combustion?
298K and 100kPa, with all reactants and products in their standard states.
58
What is the symbol for standard enthalpy change of combustion?
AcH
59
Write the chemical equation for the complete combustion of methane.
CH₄ (g) + 2 O₂ (g) → CO₂ (g) + 2 H₂O (l)
60
What products are formed during incomplete combustion?
Soot (carbon), carbon monoxide, and water.
61
How does incomplete combustion compare to complete combustion in terms of exothermicity?
Incomplete combustion is less exothermic than complete combustion.
62
What is the standard enthalpy change of neutralisation?
The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.
63
What is the symbol for standard enthalpy change of neutralisation?
Aneur H
64
What is the equation used to calculate energy change in calorimetry?
energy change = mass of solution x heat capacity x temperature change
## Footnote
Q (J) = m (g) x Cp (Jg-1K-1) x ΔT (K)
65
What does the enthalpy change of reaction (ΔH) represent?
The energy change per mole of one of the reactants
## Footnote
This value is typically derived from the energy calculated using the calorimetric method.
66
How is a calorimetric experiment generally conducted?
By mixing substances in an insulated container and measuring the temperature rise
## Footnote
This can involve a solid dissolving in a solution or two solutions reacting together.
67
What is the first step in the general method of conducting a calorimetric experiment?
Wash the equipment (cup and pipettes) with the solutions to be used
## Footnote
This ensures that no contaminants affect the results.
68
What should be done after washing the calorimetric cup?
Dry the cup after washing
## Footnote
This helps to ensure accurate temperature measurements.
69
What is the purpose of placing a polystyrene cup in a beaker during the experiment?
For insulation and support
## Footnote
This minimizes heat loss to the environment.
70
What should be done with the thermometer before adding reactants?
Clamp the thermometer into place ensuring the bulb is immersed in the solution
## Footnote
This allows for accurate temperature readings.
71
What is the procedure for measuring initial temperatures in the experiment?
Measure the initial temperatures of the solution or both solutions every minute for 2-3 minutes
## Footnote
This establishes a baseline before the reaction.
72
What should be done at minute 3 of the calorimetric experiment?
Transfer the second reagent to the cup
## Footnote
If a solid reagent is used, add the solution first, then the solid.
73
What method is used if a solid reagent is involved in the experiment?
'Before and after' weighing method
## Footnote
This allows for accurate measurement of the solid reagent's mass.
74
Why is it important to stir the mixture during the calorimetric experiment?
To ensure that all of the solution is at the same temperature
## Footnote
This helps in obtaining accurate temperature readings.
75
What additional step is taken if the reaction is slow?
Take readings at regular time intervals and extrapolate the temperature curve back
## Footnote
This helps to estimate the temperature at the time of reactant addition.
76
What should be measured before adding reactants to improve temperature accuracy?
The temperature of the reactants for a few minutes
## Footnote
An average temperature is used if both are solutions.
77
What are some errors that can occur in the calorimetric method?
* Energy transfer from surroundings (usually loss)
* Approximation in specific heat capacity of solution
* Neglecting the specific heat capacity of the calorimeter
* Incomplete or slow reaction
* Density of solution assumed to be the same as water
## Footnote
These errors can lead to inaccuracies in calculated enthalpy changes.
78
True or False: The calorimetric method assumes all solutions have the same heat capacity as water.
True
## Footnote
This is a common approximation in calorimetry.
79
Fill in the blank: The calorimetric method measures the _______ of a reaction.
enthalpy change
## Footnote
This is often expressed in terms of energy per mole of reactant.
80
What is the general method for calculating the enthalpy change of reaction, ΔH?
1. Calculate energy change using q = m x C x ΔT.
2. Work out the moles of the reactants used.
3. Divide q by the number of moles of the reactant not in excess to give ΔH.
4. Add a sign and unit (divide by 1000 to convert Jmol-1 to kJmol-1).
81
What is the heat capacity of water?
4.18 J g-1 K-1
82
What assumption is made about the heat capacity when reactants are dissolved in water?
Assumed to be the same as pure water.
83
What is the assumed density of solutions in these calculations?
1 g cm-3
84
Calculate Q for the reaction involving 25.0 cm3 of 0.200 mol dm-3 copper sulfate with a temperature increase of 7°C.
Q = 25 × 4.18 × 7 = 731.5 J
85
How do you calculate the number of moles of CuSO4 used?
moles of CuSO4 = concentration x volume = 0.2 × 25/1000 = 0.005 mol
86
What is the formula for calculating the enthalpy change per mole?
