Chapter 11: Enthalpy Change: Flashcards
(76 cards)
1
Q
What happens to atoms during a chemical reaction?
A
During a chemical reaction, bonds of the reactants are broken and rearranged to form products. NO ATOMS ARE CREATED OR DESTROYED IN A CHEMICAL REACTION, just rearranged.
2
Q
What is the law of conservation of mass?
A
In an isolated system, mass is neither created nor destroyed by chemical reactions or physical transformations. Therefore, the mass of the products in a chemical reaction must equal the mass of the reactants.
3
Q
What happens when chemical bonds are broken?
A
An input of energy is required, thus making the breaking of chemical bonds an endothermic reaction.
4
Q
What happens when chemical bonds are formed?
A
Energy is released, thus making the forming of chemical bonds an exothermic reaction.
5
Q
What to do as you study?
A
Keep the right mindset, and don’t memorize. Link to past knowledge, think of real-life applications, and possible questions.
6
Q
What to do with concepts you don’t fully grasp?
A
Watch videos on them, and improve your cards as you go.
7
Q
What is enthalpy (H)?
A
The total energy of a system at constant pressure. Sum of all kinetic and potential E. deltaH = Et = Ep + Ek
8
Q
What to mnemonic to remember?
A
BrINClHOF + P4 + S8. Also try to do more mnemonic’s when studying in the future (even if they don’t roll off the tongue, they work).
9
Q
What to do after defining all bolded words?
A
Read through notes/textbook for any additional info you missed.
10
Q
What is a formation reaction?
A
When elements combine to form a compound.
11
Q
What is a decomposition reaction?
A
When a compound separates into it’s elements.
12
Q
What is produced when carbon, C, combusts?
A
CO2(g)
13
Q
What is produced when hydrogen, H, combusts?
A
H2O(g)
14
Q
What is produced when Nitrogen, N, combusts?
A
NO2(g)
15
Q
What is produced when sulfur (remember S8 on it’s own) combusts?
A
SO2(g)
16
Q
What is produced when a metal combusts?
A
metaloxide (most common ion charge).
17
Q
What to do after finishing flashcards for one unit?
A
Look through all worked problems in notebook, and then in textbook.
18
Q
What type of energy does matter possess?
A
Matter possesses both potential and kinetic energy.
19
Q
What to do at the start of enthalpy questions?
A
Identify and label the system, and the surroundings.
20
Q
What is kinetic energy?
A
Energy due to motion of molecules (vibrational in solids, vibrational/rotational/translational in liquids, and translational in gases).
21
Q
What is the density of water?
A
- 1ml=1g, or 1L=1kg (are we assuming all entities have the same density as water?).
22
Q
What is temperature?
A
The AVERAGE kinetic energy of molecules in a substance.
23
Q
What is the ‘system’ in an enthalpy change?
A
Where the reaction occurs. It is the source of change, the substance being studied.
24
Q
What are the ‘surroundings’ in an enthalpy change?
A
The area affected by the system/the resulting solution.
25
What is implied if deltaH is positive?
The system has gained heat from it's surroundings (it is absorbing energy), which indicates an endothermic reaction.
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What is implied if deltaH is negative?
The system has lost heat to it's surroundings (it has released heat), thus indicating an exothermic reaction.
27
When is deltaH = deltaEk = Q = mcdeltat?
deltaH = mc(delta)t when there are only temperature (Ek) changes and NO phase or chemical change as this indicates the potential energy is 0.
28
What happens if you boil water, and then keep increasing the temperature?
Even if the temperature of the water is greater than 100deg, water will never have a water greater than this, as any molecule that reaches this temperature will change to vapour state. Any energy above the required threshold for vaporization will be used to break intermolecular bonds.
29
What to read over for each unit?
Learning outcomes, and then 'are you ready' section.
30
What to ask mrs.hucman?
Is enthalpy of formation opposite to enthalpy of decomposition?
31
When is (deltar)H= (delta)Ep=n(delta)rHm?
If there is only a phase change (intermolecular bonds broken) or/and a chemical change (intramolecular change) but NO temperature change to system then Ek=0 and deltaH = n(delta)rHm.
