Energetics Unknown Information

HideShow resource information
If a 100g block of iron and a 50g block of iron have the same temperature, which has a higher internal energy and why?
The 100g block because there is more of it
1 of 127
If you have a 100g block of iron and a 50g block of iron, which has a higher average kinetic energy of particles?
Neither they both have the same because they are the same temperature!
2 of 127
An endothermic reaction relates to a decrease in what?
Internal energy of the system
3 of 127
In an endothermic reaction, what is the heat energy taken in converted into?
Internal energy
4 of 127
Is it possible to measure the enthalpy of a system?
No only enthalpy change
5 of 127
Combustion is what sort of chemical reaction?
Exothermic (new products are formed from the combination of a hydrocarbon and oxygen)
6 of 127
Why is there no scale on an enthalpy level diagram?
No way of measuring initial and final enthalpy of a system
7 of 127
A negative sign with an enthalpy change means...
A decrease in enthalpy
8 of 127
Enthalpy of reaction =...
Enthalpy of products - enthalpy of reactants
9 of 127
What sort of reaction is the reaction between nitrogen and oxygen to form nitrogen (II) oxide?
Endothermic
10 of 127
Why do we use standard conditions?
To make enthalpy changes comparable and transferable
11 of 127
What are the units for enthalpy change?
kJmol-1
12 of 127
If we used half as many moles of reactants as was in the chemical reaction, how would you change the enthalpy change?
Halve it
13 of 127
Give two methods of measuring enthalpy changes
Specific heat capacity and calorimetry
14 of 127
When temperature changes are considered, what does 1K =?
1 degrees Celcius
15 of 127
What are some units for specific heat capacity?
Jg-1K-1 or Jg-1C-1 or kJkg-1K-1 or Jkg-1K-1
16 of 127
In the equation q=mcdeltaT, what is q?
The heat energy required for the constraints in the equation
17 of 127
Which substances are more difficult to heat, one with a higher or lower specific heat capacity?
Higher
18 of 127
How can specific heat capacity be applied to a cooling substance?
When a 1g substance cools by 1K, the same amount of joules are given out as the specific heat capacity
19 of 127
What is the principle of calorimetry?
The heat given out by a substance can be used to heat another substance of known specific heat capacity and mass so q=mcdeltaT can be used
20 of 127
What is the specific heat capacity of water?
4.18 Jg-1C-1
21 of 127
The heat given out in a calorimetry experiment not only heats the water. What else does it heat?
The can containing the water and the surrounding air
22 of 127
How could you improve the systematic errors in a calorimetry experiment to ensure the water is heated properly?
Draught exclude the can to prevent convection currents. Insulate the can to reduce heat loss. Calculate the specific heat capacity of the can and take that into consideration
23 of 127
What are other major issues with calorimetry experiments as well as heat loss?
Incomplete combustion which gives out less heat and evaporation of water and alcohol
24 of 127
What is a bomb calorimeter?
Calorimeter is heavily insulated and flame is ignited electronically with plentiful supply of oxygen
25 of 127
The enthalpy change of neutralisation is always...
Exothermic
26 of 127
What does infinite dilution mean?
Any further dilution of the solution produces no further enthalpy change
27 of 127
The enthalpy change of solution is...
Exothermic or endothermic
28 of 127
What do we assume 200cm3 has a mass of?
220g - assume density is the same as water where 1cm3 has mass of 1g
29 of 127
When calculating enthalpy of neutralisation, what major assumption is made other than density of solutions?
The solution has the same specific heat capacity as water because it is composed of mostly water
30 of 127
Why might the literature values for enthalpy changes be different to experimental values?
Assumptions made about density and specific heat capacity. Heat lost to surroundings and calorimeter. Specific heat capacity of polystyrene cup not taken into account
31 of 127
If you use half the mass as you did before, how does the temperature change in calorimetry?
It doesn't! You use half the volumes of reactants but only half the reaction mixture is being heated so the temperature change is the same
32 of 127
If you add a solid to a liquid in an enthalpy change experiment, what do you use as the mass in the q equation?
