Energetics Flashcards
(116 cards)
1
Q
What is the total energy contained within a substance called?
A
Enthalpy of the substance (Symbol: H)
Enthalpy is a thermodynamic property that reflects the total heat content of a system.
2
Q
What accompanies chemical reactions?
A
Enthalpy changes, usually in the form of a heat change.
Enthalpy changes indicate whether a reaction absorbs or releases heat.
3
Q
What is an exothermic reaction?
A
A reaction where the enthalpy of the products is less than that of the reactants, releasing energy.
Exothermic reactions increase the temperature of the reaction mixture.
4
Q
What happens to the temperature of the reaction mixture in an exothermic reaction?
A
The temperature increases.
This increase is due to the release of energy from the reaction.
5
Q
What is the enthalpy of reaction for an exothermic reaction?
A
Negative.
This indicates energy is released during the reaction.
6
Q
What is an endothermic reaction?
A
A reaction where the enthalpy of the products is more than that of the reactants, absorbing energy.
Endothermic reactions require heat from the surroundings.
7
Q
What happens to the temperature of the reaction mixture in an endothermic reaction?
A
The temperature decreases.
This decrease is due to the absorption of heat from the surroundings.
8
Q
What is the enthalpy of reaction for an endothermic reaction?
A
Positive.
This indicates energy is absorbed during the reaction.
9
Q
Fill in the blank: In an exothermic reaction, the chemicals ______ energy.
A
lose
The loss of energy results in the release of heat.
10
Q
Fill in the blank: In an endothermic reaction, the chemicals ______ energy.
A
gain
The gain of energy results in the absorption of heat.
11
Q
True or False: The enthalpy change for an exothermic reaction is positive.
A
False.
The enthalpy change for an exothermic reaction is negative.
12
Q
True or False: Endothermic reactions feel warmer to the touch.
A
False.
Endothermic reactions feel colder as they absorb heat from their surroundings.
13
Q
What do standard conditions refer to in chemical reactions?
A
The specific conditions under which enthalpy changes are measured.
Standard conditions typically include a temperature of 25°C and a pressure of 1 atm.
14
Q
What is the standard temperature defined in standard conditions?
A
25°C
Standard temperature is often reported as 298 K.
15
Q
What is the pressure defined in standard conditions?
A
1 atmosphere
16
Q
What is the symbol for standard enthalpy change?
A
ΔH°
17
Q
What does the term ‘standard state’ refer to?
A
The physical state of a substance under standard conditions
18
Q
What is the standard enthalpy of reaction symbol?
A
ΔH
19
Q
What is the energy released by the reaction referred to as?
A
Standard enthalpy of reaction
20
Q
What is the characteristic of an exothermic reaction regarding enthalpy?
A
ΔH - ve
21
Q
What is the characteristic of an endothermic reaction regarding enthalpy?
A
ΔH + ve
22
Q
What is activation energy?
A
The energy needed to break the reactant bonds
23
Q
In an exothermic reaction, what happens after the activation energy is overcome?
A
The exothermic nature of the reaction breaks more reactant bonds
24
Q
In an endothermic reaction, what is required to keep the reaction going?
A
A sustained amount of energy
25
Fill in the blank: A way of thinking of the energy needed for an exothermic reaction is like a cyclist riding a bike up a small hill to gain a large amount of _______.
free wheeling energy
26
Fill in the blank: In a closed system, the amount of energy is _______.
fixed
27
True or False: In a closed system, you can create more energy.
False
28
What happens to energy in a closed system?
It can be converted from one form to another
29
What is the role of the activation energy in a reaction?
To overcome the energy barrier
30
What does it mean for products to be higher in energy than the reactants?
It indicates an endothermic reaction
31
What does it mean for products to be lower in energy than the reactants?
It indicates an exothermic reaction
32
What is the process of measuring enthalpy changes called?
Calorimetry
33
What apparatus is used to measure the temperature change during a reaction?
Calorimeter
34
What type of calorimeter can be used to measure many enthalpy changes?
Coffee cup calorimeter
35
How can the accuracy of enthalpy measurements be improved?
By reducing the amount of heat lost to or gained from the surroundings
36
What is the relationship between enthalpy change (ΔH), temperature change (ΔT), and mass (m)?
ΔH = -m c ΔT
37
What indicates an exothermic reaction in terms of enthalpy change?
ΔH < 0
38
What indicates an endothermic reaction in terms of enthalpy change?
ΔH > 0
39
What happens to the temperature of the solution in an exothermic reaction?
The temperature increases (ΔT > 0)
40
What happens to the temperature of the solution in an endothermic reaction?
The temperature decreases (ΔT < 0)
41
What is the assumed density of an aqueous solution compared to water?
Similar to the density of water
42
What is the specific heat capacity of pure water?