ΔH = Q / number of moles
87
Calculate ΔH for the reaction where Q = 731.5 J and moles of CuSO4 = 0.005 mol.
ΔH = 731.5 / 0.005 = 146300 J mol-1 = -146 kJ mol-1
88
In an exothermic reaction, how is ΔH represented?
With a minus sign, e.g., -146 kJ mol-1
89
What is the total volume of solutions used when 25.0 cm3 of 2.00 mol dm-3 HCl is neutralized by 25.0 cm3 of 2.00 mol dm-3 NaOH?
50.0 cm3
90
Calculate Q for the neutralization reaction with a temperature increase of 13.5°C.
Q = 50 × 4.18 × 13.5 = 2821.5 J
91
How do you calculate the number of moles of HCl used in the neutralization reaction?
moles of HCl = concentration x volume = 2 × 25/1000 = 0.05 mol
92
What is the enthalpy change of neutralization for the reaction where Q = 2821.5 J and moles of HCl = 0.05 mol?
ΔH = 2821.5 / 0.05 = 56430 J mol-1 = -56.4 kJ mol-1
93
What must be remembered when reporting ΔH in these calculations?
Include the sign, unit, and 3 significant figures.
94
What is the mean bond enthalpy?
The enthalpy change when one mole of gaseous covalent bonds is broken, averaged over different molecules.
95
Why are mean bond energy values positive?
Energy is required to break a bond.
96
What does the mean bond energy account for in a compound?
It averages the different bond energies of each bond in the compound.
97
When does the definition of mean bond energy apply?
When the substances start and end in the gaseous state.
98
What happens during bond breaking and bond making?
Bond breaking absorbs energy; bond making releases energy.
99
In an exothermic reaction, what is true about energy release and absorption?
More energy is released when making bonds than is absorbed when breaking bonds.
100
In an endothermic reaction, what is true about energy absorption and release?
More energy is absorbed when breaking bonds than is released when making bonds.
101
How is the change in enthalpy (ΔH) calculated for reactions involving gases?
ΔH = E bond energies broken - E bond energies made.
102
Why are ΔH values calculated using mean bond energies less accurate?
Because the mean bond energies are not exact.
103
What is the formula for calculating the enthalpy of combustion of propene?
ΔH = [E(C=C) + E(C-C) + 6 x E(C-H) + 4.5 × E(O=O)] - [6 x E(O=C) + 6 x E(O-H)]
104
What is the formula used to calculate the enthalpy change (ΔH) in this context?
ΔH = L bond energies broken - E bond energies made
## Footnote
This formula accounts for the difference in energies of bonds broken and formed during a reaction.
105
What does Hess's Law state?
Total enthalpy change for a reaction is independent of the route taken
## Footnote
This principle is based on the conservation of energy, a concept from the first law of thermodynamics.
106
What are the two routes shown in the energy level diagram example involving the reaction 2H (g) + 2Cl (g)?
Arrow 'a' and arrows 'b'
## Footnote
These arrows represent different pathways a reaction can take, illustrating the concept of Hess's Law.
107
In the equation ΔH = a - b, what do 'a' and 'b' represent?
'a' represents the enthalpy change of one route and 'b' represents the enthalpy change of another route
## Footnote
This rearrangement allows for calculating the enthalpy change for a reaction.
108
What is the significance of Hess's law cycles?
Used to measure enthalpy change for reactions that cannot be measured directly
## Footnote
Alternative reactions are conducted to determine the enthalpy change experimentally.
109
Why is it difficult to measure the enthalpy change for the formation of a hydrated salt directly?
It's impossible to add the exact amount of water and measure temperature change easily
## Footnote
This complicates direct experimental measurement, hence the use of Hess's Law.
110
Fill in the blank: Hess's Law is a version of the _______.
first law of thermodynamics
## Footnote
This law emphasizes that energy is always conserved in chemical reactions.
111
What is Hess's law used for?
To determine enthalpy changes from enthalpy changes of formation.
## Footnote
Hess's law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps.
112
What is the formula for calculating the enthalpy change of a reaction?
ΔH reaction = E ΔH products - E ΔH reactants.
## Footnote
This formula applies to both formation and combustion reactions.
113
How do you calculate the enthalpy change of combustion using Hess's law?
ΔH combustion = E ΔH products - E ΔH reactants.
114
True or False: Elements in their standard states have an enthalpy of formation of 0.
True.
115
What is the chemical equation for the formation of propene?
3C (s) + 3H2 (g) → C3H6 (g)
## Footnote
This equation represents the synthesis of propene from carbon and hydrogen.