Just the mass of the liquid because heat is transferred to it
33 of 127
What do you use mass of in the q equation?
Whatever heat is transferred to
34 of 127
What is an issue with endothermic reactions and calorimetry?
Vigorous stirring greatly improves the experiment because substances could be slow to dissolve and therefore take in energy from the surroundings in more time so the temperature drop is not as large
35 of 127
What is another issue with dissolving enthalpy reactions?
Mass of substance dissolved not taken into account when calculating heat energy
36 of 127
Why do you have to extrapolate graphs in enthalpy reactions?
Reactions do not occur instantaneously so sometimes the reaction mixture heats up but it is also cooling at the same time. Extrapolating calculates an estimate for the temperature change that would have occurred if a reaction was instantaneous
37 of 127
What do we have to consider with extrapolation?
It is a matter of judgement and could introduce errors
38 of 127
Why do we use Hess' law?
For some experiments where it is not possible to design them to calculate the enthalpy change
39 of 127
Why does Hess' law work?
The route between two substances cannot affect the amount of enthalpy those substances have so the enthalpy is constant for a certain set of conditions
40 of 127
When you add something to both sides of the top equation in a Hess' law cycle, what happens to the enthalpy change?
Does not change if the same thing is added to both sides
41 of 127
Equations for the enthalpy change of formation show the formation of how much product?
1 mole
42 of 127
Why is graphite often used as the carbon source for formation reactions?
Most stable form of carbon
43 of 127
What is the enthalpy change for the formation of a substance in its standard state and why?
Zero because no heat energy is taken in or given out when one mole of an element in its standard state is formed
44 of 127
What must always be included in enthalpy change equations? Why?
State symbols because the state of substances affects the enthalpy change of the reaction
45 of 127
What are enthalpy change of formation reactions?
Can be either exothermic or endothermic
46 of 127
Enthalpy change has the same units as...
Gibbs free energy
47 of 127
What is the first thing to do before setting up a Hess' law cycle?
Write out the equations if not done for you
48 of 127
How can a general equation for Hess' law involving enthalpies of formation be given?
The sum of the enthalpies of formation of products - the sum of enthalpies of formation of reactants
49 of 127
Is 1/2H2 correct in a bond enthalpy equation for HCl?
No H(g) is correct because that represents one mole of hydrogen while 1/2H2 represents half a mole of H2 molecules
50 of 127
What state do bond enthalpies only apply to?
Gaseous states
51 of 127
Why does this reaction not represent bond enthalpy? Br2(l) -->2Br(g)
This equation can be split into two stages: One is the enthalpy of vaporisation forming gaseous bromine molecules and the other is the bond enthalpy to produce single atoms of bromine
52 of 127
The reaction Br2(l) --> 2Br(g) is what sort of reaction?
Atomisation
53 of 127
Are bond enthalpies exothermic or endothermic?
Endothermic because it is about breaking bonds
54 of 127
Why might there be some inaccuracies in calculating bond enthalpies?
The values in table for bond enthalpies are averages because e.g. the C-H bond in different compounds has a different bond energy
55 of 127
When are bond enthalpy calculations most accurate?
When making or breaking a few bonds as possible
56 of 127
What is a general equation for calculating bond enthalpies?
Sum of bonds broken (reactants) - sum of bonds made (negative sign relates to exothermic bond making reaction already so input all values as positive)
57 of 127
If you had 4 C-H bonds on one side of the reaction and 4 C-H bonds formed in the products, do you need to worry about these bond enthalpies when calculating the bond enthalpies of a reaction?
No
58 of 127
Is using the enthalpy change of combustion or formation (experimental data) more or less reliable than using bond enthalpy data for a reaction?
More
59 of 127
Why is experimental data more reliable than bond enthalpy data?
Experimental data is specific to particular substances while bond enthalpies are averages
60 of 127
What sort of bond is present in an oxygen molecule?