32
What is calorimetry?
Calorimetry is the process of measuring the amount of heat released or absorbed during a chemical reaction using an isolated system called a calorimeter.
33
What 2 laws of thermodynamics is calorimetry based on?
1. Energy cannot be created or destroyed, only transferred. This implies that heat 'lost'= heat 'gained' elsewhere.
2. Heat flows from hot to cold.
34
What to do when studying and having extra time?
Delve deeper into the concepts (more than you need to know).
35
What is thermal energy?
The total kinetic energy of the entities of a substance.
36
What is thermochemistry?
The study of energy changes (produced/absorbed) by a chemical system during a chemical reaction.
37
What do chemists require to study energy changes?
An isolated system, in which neither matter nor energy can move in or out.
38
How do we measure the quantity of thermal energy (Q) transferred in a chemical reaction?
Q=mc(delta)t, where m is mass, c is specific heat capacity (c and m MUST BE IN SAME UNITS) and t is the change in temperature.
39
What is specific heat capacity?
The quantity of energy required to raise the temperature of a unit mass (1g or 1kg) by one degree celsius. Measured in J/g x C(deg) or Kj/kg x C(deg). Assuming all have 4.19?
40
What is a joule?
The unit of work, or energy.
41
What are the assumptions made in calorimetry about the system of the reaction?
1. All the energy lost or gained by the chemical system is gained or lost by the calorimeter. In other words, the total system is isolated. Also all of the material of the system is conserved.
42
What assumptions are made in calorimetry about specific heat capacity of entities involved?
The specific heat capacity of water is 4.19 J/(gxCdeg), the same as the specific heat capacity of dilute aqueous solutions.
43
What assumption is made in calorimetry about the density of aqueous solutions?
The density of dilute aqueous solutions is the same as that of water; 1.00g/ml. This implies the volume of aqueous solutions can be immediately converted to mass without calculation.
44
What assumption is made in calorimetry about the thermal energy transferred?
The thermal energy gained or lost by the rest of the calorimeter (other than water) is negligible. In other words, the container, lid, thermometer, and stirred do not gain or lose thermal energy.
45
What are some of the kinetic energies of chemical systems?
Moving electrons within atoms, the vibration of atoms connected by chemical bonds, and the rotation/translation of molecules that are made up of these atoms.
46
What are some of the sources of the chemical potential energy of a chemical system?
Energy stored in covalent and/or ionic bonds between the entities, and energy stored in intermolecular forces between entities.
47
What does an enthalpy change ((delta)H) communicate?
The difference between the enthalpy of the products and of the reactants.
48
How do we define an exothermic reaction?
A reaction that produces/releases energy ((delta)H is negative).
49
How do we define an endothermic reaction?
A reaction that absorbs energy from it's surroundings ((delta)H is positive because the system is gaining energy).
50
What is the molar enthalpy of reaction?
The enthalpy change in a chemical system per unit chemical amount (mole) of a specified chemical undergoing change in the system at constant pressure.. Expressed in Kj/mol.
51
What is the formula fo the enthalpy change of a reaction?
(delta(r))H=n(delta(r))Hm, where rHm is the molar enthalpy of reaction, and n is the chemical amount, in mols.
52
How to find molar quantity of a gas given it's volume?
Figure out whether the reaction is happening at SATP or STP, and remember that there are 24.8L of gas per mole at SATP and 22.4L of gas per mole at STP.
53
What is 'Heat'?
Thermal energy transferred between systems. Heat is NOT possessed by a system, as it is defined as energy in transition between systems.
54
What is chemical potential energy?
The energy present in the chemical bonds of a substance.
55
How do phase changes affect chemical potential energy?
As you go down states of matter, chemical potential energy decreases, since the strength of intermolecular forces is decreasing (they've been broken to undergo phase change)???????
56
What are the 4 ways of communicating enthalpy change?
1. By stating the molar enthalpy of a specific reactant in a reaction. 2. By stating the enthalpy change for a balanced reaction equation. 3. By including an energy value as e term in a balanced reaction equation. 4. By drawing a chemical potential diagram.
