Energy and Rates of Reaction

Question 1

System and Surroundings:

Answer 1

• System – the collection of atoms and molecules involved in a chemical reaction.
• Surroundings – anything else around the chemical reaction

Question 2

Energy (Enthalpy and Heat of Reaction)

Answer 2

• ENTHALPY (H) – chemical potential energy.
• HEAT OF REACTION (ΔH) – The change in enthalpy between reactants and products.

Question 3

Exothermic Reactions

Answer 3

• Energy is released to the surroundings.
• The reactants have more enthalpy than the products.
• Combustion & neutralisations.

Question 4

Endothermic Reactions

Answer 4

• Energy is absorbed.
• The reactants have less enthalpy than the products.
• Energy is taken in from the surroundings

Question 5

Enthalpy

Answer 5

• ΔH = enthalpy of products – enthalpy of reactants
• ΔH = Hproducts - Hreactants

Question 6

Exothermic and Endothermic Equations

Answer 6

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.

Question 7

Bonding and Energy

Answer 7

• 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.

Question 8

Heat of Reaction

Answer 8

• 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.

Question 9

Physical and Chemical

Answer 9

• 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.

Question 10

Types of Enthalpy

Answer 10

• 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)

Question 11

Reaction Rates

Answer 11

• 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.

Question 12

What happens in chemical reactions

Answer 12

• 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

Question 13

Collision Theory

Answer 13

• 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.

Question 14

Energy Profile Diagram

Answer 14

• 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

Question 15

Rate of Reaction

Answer 15

• 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

Question 16

How to speed up a chemical reaction

Answer 16

1. Increase the frequency or number of collisions per second
   – the more often particles collide, the more successful collisions there will be
2. 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

Question 17

Total Number of Collisions vs. % successful collisions

Answer 17

• 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

Question 18

Factors Affecting Reaction Rate

Answer 18

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

Question 19

Nature of Reactants

Answer 19

1. If a reaction doesn’t involve bonding arrangements it is likely to be rapid at room temperature.
   - Ag+(aq) + Cl-(aq)  AgCl(s)
2. 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

Question 20

Sub-Division

Answer 20

• 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!

Question 21

Concentration

Answer 21

• 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

Question 22

Temperature

Answer 22

• 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

1. 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
2. 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

Question 23

Catalysts

Answer 23

• 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