Double
61 of 127
Why might a reaction be more exothermic or endothermic?
If there are stronger or more bonds on one side of the equation
62 of 127
Why are bond enthalpies only related to gases?
In solids and liquids we would then have to think about intermolecular forces and convert solids and liquids to gases before using bond enthalpies
63 of 127
For calculating the bond enthalpies of liquids, what other enthalpy reaction is needed?
Enthalpy of vaporisation
64 of 127
In an endothermic energy level diagram, what is a higher/ less stable?
Products
65 of 127
What are the axes on an energy level diagram?
X axis = Reaction coordinate. Y axis = potential energy
66 of 127
What is higher/ less stable in an exothermic energy level diagram?
Reactants
67 of 127
What wavelengths of UV light do oxygen molecules absorb to break their bonds?
Less than 242nm (higher energy)
68 of 127
What wavelength of UV light do ozone molecules absorb to break the bonds?
Less than 330nm (lower energy)
69 of 127
Why do oxygen molecules absorb higher energy UV light than ozone molecules?
Double bond between oxygen atoms is stronger than the bond between ozone oxygen atoms
70 of 127
What spectrum of UV light is needed to break bonds in oxygen molecules?
UV-C
71 of 127
What spectrum of UV light is needed to break bonds in ozone?
UV-B
72 of 127
What sort of reaction is standard enthalpy change of atomisation?
Endothermic
73 of 127
In an equation for enthalpy change of atomisation, what must you ensure is formed?
Only one mole of the gaseous atoms from their element
74 of 127
What states do ionisation energy and electron affinity involve?
Gases
75 of 127
What type of reaction are ionisation energies?
Endothermic
76 of 127
What sort of reaction is first electron affinity? Why?
Exothermic because it is favourable to bring an electron from infinity to where it feels attraction from the nucleus
77 of 127
What sort of reaction is second electron affinity? Why?
Endothermic because it is unfavourable to add an electron to an ion so more energy must be added
78 of 127
If lattice enthalpy is defined as the breaking of a lattice, what sort of reaction is it?
Endothermic
79 of 127
If lattice enthalpy is defined as the making of a lattice, what is it defined as?
Exothermic
80 of 127
In a Born-Haber cycle, what does an upward arrow suggest?
Endothermic reaction
81 of 127
In a Born-Haber cycle, what does a downwards arrow suggest?
Exothermic
82 of 127
In a Born-Haber cycle, what does an upward arrow suggest?
Endothermic reaction
83 of 127
In a Born-Haber cycle, what does a downwards arrow suggest?
Exothermic
84 of 127
In Born-Haber cycles, what type of reaction is lattice enthalpy?
Endothermic (broken apart into constituent gaseous ions)
85 of 127
What goes at the very bottom of a Born-Haber cycle?
The chemical formula for the ionic compound
86 of 127
What is the order of reactions in a Born-Haber cycle starting at the enthalpy change of formation?
Enthalpy change of formation, atomisation of metal, ionisation energy (ies) of metal, atomisation of non-metal, electron affinity (ies) of non-metal, lattice enthalpy
87 of 127
Which equations are the only exothermic equations in a Born-Haber cycle?
Standard enthalpy change of formation, standard enthalpy change of 1st electron affinity
88 of 127
In Born-Haber cycles like the formation of MgCl2, what must be done to the atomisation and first electron affinity calculations for chlorine?
Multiply them by 2 because 2 chlorine atoms involved
89 of 127
The higher the charge on the ions... Why?
The greater the lattice enthalpy because they attract each other more strongly
90 of 127
What is proportional to the product of the charges on ions in a lattice?
The force between ions
91 of 127
The smaller the ions... Why?
The larger the lattice enthalpy because the distance between nuclei is smaller so the stronger the attraction
92 of 127
What causes a larger effect on lattice enthalpy?
Charge on ions as opposed to size of ions
93 of 127
A high lattice enthalpy means...
A high melting point
94 of 127
What are the two stages of dissolving an ionic compound?
1) lattice enthalpy 2) standard enthalpy change of hydration
95 of 127
Hydration enthalpies are always...
Exothermic
96 of 127
What is the lattice enthalpy always in the enthalpy of hydration?
Endothermic
97 of 127
Enthalpy change of solution =...
