5.1 - 6.3. Chemical energetics & reactions

Question 1

exothermic

Answer 1

transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings

bond making, negative enthalpy change

Question 2

endothermic

Answer 2

absorbs thermal energy from the surroundings leading to a decrease in the temperature of the surroundings

bond breaking, positive enthalpy change

Question 3

activation energy

Answer 3

the minimum energy colliding particles must have to react

Question 4

enthalpy change positive, vice versa

Answer 4

enthalpy change positive, total energy in the bonds of the reactants less than the total energy in the bonds of the products, energy was taken in by the reaction mixture to break bonds i.e. the reaction is endothermic

vice versa

Question 5

concentration

Answer 5

higher concentration -> more particles per unit volume -> more frequent collisions -> higher rate of reaction

Question 6

pressure

Answer 6

only for gases; higher pressure -> more frequent collisions -> higher rate of reaction

Question 7

surface area

Answer 7

only for solids; higher surface area -> more reactions that can occur at a given moment -> higher rate of reaction

Question 8

temperature

Answer 8

higher temperature -> higher average kinetic energy of particles -> particles move faster -> more frequent collisions -> higher rate of reaction

Question 9

catalyst

Answer 9

presence of catalyst -> provides alternate reaction pathway that requires less activation energy -> more reactant particles collide with sufficient energy to react -> higher rate of reaction

catalysts are not used up or changed during a reaction i.e. reusable

Question 10

graph theory

Answer 10

• The steeper the slope, the higher the rate of
  the reaction
• The curve is steepest initially so the rate is
  quickest at the beginning of the reaction
• As the reaction progresses, the concentration
  of the reactants decreases and the rate
  decreases shown by the curve becoming less
  steep
• When one of the reactants is used up, the
  reaction stops, the rate becomes zero and the
  curve levels off to a horizontal line
• The amount of product formed in a reaction is
  determined by the limiting reactant:
  ◦ If the amount of limiting reactant increases,
  the amount of product formed increases
  ◦ If the amount of the reactant in excess
  increases, the amount of product remains the
  same
• Drawing a tangent to the slope allows you to
  show the gradient at any point on the curve

Question 11

haber

Answer 11

H2 & N2 obtained via steam reforming of methane and fractional distillation of liquified air respectively.
Gases pumped into compressor through pipe
Gases compressed to 200atm in compressor
Pressurized gases pumped into tank with iron catalyst & temperature of 450C
Some H2 and N2 react, unreacted gas and ammonia pass into cooling tank & ammonia is liquified, removed and sent to storage vessels
Unreacted H2 & N2 recycled back into system to react again

Question 12

contact

Answer 12

S + O2 -> SO2 (non-reversible)
Burning sulfur to oxidize or roasting sulfide ores
2SO2 + O2 <-> 2SO3 (reversible)
Oxidation of sulfur dioxide to sulfur trioxide in the presence of a vanadium(V) oxide, V2O5, catalyst

Question 13

enthalpy

Answer 13

enthalpy of products - enthalpy of reactants
moles do affect