Energy and Rates of Reaction Flashcards
1
Q
System and Surroundings:
A
- System – the collection of atoms and molecules involved in a chemical reaction.
- Surroundings – anything else around the chemical reaction
2
Q
Energy (Enthalpy and Heat of Reaction)
A
- ENTHALPY (H) – chemical potential energy.
- HEAT OF REACTION (ΔH) – The change in enthalpy between reactants and products.
3
Q
Exothermic Reactions
A
- Energy is released to the surroundings.
- The reactants have more enthalpy than the products.
- Combustion & neutralisations.
4
Q
Endothermic Reactions
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- Energy is absorbed.
- The reactants have less enthalpy than the products.
- Energy is taken in from the surroundings
5
Q
Enthalpy
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- ΔH = enthalpy of products – enthalpy of reactants
- ΔH = Hproducts - Hreactants
6
Q
Exothermic and Endothermic Equations
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Exothermic Thermochemical Equation
- CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + energy
- Exothermic reactions release heat energy from the reacting system to the surroundings and have a –ve change in enthalpy.
Endothermic Thermochemical Equation
- C(s) + 2S(g) + energy -> CS2(g)
- Endothermic reactions absorb heat energy into the reacting system from the surroundings and have a +ve change in enthalpy.
7
Q
Bonding and Energy
A
- Stored chemical potential energy depends on the bonding involved in the substance.
- Energy changes are a result of bonds breaking and bonds forming.
- The amount of energy needed to break a bond is the same as the amount needed to form that same bond.
- Bond breaking processes require energy so is endothermic.
- Bond forming processes release energy so is exothermic.
- If bonds formed are weaker and fewer than bonds broken, energy will be absorbed.
- If bonds formed are stronger and more numerous than those broken, energy will be released.
- Summary:
- Reactions are exothermic when there are more bonds formed or they are stronger than the bonds broken.
- Reactions are endothermic when there are fewer bonds formed or they are weaker than the bonds broken.
8
Q
Heat of Reaction
A
- ΔH = Hproducts - Hreactants
- We don’t actually measure enthalpy of a substance but we do measure the change during a reaction.
- ΔH is the heat of reaction
- -ΔH = exothermic rxn (energy released)
- +ΔH = endothermic rxn (energy absorbed)
- Values given per mole of 1 of the species
- Heat of formation - the enthalpy change when one mole of a compound is formed from the elements in their stable states.
9
Q
Physical and Chemical
A
- Generally the physical process have smaller energy changes than chemical processes.
- H2(g) + ½ O2(g) H2O(l) H = -286kJ
- H2O(g) H2O(l) H = -44kJ
- Has to do with bonding involved.
10
Q
Types of Enthalpy
A
- Heat of combustion
- CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) ΔH = -803kJ
- Heat of fusion
- NaCl(s) -> Na+(l) + Cl-(l) ΔH = +28kJ
- Endothermic
- Melting - heat energy absorbed from surroundings
- Heat of vaporisation
- CH3CH2OH(l) -> CH3CH2OH(g) ΔH = +43.3kJ
- Endothermic, increase in EP
- Condensation – exothermic (decrease in EP)
11
Q
Reaction Rates
A
- Reaction rate can be determined:
- Rate of disappearance of reactants
- Rate of appearance of products
- The average rate of a reaction would be obtained by measuring the change over a series of time intervals.
- A graph of the reaction can be plotted to show the change in the rate of a reaction as it proceeds.
- The rate of reaction is quickest at the beginning of the reaction, as shown by the steepness of the graph. However, as the reaction proceeds the rate decreases.
- The instantaneous rate of a reaction can be determined using this graph. It is achieved by determining the slope of the tangent to the curve at a particular time.
- Example: CaCO3(s) + 2CH3COOH(aq) -> Ca2+(aq) + 2CH3COO-(aq) + CO2(g) + H2O(l)
1. The mass of CaCO3(s) decreases.
2. The concentration of CH3COOH(aq) decreases.
3. The amount of formed Ca2+(aq) and CH3COO-(aq) increases.
4. The volume of CO2(g) produced increases.
12
Q
What happens in chemical reactions
A
- When chemicals react:
- bonds are broken
- bonds are formed
- For this to occur, particles must collide in a certain way
- This is called collision theory
13
Q
Collision Theory
A
- For a chemical reaction to occur:
- Reactant particles must collide.
- Sufficient energy (activation energy, Ea) to disrupt bonds within reactant particles.
- An orientation that is suitable for the breaking and formation of bonds.
14
Q
Energy Profile Diagram
A
- The activation energy is the minimum amount of energy required for a successful collision to occur.
- This is equivalent to the amount of energy needed to disrupt the bonds present in the reactants.
- Show the potential energy (EP) changes during a reaction
- Collision – kinetic energy (EK) transformed into EP
- Initially, EP increases
- Due to breaking bonds & rearrangement of atoms
- If sufficient energy in collision (activation energy, Ea) then activated complex or transition state reached
- Activated complex:
- Highest EP
- Bond breaking & forming
- Arrangement of atoms that is unstable
- Exists for an instant before reaction ends
15
Q
Rate of Reaction
A
- Rate of reaction depends upon the number of molecules with enough energy to overcome the activation energy.
