Thermo chem Flashcards
(41 cards)
1
Q
The first law of thermodynamics states that
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- Total energy of the universe is constant
- Energy can be converted to other forms however total energy of any system is conserved( It cannot be created nor destroyed)
2
Q
Two fundamental type of energy
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- Kinetic Energy (Ek): The energy of the motion of particle
- Potential Energy (Ep): Energy is that is stored in the bond of chemical
3
Q
The Second Law Of Thermodynamic States That
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In the absence of energy input the system will be in disordered
heat will always transfer from hot object to cooler object until thermal equilibrium is reached
4
Q
TEMPERATURE CHANGE
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5
Q
What is temperature?
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Its is the measure of kinetic energy of the particles of a substance
- Faster the particle moves = Higher temperature
6
Q
Three factor the amount of energy needs to heat a substance
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The mass of substance
The change in temperature
The type of substance (heat capacity)
Q= MCT
7
Q
ENERGY AND ENTHALPY
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8
Q
What is enthalpy
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- Sum of all forms of kinetic and potential energy in a system (Ek + Ep)
Enthalpy of individual substance cannot be measured (Ek can with thermometer but how to measure Ep)
Change in enthalpy occur whenever heat is released or absorbed in a physical, chemical change or nuclear change this can be measured
9
Q
Rules for Enthalpy
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- measured in J or KJ
- subscripts are used to denote type of process for the reaction
rH = Enthalpy Reaction
The symbol “*” (its an o) used to denote change taking in standard conditions
rH* = Standard enthalpy of reaction
10
Q
Molar enthalpy
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- Molar enthalpy is the enthalpy change per mole of a substance
- Molar enthalpy is also designated as (triangle)H although ill be just using H in the formula
- Can be measured in J/mol or KJ/mol
11
Q
D. Energy Change In Chemical Reactions
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- Change in potential energy take place during chemical reaction when one substance is converted to another substance
- Chemical bond are a source of stored potential energy
- Brea king bond required energy
- Forming bond releases energy
12
Q
Photosynthesis
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6CO2(g) + H2O + energy = C6H12O6(s) + 6O2(g)
13
Q
Endothermic
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> If the energy added to break the bond is greater than the energy released when the new bond is formed then the reaction is endothermic
- Designated as positive values since energy is being added to the system
14
Q
A
15
Q
Exothermic
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> If the energy added to break the bond is less than the energy released when the new bond are formed then the reaction is exothermic
- designated as negative value since energy is being lost in the system
16
Q
Important !
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The amount of energy lost or gained is directly proportional to the amount of substance that react
eg if 100g of a substance burning will release twice as much energy comapared to when 50g of that same substance burns
‘
17
Q
COMMUNICATING ENTHALPY CHANGES
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18
Q
Exothermic reaction (energy released )
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- rH Notation
- The heat of reaction can be given as a (triangle0 rH value outside of the equation
- The sign on (triangle) rH is negative since the enthalpy of the system is decreasing - Inside The Equation
- In an exothermic reaction enthalpy is included as a product
X + X = X + X + energy - Potential Energy Diagram
- Shows the potential energy of the reactants and the product of a chemical reaction
- Reactant have more potential energy than the product is an exothermic reaction
The difference between reactant and product is the (triangle)rH
19
Q
Endothermic (energy absorbed)
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- rH Notation
- The sign on rH is positive since the enthalpy of the system is increasing - Inside the Equation
- In an endothermic reaction enthalpy is included as a reactant - Potential Energy Diagrams
- Reactant have less potential energy than the product in an endothermic reaction
- The difference between the reactant and the product is the rH
20
Q
CALCULATING ENTHALPY CHANGE
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(triangle)rH = nrH
- Molar enthalpy, kJ/mol aand the number of moles of substance can be used to calculate the enthalpy change of chemical change.
- Number of mole can come from the balancing in the chemical reaction or from the formula n = m/M
positive endothermic
negative exothermic
21
Q
Calorimetry
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- Calorimetry is a technological process of measuring energy changes using an isolated system
- The isolated system used to determine the heat involved in a phase change or in a chemical reaction is called calorimeter
22
Q
Steps for Using a Simple Calorimeter
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- Measure mass or volume of water (mass small can - mass of can when empty)
- Measure initial mass of reactant
- Measure the initial temperature of the water in the calorimeter
- Add the reactant to the calorimeter
- Allow reaction to proceed, stirring the solution to ensure even temperature.