4.2 J g⁻¹ °C⁻¹
43
What is the density of pure water?
1 g cm⁻³
44
Fill in the blank: The enthalpy change for a reaction (ΔH) is related to the temperature change in the solution (ΔT) and the mass of the solution (m) by the equation ΔH = -m c _______.
ΔT
45
What does the term standard enthalpy of neutralisation, AH,® refer to?
The enthalpy change that occurs when one mole of water is produced by a neutralisation reaction carried out under standard conditions.
46
How is the enthalpy of neutralisation determined?
Using a simple coffee cup calorimeter.
47
What is the enthalpy of neutralisation for hydrochloric acid by sodium hydroxide?
The enthalpy change that occurs when one mole of water is produced by the reaction between hydrochloric acid and sodium hydroxide.
48
Write the balanced chemical equation for the reaction between hydrochloric acid and sodium hydroxide.
HCl + NaOH → NaCl + H2O
49
What is the ionic equation for the neutralisation reaction between H+ and OH-?
H+ + OH- → H2O
50
What does the enthalpy of neutralisation for nitric acid by sodium hydroxide refer to?
The enthalpy change that occurs when one mole of water is formed by the reaction between nitric acid and sodium hydroxide.
51
Write the balanced chemical equation for the reaction between nitric acid and sodium hydroxide.
HNO3 + NaOH → NaNO3 + H2O
52
What is the approximate enthalpy of neutralisation for strong acids such as hydrochloric acid and nitric acid?
-57 kJ/mol
53
Do the nature of spectator ions in the reaction mixture significantly affect the enthalpy change?
No, they do not significantly affect the enthalpy change.
54
What is produced for every mole of acid neutralised for strong acids like hydrochloric and nitric acid?
One mole of water.
55
If the formula of an acid contains more than one hydrogen ion (H+), how many moles of water are produced when one mole of the acid is neutralised?
One mole of water for every hydrogen ion in the formula.
56
How many moles of water are produced when one mole of sulfuric acid (H2SO4) is neutralised?
Two moles of water.
57
What does the enthalpy change correspond to when one mole of sulfuric acid is neutralised?
Twice the enthalpy of neutralisation for sulfuric acid.
58
What does standard enthalpy of formation (ΔH°f) refer to?
The enthalpy change when one mole of a substance is formed from its elements under standard conditions.
59
What is the standard enthalpy of formation for propane (C3H8)?
The standard enthalpy change when one mole of propane is formed from carbon (graphite) and hydrogen gas under standard conditions.
60
Write the formation reaction for propane.
3C(s) + 4H2(g) → C3H8(g)
61
What is the standard enthalpy of formation for sodium chloride (NaCl)?
The standard enthalpy change when 1 mole of sodium chloride is formed from sodium metal and chlorine gas under standard conditions.
62
Write the formation reaction for sodium chloride.
Na(s) + ½Cl2(g) → NaCl(s)
63
What is the standard enthalpy of formation for an element?
Zero.
64
How can the standard enthalpy change for a reaction be calculated?
By subtracting the total enthalpy of formation for the reactants from the total enthalpy of formation for the products.
65
What is the formula to calculate the standard enthalpy change for a reaction?
ΔH° = ΔH°(products) - ΔH°(reactants)
66
What does the term standard enthalpy of combustion (ΔH°) refer to?
The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions
## Footnote
Standard conditions typically refer to a temperature of 298 K and a pressure of 1 atm.
67
What is the standard enthalpy of combustion for butane represented by?
The enthalpy change that occurs when 1 mole of butane (C4H10) burns to form carbon dioxide and water under standard conditions
## Footnote
The reaction is: C4H10(g) + 6.5 O2(g) → 4 CO2(g) + 5 H2O(l).
68
What products are formed when hydrocarbons like butane are burnt?
Carbon dioxide and water
## Footnote
The combustion process converts carbon and hydrogen in the hydrocarbons to these stable compounds.
69
Why is the combustion of hydrocarbons considered highly exothermic?
A great deal of energy is released when carbon dioxide and water are formed
## Footnote
This makes hydrocarbons very effective fuels.
70
What apparatus is used to determine the enthalpy change in combustion?
Apparatus typically includes a thermometer, a spirit burner, and a container for water
## Footnote
The lid prevents heat loss by evaporation and the screen minimizes heat loss.
71
What are some errors that may occur during the measurement of enthalpy change in combustion?
* Energy/heat lost to surroundings
* Water may have evaporated
* Size of flame may vary
* Energy/heat lost to the glass of liquid/bulb of thermometers.
72
Fill in the blank: The combustion of hydrocarbons is highly _______.
[exothermic]
73
What is the formula to calculate the heat produced when burning a fuel?
AH = m c AT
## Footnote
AH represents the heat produced, m is the mass of the liquid, c is the specific heat capacity, and AT is the rise in temperature.
