[18.5] rate constants and temperature

Question 1

why are so few collisions successful?

Answer 1

• steric factor: molecules must approach each other in the correct orientation
• molecules must have more energy than activation energy

Question 2

what does the arrhenius equation show?

Answer 2

• that the relationship between reaction rate and temperature is exponential
• eg. small increase in temperature causes large increase in reaction rate
• gives us a simple and accurate method of measuring Ea

Question 3

when temperature is increased, what 2 factors contribute to the increased rate and rate constant?

Answer 3

• increasing temperature shifts the boltzmann distribution to the right, increasing the proportion of particles that exceed the Ea
• as temp increases, particles move faster and collide more frequently

Question 4

what is the arrhenius equation?

Answer 4

k = Ae ⁻ᴱᵃ / ᴿᵀ

Question 5

what do the different components of the arrhenius equation stand for?

Answer 5

• k = rate constant
• A = pre-exponential frequency factor
• e = natural log constant (2.718…)
• -Ea = activation energy (J mol⁻¹)
• R = gas constant (8.314)
• T = temperature (K)

Question 6

what do high Ea and high T mean for the rate constant?

Answer 6

• high Ea = low rate constant
• high T = high rate constant

Question 7

how can the e be inversed in the arrhenius equation?

Answer 7

• because there is an exponential in the equation, you can ‘inverse’ the e into a natural log, ln
• ln k = ln A - Ea / RT

Question 8

graphing arrhenius

Answer 8

• y = ln k, m = - Ea / R, x = 1 / T, c = ln A
• a plot of ln k (y-axis) against 1 / T (x-axis) gives downward straight line
• gradient m = - Ea / R
• incercept c of ln A on y axis (extrapolate)