rate of reaction

Question 1

how is rate of reaction calculated?

Answer 1

rate of reaction =
amount of reactant used or product formed
÷
time

Question 2

why do reactions slow down

Answer 2

the reactants have been used up and dont have alot of molecules left to react

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

what is collision theory

Answer 4

chemical reactions can only take place when reacting particles collide with each other
collisions must have enough energy

Question 5

what is the rate of a reaction determined by?

Answer 5

the frequency (number per second) of successful collisions

Question 6

How to find a rate of reaction at some

time, z, from a graph of amount of reactant vs time?

Answer 6

• Pick a point corresponding to the time z, 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 7

state five factors affecting the rate of a chemical reaction

Answer 7

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

Question 8

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

Answer 8

concentration is proportional to the rate of a reaction
conc. increases = faster reaction,
More reactants = more frequent collisions

Question 9

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

Answer 9

• It increases the number of gas molecules in the same volume
• increases the frequency of collisions and therefore increases the rate of reaction.

Note that volume and pressure are inversely proportional to each other.

Question 10

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

Answer 10

Temp increases = faster reaction
As temp increases, kinetic energy of particles increases, more energetic collisions, they move faster, so they collide more frequently

However, there is no straight line relationship between rate and temperature, they are not directly proportional to each other

Question 11

describe and explain the effect of increasing surface

area

Answer 11

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.

Question 12

What is a catalyst and how does it work? How does it

affect the reaction profile?

Answer 12

a catalyst increases the rate of reaction but is not used up.

It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy).

Question 13

what is activation energy?

Answer 13

the minimum amount of energy the particles must have in order to react

Question 14

What is an enzyme?

Answer 14

enzyme is a molecule that acts as a catalyst in a biological system.