- The energy profile diagram can be related to the EK distribution diagram for a reaction at a particular temperature.
- The following graph indicates that only a small proportion of molecules in this system have energies greater than that of the activation energy barrier.
- Few collisions would have sufficient energy to be successful – slow reaction rate.
- Reverse Reaction: energy profile diagrams can be reversed and information about the reverse reaction can be determined.
- Forward Reaction:
- the reaction is exothermic
- the heat of reaction, ∆H, is -572 kJ
- the activation energy, is Ea kJ
- Reverse Reaction:
- the reaction is endothermic
- the heat of reaction, ∆H, is +572 kJ
- the activation energy, is (Ea + 572) kJ
16
Q
How to speed up a chemical reaction
A
- Increase the frequency or number of collisions per second
– the more often particles collide, the more successful collisions there will be - Increase the amount of energy the particles have
– if particles have more energy, a greater percentage of collisions will have sufficient energy to be successful
17
Q
Total Number of Collisions vs. % successful collisions
A
- 1000 collisions per second & 60% are successful, there will be 600 successful collisions
- 2000 collisions per second and 60% are successful, there will be 1200 successful collisions – Faster Reaction Rate
- still 1000 collisions per second but 80% are successful, there will be 800 successful collisions – Faster Reaction Rate
18
Q
Factors Affecting Reaction Rate
A
The rate of reaction is affected by five factors:
1. The nature of reactants.
2. Concentration of reactants (solutions) or pressure (gases).
3. State of subdivision of reactants.
4. Temperature.
5. Presence of a Catalyst
19
Q
Nature of Reactants
A
- If a reaction doesn’t involve bonding arrangements it is likely to be rapid at room temperature.
- Ag+(aq) + Cl-(aq) AgCl(s) - If a reaction involves breaking bonds, it is likely to be slow at room temperature.
- CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
* covalent bond-breaking and covalent bond-forming, like the combustion of methane in oxygen, are likely to be slower than those involving the combination of oppositely charged ions to form a precipitate.
* If only weak bonds or no bonds are broken – very low activation energy – a large number of particles will have enough energy for a successful collision
20
Q
Sub-Division
A
- The smaller the particles in a reaction, the faster the reaction rate
- Smaller pieces of wood burn faster than larger blocks of wood
- Smaller pieces – exposes more of the reactant surface
- Smaller particles, greater surface area exposed during reaction
- The smaller the particles in a reaction, the faster the reaction rate.
- Smaller particles means greater surface area exposed during reaction.
- If more reactant particles are exposed, there will be more particle collisions per second
- the same percentage of collisions are successful
- aerosols/mists increase surface area of liquids
- LARGE PIECES: only 50% of collisions have enough energy and there are 1000 collisions per second
- 50% x 1000 = 500 successful collisions per second
- SMALLER PIECES: the number of collisions increases to 10 000 collision per second but still only 50% are successful
- 50% x 10 000 = 5 000 successful collisions per second
- more collision per second results in a faster reaction rate!
21
Q
Concentration
A
- As concentration increases, there are more particles in a given volume.
- Having more reactant particles results in more particle collisions per second but does not change the % of successful collisions (collisions with enough energy to react)
- more collision per second results in a faster reaction rate
Gas Pressure (Conc. of Gases):
* raising gas pressure can be done by:
- decreasing volume
- adding more gas particles (increasing concentration of gas particles)
* this results in a greater rate/frequency of particle collisions
* leading to faster reaction rates
22
Q
Temperature
A
- Temperature is the only factor that has a two-fold effect on reaction as it:
1. Increases the frequency or number of collisions per second
2. Increase the amount of energy the particles have
- Increases the frequency or number of collisions per second
- particles at higher temperatures have more kinetic energy than particles at lower temperatures
- kinetic energy is ‘moving energy’
- therefore the particles are moving faster
- this means that they will collide more often resulting in an increase in the frequency of particle collisions - Increase the amount of energy the particles have
- this effect is more significant
- when particles with more kinetic energy collide, a greater percentage of collisions will have sufficient energy to be successful
- there is an increase in the percentage of successful collisions
- results in an increase in reaction rate
23
Q
Catalysts
A
- A substance that provides an alternative reaction pathway with lower Ea
- Now there are more particles with the required activation energy to react
- Catalysts are unchanged in the reaction
- More particles with sufficient energy, frequency of successful collisions increases, increase reaction rate
- Only a small amount is required
- Useful in lowering the energy requirements (T & P) in industrial chemical reactions
- Industrial manufacture of chemicals
- N2(g) + 3H2(g) 2NH3(g)
- Fe catalyst allows the bonds in N2 and H2 molecules to be broken more easily
- Removal of pollutants from combustion reactions – e.g. catalytic converters
