Chapter 18 Key concepts

Question 1

how to determine the order of reaction

Answer 1

they cant be found directly from the chemical equation so monitor how a physical quantity changes over time

Question 2

what is continuous monitoring and give 3 example methods

Answer 2

concentration time graphs plotted from continuous measurements taken during the course of the reaction.
e.g. gas collection
and mass loss. if no gas made then colour change. either by eye or colorimeter

Question 3

what is a colorimeter and how does a colorimeter work?

Answer 3

measures the light intensity of light passing through a sample.

wavelength passed through a coloured solution is controlled using a filter (complementary colour to the colour being absorbed in the reaction) the amount of light is then measured. absorbance is recorded which is directly linked to the concentration of the solution

Question 4

Describe the first order concentration time graph

Answer 4

downward curve with decreasing gradient over time
reaction gradually slows down
half life is constant

Question 5

Describe the second order concentration time graph

Answer 5

also a downward curve

steeper at the start but tailing off more slowly

Question 6

Describe the zero order concentration time graph

Answer 6

straight line with a negative gradient
reaction rate nah change during the course of the reaction
GRADIENT OF THE GRAPH IS EQUAL TO K

Question 7

How do you confirm that its a first order reaction

Answer 7

measure successive half lives. if they’re the same then the reaction is first order with respect to the reactant

• draw a tangent at a concentration of your choice so the rate can be calculated at this con
• if you chose 1moldm3 then check the time take from 1-0.5. 0.5-0.25 etc.
• constant half life means first order

Question 8

Two methods of working out k for a first order reaction

Answer 8

1. calculate rate constant from rate
work out gradient of tangent. that gives you the rate
use your chosen con
rate/con = k
*not ideal as very difficult to judge how steep tangent should be drawn
2.Calculate rate constant from half life
k=ln2/halflife. 
*much more accurate

Question 9

How can you plot a rate con graph and Why are rate con graphs important?

Answer 9

Offer a route into the direct link between rate and con in. The rate equation.measure the rate at different con

Question 10

Zero order rate con

Answer 10

Horizontal straight line graph with zero gradient.
y intercept is k.
Rate doesn’t change with increasing concentration

Question 11

First Order rate con

Answer 11

Makes a straight line graph through the origin
Rate is directly proportional to concentration here
k is the gradient

Question 12

Second order rate con

Answer 12

Upward curve starting from the origin with increasing gradient
-to get k, plot another graph, rate by the concentration squared. That would make a straight line through the origin.that gradient is k

Question 13

clock reaction

Answer 13

More convenient way of obtaining the initial rate of a redaction by taking a single measurement .
Time t from the start of a experiment is measured for a visual change to be observed usually a colour or ppt

*provided no sig change in in rate during the time. So average rate of reaction over this time is same as initial rate. Hence initial rate proportional to 1\t. Repeat clock reaction bare times with diff con and values of 1/t calculated for each experiment

Question 14

How to work out the initial rate

Answer 14

Measure the gradient of a tangent at t=0 on a con time graph

Question 15

How accurate are clock reactions

Answer 15

You’re measuring average rate during the first part of the reaction

• assume that average rate of reaction is constant and same as the initial rate
• the shorter the period of time over which an average rate is measured , the less the rate changes over that time period

Question 16

What, when you increase the temperature are the mAin two factors that lead to increased rate and rate constant

Answer 16

Shifts Boltzmann Distribution to the right, increasing proportion of particles that exceed the Ea
Particles move faster and collide more frequently (comparatively smaller effect than the former)

Question 17

What does -Ea/Rt (exponential factor) represent

Answer 17

The proportion of molecules that exceed the Ea and have sufficient energy for a reaction to take place

Question 18

What does A,the pre exponential factor/frequency factor represent?

Answer 18

The frequency of collisions with the correct orientation. Slightly increases with temperature but essentially constant over a small temperature range. Effectively rate as if there was no Ea