57
What phase changes are endothermic, and which ones are exothermic?
Going down states of matter is endothermic, since energy (heat) must be applied to do so. Going up states of matter is exothermic because energy (heat) must be lost by the entity for this to happen.
58
What superscript is used for standard molar enthalpy of reaction, and what does this imply?
The degree sign is used as a superscript for standard molar enthalpy of a reaction, and this implies that the reaction is happening at 25 degrees and 101kpa. For a 'standard' value, the initial and final conditions of the chemical system must be in the standard state(if not, they are allowed to cool until they are at standard conditions).
59
How do you obtain enthalpy change from a chemical equation?
It is obtained from the chemical amount (molar coefficient) multiplied by the molar enthalpy of reaction (for that specific chemical).
60
What to look through?
Old thermochem worksheet. Also see all labs, quizzes, and unit tests monday.
61
What does the energy diagram of an exothermic reaction look like?
The reactants should have higher energy than the products, since energy is being released as the reaction progresses.
62
What does the energy diagram of an endothermic reaction look like?
The products should have higher energy than the reactants, since energy is being absorbed as the reaction progresses.
63
How is evidence of a change in enthalpy of a chemical system provided by?
A temperature change of its surroundings.
64
How to find the enthalpy change of a reaction from a potential energy diagram?
The difference between the final-inital energies in the diagram represents the enthalpy change (positive or negative) of the reaction.
65
What is Hess' law?
A way of finding enthalpy change without experimental evidence. It states that: The addition of chemical equations yields a net chemical equation whose enthalpy change is the sum of the individual enthalpy change; (delta)rH= Sum of (delta)rH^deg (standard conditions).
66
What happens when a chemical equation is reversed?
The sign of (delta)rH changes.
67
What happens if the coefficients of a chemical equation are altered by multiplying or dividing by a constant factor?
(delta)rH is altered by the same factor.
68
What are the steps to determining an enthalpy change of a reaction using Hess' law?
1. Write the net reaction equation if not given. 2. Manipulate given equations so they will add to yield net equation. 3. Apply modifications you have made to each reaction to individual enthalpies (whether multiply/divide, or reverse). 4. Cancel out/add both sides to yield net equation. 5. Add ADJUSTED enthalpy changes to obtain net enthalpy change (in KJ). 6. If required, determine molar enthalpy for a specific entity (in KJ/mol), using it's molar coefficient in net equation.
69
What is the reference energy state?
Elements are defined as the reference point at which potential energy at SATP is defined as 0. Much like 0 degree temperature, remember that a reference point doesn't imply they can't have an enthalpy (much like a temperature of 0 degrees doesn't imply the average kinetic energy of the molecules is 0).
70
What is thermal stability?
The tendency of a compound to resist decomposition when heated. The lower the standard molar enthalpy of formation, the greater the thermal stability.
71
What to do before asking/searching the reason for a concept?
Think of the properties of the things involved in the concept, and come up with a logical hypothesis.
72
What is the formula of Hess' Law which allows you to find the net enthalpy change for a chemical reaction?
net (delta)rH= Sum of enthalpies of products (individual formation molar enthalpy multiplied by molar coefficient) minus the sum of enthalpies of reactants (individual formation molar enthalpies multiplied by molar coefficient).
73
How to find the enthalpy change ((delta)H) of a chemical system during a heat transfer?
(delta)H= -(Q). The change in enthalpy of a chemical system is equal and opposite to the thermal energy transferred or absorbed by it's surroundings (mcdeltat).
74
What to do in thermal energy transfer question where you're given weight of calorimeter?
Do separate q=mct where you assume it has the same temperature change as it's surroundings, and add to the thermal energy transfer of it's surroundings.
75
What to do in thermal energy transfer question where 2 solutions are combined?
The surroundings= combined volume of 2 solutions. If the question asks for the molar enthalpy of one of the 2 solutions, divide it's moles present by -(Q).
76
How to find industry names if you forget them on diploma?
They're on your formula sheet.