- Measure the final temperature of the water in the calorimeter (maximum temperature for the endothermic reaction)
- Final mass of reactant (triangle)m = initial mass - final mass
23
Q
Calorimeter points
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- Calorimetry is based on the First and Second law of Thermodynamics.. energy is conserved and energy is transferred from hotter objects to cooler objects until thermal equilibrium is reached
- It is assumed that no energy is gained or lost by the system except for the energy required to released by the chemical change
24
Q
Calculation are based on the Principle of Heat Transfer
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Heat loss = Heat gain
Exo: Hl(rxn) = HG(water)
Endo; Hl(water) = HG(rxn)
Endo is when water temperature decreased
Exo is when water temperature increase
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How to calculate kJ/g
- Use the formula mH instead on nh to give KJ/g
- You can also convert KJ/mol to KJ/g by dividing by the molar mass of the substance
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Hess' Law: Prediciting Enthalpy
- Because of the law of conservation of energy, the heat of reaction is the same whether the reactant are converted to the product in a singular reaction or in a series of reaction
- G.H. Hess suggested that if two or more thermochemical equations are added to give a final equation then the enthalpies can be added to give the enthalpy for the final equation
Sometimes the heat of reaction for chemical change is not easily measured due to time of reaction, cost, rarity of reactant etc. so we use Hess's Law to calculate (triangle)rH
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STEPS TO CALCULATE rH
1. Write the net reaction, if it is not given
2. Manipulate the given equations so they will add to yield the net equation
- if you multiply or divide an equation, multiply or divide the rH by the same factor
- if you flip an equation, flip the sign on rH
3.Cancel the reactants and products where possible to simplify( you must end up with your net equation)
4. Add the component enthalpy changes to get the net enthalpy change
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Hess's Law Formula
It stes that rH is the difference between the standard heat of formation of the reactant and product
rH*= fH*(product) - fH* (reactant)
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Standard Heat of formation rH
sometimes it is not easy to measure the heat change for a reaction ( to slow/ expensive)
- in this case, rH cn be deterimined by using heat formation
- heat formation (fH*) are change in the Ep that occurs when compounds are formed their elements
- fH* for elements cannot be directly measured therefore they are designated as zeros... all other fH* value are in reference to this
- The fH* is an indirect measure of the stability of a compound
- The more exothermic the formation, the more stable the compound is( means add energy to decompose it)
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ENERGY EFFICIENCY
Most canada energy(electricity) comes from chemical processes such as the combustion of fossil fuel
- Electricity is also generated through nuclear processes
- Both methods involves changing water into steam (a phase change) which turns turbines to generate electrical energy
- How does the energy from the physical, chemical, and nuclear process compare;
- Physical changes involve the breaking and forming of intermolecular forces
- chemical change involve in breaking and forming of chemical bonds
- nuclear changes involve changes within the nuclei of atoms
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Efficiency
% efficie = Energy output x 100
Energy input
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More knowledge
In any process the more energy conversion that take place the less efficient the process because of heat loss in transfer
- Gas furnace( natural gas) is about 90% efficient since it is used to directly supply heat
- Natural gas power is only 37% efficient because there are several energy conversion that take place before electricity is generated ( water to steam to kinetic energy to mechanical energy to electrical energy
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Fueling society
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Advantages of fossil Fuels
1. Realitivly low cost
2. Available
3. Plant set up, vehicle design, expertise affordable
4. Used all over the world
5. Deposits are larger
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Disadvantage
1. Release oof gases that contribute to the greenhouse effect and acid rain when burned
2. mining is detrimental to the environemnt
3. non renewable
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Important
1. Nuclear process produces radioactive waste that last for thousand of years
2. wind renewable energy sources( sun directly but are not free of problem there noisy and dangerous to birds )
3. Hydroelectric power is also renewable but damming river affects ecosystem on both side of the dams
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MORE IMPORTANT
THE amount of CO2g released by a fuel determine how clean it is.
fuels that uses renewable energy sources( solar, wind, geotherm etc)
Methane is the cleanest fossil fuel and coal is least clean (less carbon more clean)
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BOND ENERGY AND ACTIVATION ENERGY
Bond energy is the energy required to break a chemical bond or the energy released when a bond is formed
the change in enthalpy of a reaction represents the net effect from the breaking the bonds in the rectant(s) and forming the bonds of the product(s)
in exothermic reactions, bond breakaing absorbs less energy than the bond formation gives off resulting in a -rH
37
Endothermic
reaction bond breaking absorbs more energy than the bond formation gives off, resulting in a +rH
- the energy barrier that must be overcome for a chemical reaction is called activation energy - the height from reactant to the top of activiated.
the atom in the reactant have to be pulled apart in order for them to bond in a different configuration and become the products
The activiation energy is always higher than the energy contained in the reactant and the products, however the amount of activiation energy is necessary is dependent on the reaction
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CATALYSTS
1. its a substance that increase the rate of chemical reaction without being consumed itself
2. catalyst provide alternate pathway for chemical reaction
3. They lower the activation energy required for a reaction to take place which results in the production of a greater yield of product in a given length of time
39
Enzymes
compound that act as a catalyst in a living system are called enzymes
called biological catalysts
chemical reaction in the body occur at very low temperature (37*C) and without catalyst many would be too slow
