C6 Rate of Reaction (Part 1)

Question 1

How is rate of reaction calculated?

Answer 1

Rate of reaction = amount (e.g. grams, cm3) of reactant used or product formed
divided by time

Rate of reaction (mol/s) = Moles of reactant used or product formed divided by time

Question 2

What are the various units for

rate of reaction?

Answer 2

Can include g/s or cm3/s or mol/s

Generally, mass/time, volume/time, moles/time

Question 3

Name three common ways of

measuring rate of reaction

Answer 3

• Loss in mass of reactants
• Volume of gas produced
• Time for a solution to become opaque

Question 4

Describe measuring the rate

by monitoring mass loss

Answer 4

Place the reaction flask on a balance. In these reactions (e.g. metal carbonate +
acid) a gas is given off, so record the decrease in mass in time intervals (note
hydrogen is too light). Plot a graph of mass vs time.

Question 5

Describe measuring the rate
by monitoring the volume of a
gas

Answer 5

Connect a gas syringe to a reaction flask and measure the volume of a gas
formed in time intervals. Plot a graph of volume vs time.

Question 6

Describe measuring the rate
by monitoring the
disappearance of a cross

Answer 6

Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this
cross. Mix the reagents, and measure how long it takes for a cloudy mixture to
conceal a cross.

Question 7

How to find a rate of reaction at
some time, t, from a graph of
amount of reactant vs time?

Answer 7

• Pick a point corresponding to the time t, and find the tangent to the curve at
  this point.
• The tangent is the gradient of this graph - it tells you how fast the reaction
  proceeds at this point. The steeper the tangent line, the faster the rate.
  Gradient of tangent can be expressed in change in y values over change in x
  values.

Question 8

State five factors affecting the

rate of a chemical reaction

Answer 8

• Concentration of reactants
• Pressure of gases (volume)
• Surface area
• Temperature
• Catalysts

Question 9

What is the collision theory?

Answer 9

Chemical reactions can occur only when reacting particles collide with each other
with sufficient energy (more than or equal to activation energy).

Question 10

Describe and explain the
effect of increasing
temperature on the rate of
reaction

Answer 10

T increases = faster reaction
As T increases, kinetic energy of particles increases, i.e. more energetic
collisions
Also, they move faster, so they collide more frequently
However, there is no straight line relationship between rate and temperature,
i.e. they are not directly proportional to each other

Question 11

Describe and explain the effect
of increasing concentration on
the rate of reaction

Answer 11

Conc. increases = faster reaction,

More reactants = more frequent collisions

Question 12

Describe and explain the effect
of increasing pressure of a gas
on the rate of reaction

Answer 12

Increasing the pressure of reacting gases, is the same as increasing concentration. It increases the number of gas molecules in the same volume and
so increases the frequency of collisions and therefore increases the rate of reaction.

Note that volume and pressure are inversely proportional to each other.
Increasing the volume retards the reaction.

Question 13

Describe and explain the
effect of increasing surface
area

Answer 13

If solid reactants are in smaller pieces, they have a greater surface area. Increasing the surface area of solid reactants increases the frequency of collisions
and so increases the rate of reaction, e.g. block of magnesium reacts slower with acid then magnesium powder