Lattice enthalpy + enthalpy change of hydration for positive ions + enthalpy change of hydration for negative ions
98 of 127
What must be done to the enthalpy of hydration for the enthalpy of solution of MgCl2?
Multiply it by 2
99 of 127
The bigger the ion... (enthalpy change of hydration) Why?
The smaller the enthalpy of hydration because weaker electrostatic attraction between ions and water
100 of 127
The larger the charge... (hydration enthalpy) Why?
The greater the hydration enthalpy because stronger electrostatic attraction with water molecues
101 of 127
What is more significant in deciding which ions have a higher hydration enthalpy?
Charge of ions
102 of 127
Can you calculate entropies from experimental data?
Yes unlike enthalpies
103 of 127
A decrease in number of moles of gas means...
A decrease in disorder
104 of 127
To determine if there has been an increase or decrease in level of disorder, what can you look at?
The change in the number of moles of gas
105 of 127
What if the number of moles of gas is the same on both sides of the equation?
No entropy change
106 of 127
Entropy change =...
Total standard entropy of products - total standard entropy of reactants
107 of 127
Does a spontaneous reaction have to happen quickly? Why?
No because it relates to if an outside influence is needed or not not the speed
108 of 127
What is more useful; low entropy or high entropy? Why?
Low entropy because it is concentrated and can bring about a change
109 of 127
Where will a system move in terms of entropy?
Low to high entropy
110 of 127
In an exothermic reaction, what happens to the entropy of the surroundings?
Increases
111 of 127
In the delta G = Delta H -T delta S equation, what is Delta S?
Entropy change of the universe
112 of 127
When calculating the temperature at which a reaction will become spontaneous, what must we consider?
The values of enthalpy change and entropy change alter with temperature but to do the calculations, you may continue to use the values at a certain temperature so the calculation is only approximate
113 of 127
How could you make a reaction spontaneous that isn't?
Electrolysis with a continuous passage of electric current
114 of 127
Will an exothermic reaction ever be spontaneous?
Yes at a certain temperature
115 of 127
Reactions with a positive deltaS...
Become more spontaneous as temperature increases
116 of 127
Reactions with negative deltaS...
Become less spontaneous as temperature increases
117 of 127
An equilibrium mixture always has a lower Gibbs free energy than what?
Pure reactants or pure products
118 of 127
The formation of equilibrium is what?
Negative Gibbs free energy
119 of 127
Equilibrium marks what?
Maximum entropy and minimum value for Gibbs free energy (not change of Gibbs free energy)
120 of 127
What does Gibbs free energy depend on?
Amount of each substance present
121 of 127
What is equal at equilibrium?
Gibbs free energy of reactants and products
122 of 127
What is the value of free energy change at equilibrium?
0 - no desire to move towards reactants or products
123 of 127
If free energy change is negative, where does equilibrium lie?
Closer to products (reaction is spontaneous and proceeds in forwards direction)
124 of 127
If free energy change is positive where does equilibrium lie?
Closer to reactants (non-spontaneous and hard to proceed in forwards direction)
125 of 127
On a composition versus Gibbs free energy graph, what is free energy change?
The difference between the Gibbs free energy of reactants and products
126 of 127
In a composition versus Gibbs free energy graph showing the minimum closer to products, what is the value of Gibbs free energy change going down each slope towards the minimum point?
Gibbs free energy change is negative on both slopes
127 of 127

Other cards in this set

Card 2

Front

If you have a 100g block of iron and a 50g block of iron, which has a higher average kinetic energy of particles?

Back

Neither they both have the same because they are the same temperature!

Card 3

Front

An endothermic reaction relates to a decrease in what?

Back

Preview of the front of card 3

Card 4

Front

In an endothermic reaction, what is the heat energy taken in converted into?

Back

Preview of the front of card 4

Card 5

Front

Is it possible to measure the enthalpy of a system?

Back

Preview of the front of card 5
View more cards

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Energetics resources »