74
What does the specific heat capacity (c) represent?
The amount of energy needed to raise the temperature of 1 g of the liquid by 1°C
## Footnote
Units of specific heat capacity are J g-1°C.
75
True or False: A change in temperature of one Kelvin is equal to a change of one degree Celsius.
True
76
What are the units for specific heat capacity when expressed in relation to Kelvin?
J g K-1
## Footnote
This shows that the units can be written in terms of Kelvin instead of degrees Celsius.
77
Fill in the blank: The accuracy of the enthalpy change calculated can be improved by maximizing the amount of heat _______.
transferred to the liquid being heated
78
What method can be used to ensure uniform heating of the liquid in the enthalpy measurement?
Stirring the liquid
## Footnote
This helps to distribute heat evenly throughout the liquid.
79
What is the role of a 'spirit burner' in measuring enthalpy changes?
It allows for controlled burning of liquid fuels like ethanol.
80
What is the relationship between the rise in temperature (AT) and the heat produced (AH)?
Directly proportional, as described by the formula AH = m c AT.
81
What factors affect the accuracy of enthalpy change measurements?
* Maximizing heat transfer
* Stirring the liquid
82
What is Hess's Law?
The total enthalpy change accompanying a chemical change is independent of the route by which the chemical change takes place provided the initial and final conditions are the same.
83
What is the significance of Hess's Law in thermodynamics?
It allows the calculation of enthalpy changes that are difficult to measure directly.
84
What does it mean if reactants can be converted into the same products by more than one route?
The total energy for each route is the same, leading to the concept of Enthalpy cycles.
85
True or False: Hess's Law suggests that energy can be gained by taking different routes in a chemical reaction.
False
86
What is the first step to use enthalpy cycles?
Write a balanced chemical equation for the reaction.
87
What is the second step in constructing an enthalpy cycle?
Construct the Enthalpy cycle.
88
Fill in the blank: The values of each _______ should be looked up and written in for each compound/element in the enthalpy cycle.
ΔH
89
What should be done after deciding on the routes in an enthalpy cycle?
Draw them on the cycle.
90
What is the purpose of adding up the ΔH values for each route in an enthalpy cycle?
To find the total enthalpy change for the reaction.
91
Why is it important that the initial and final conditions remain the same in Hess's Law?
To ensure that the total enthalpy change is consistent across different routes.
92
What is Hess's law?
Route 1 = Route 2
93
How is enthalpy change for a chemical reaction calculated?
By calculating the difference between the energy needed to break bonds in the reactants and the energy released when bonds are formed in the products.
94
What is average bond enthalpy?
The energy required to break one mole of a given bond averaged over many compounds.
95
What indicates an exothermic reaction in terms of bond energy?
If the energy released by making bonds is greater than the energy needed to break bonds.
96
What indicates an endothermic reaction in terms of bond energy?
If the energy needed to break bonds is greater than the energy released by making bonds.
97
Fill in the blank: The formula for enthalpy change (ΔH) is _______.
ΔH = Ebreaking - Emaking
98
What does ΔH < 0 signify?
An exothermic reaction.
99
What does ΔH > 0 signify?
An endothermic reaction.
100
In an exothermic reaction, what happens to the bonds?
Energy is released when bonds are made.
101
In an endothermic reaction, what happens to the bonds?
Energy is absorbed when bonds are broken.
102
What is the relationship between making and breaking bonds and enthalpy change?
The relationship is illustrated by the difference in energy needed to break bonds and the energy released in making bonds.
103
What does average bond enthalpy refer to?
The energy needed to break one mole of a particular type of bond averaged over many compounds.
104
How can the energy needed to break a bond be estimated?
By the average bond enthalpy for the particular type of bond.
105
What symbol is used for 'sum of or adding together'?
Σ
106
What is the formula for calculating enthalpy change using bond enthalpies?
ΔH = Σ (Bond enthalpies of bonds broken) - Σ (Bond enthalpies of bonds formed)
107
What does breaking weak bonds imply in terms of bond enthalpy?
Breaking weak bonds requires less energy compared to strong bonds.
108
In the example, what is the bond energy of C-H?
413 kJ/mol
109
In the worked example, what is the total bond energy for the reactants?
1652 kJ/mol
110
What is the bond energy of C=0?
805 kJ/mol
111
What is the bond energy of H in the products?
463 kJ/mol
112
What is the total bond energy for the products in the example?
1852 kJ/mol
113
Fill in the blank: The energy required to break bonds is represented as _______.
[Bond enthalpies of bonds broken]
114
True or False: The total bond energy for the reactants is greater than that of the products in the example.
False
115
What is the total energy change calculated in the worked example?
-200 kJ/mol
116
Why are average bond enthalpies different from those determined using Hess’ law.
Due to average bond enthalpies
