6 Chemical reactions

Question 1

Physical change

Answer 1

A change that does not alter what the substance is, but the physical state changes, for example melting.

Question 2

Reactants

Answer 2

The substances that exist at the start of a chemical reaction. In a symbol equation, reactants are shown on the left-hand side.

Question 3

Products

Answer 3

The substances formed during a chemical reaction. In a symbol equation, products are shown on the right-hand side.

Question 4

Chemical change

Answer 4

A change that alters what the substance is during the process of a chemical reaction, so that the reactants are different from the products.

Question 5

Effervescence

Answer 5

Bubbles form when a gas is formed in a liquid. The scientific name for this is effervescence.

Question 6

What does redox reaction stand for

Answer 6

Reduction–oxidation, Redox reactions are reactions involving simultaneous, oxidation and reduction reactions.

Question 7

Oxidation

Answer 7

Is the gain of oxygen Or the loss of electrons

Question 8

Reduction

Answer 8

Is the loss of oxygen Or the gain of electrons

Question 9

Redox reaction OILRIG

Answer 9

Oxidation
Is
Loss of electrons
Reduction
Is
Gain of electrons

Question 10

Oxidation state

Answer 10

A number assigned to an element showing the number of electrons lost or gained (or even shared) in a compound or ion.

Question 11

Redox reaction and oxygen state

Answer 11

Oxidation can also be defined as an increase in oxidation number and reduction is a decrease in oxidation number.

Question 12

Half equation

Answer 12

A symbol equation that only shows the atoms and ions that react together.
These equations show the individual oxidation and reduction reactions in terms of loss and gain of electrons.

Question 13

Oxidising agent

Answer 13

A substance which oxidises another substance and itself is reduced.

Question 14

Reducing agent

Answer 14

A substance which reduces another substance and itself is oxidised.

Question 15

Oxidation number can appear in one different way what is it

Answer 15

These can appear in the name of ions and are shown as Roman numerals.

Question 16

what is the oxidation number of an element in there uncombined state

Answer 16

0 Zero

Question 17

The oxidation number of a monatomic atom

Answer 17

Is the same as the charge on the ion

Question 18

What should the sum of the oxidation number in a compound equal to

Answer 18

0 Zero

Question 19

What should the sum of the oxidation numbers in an ion equal to

Answer 19

The charge on the ion

Question 20

A good oxidising agent is

Answer 20

Acidified potassium maganate(Vll)

Question 21

A good reducing agent is

Answer 21

Aqueous potassium/ sodium Iodide
(Iodide ion)

Question 22

When Acidified potassium manganate(VII) is reduced what colour does it change from and to

Answer 22

Purple to colourless

Question 23

when an Iodide ion is oxidised what colour does it change from and to

Answer 23

colourless to brown

Question 24

Usual oxidation states for group 1 and 2 metals and Hydrogen, oxygen, fluorine and chlorine

Answer 24

Element Usual oxidation state
Group I metals +1
Group II metals +2
Hydrogen +1
Oxygen −2
Fluorine −1
Chlorine −1

Question 25

Rate of reaction

Answer 25

The speed at which a reaction takes place.

Question 26

Calculating the rate of reaction

Answer 26

Gradient = Rate
Rate of reaction= Mass loss in grams ÷ time in seconds
Rate= Volume of gas incm3 ÷ time in seconds

Question 27

Factors affecting the rate of a reaction

Answer 27

Concentration of solutions – when one or more of the reactants are in solution, an increase in concentration increases the rate of reaction.

Pressure of gases – when one or more of the reactants are in the gas state, an increase in pressure increases the rate of reaction.

Surface area – when one or more of the reactants are in the solid state, an increase in surface area increases the rate of reaction.

Temperature – an increase in temperature increases the rate of reaction.

Catalyst - increases the rate of reaction.

Question 28

The effect of concentration of solutions

Answer 28

If the concentration of a solution increases, then the number of particles in a given volume also increases.

This increases the rate of the reaction as there are more reactant particles available.

Question 29

The effect of pressure of gases

Answer 29

The effect of increasing pressure is similar to concentration but refers to gases.

If the pressure increases, often by decreasing the volume, then the number of particles in a given volume also increases an so the rate of reaction increases.

Question 30

The effect of surface area of solids

Answer 30

When the particle size of a solid decreases, the surface area of the solid increases.

This puts more particles in contact with the other reactant.

The chance of successful collisions increases, therefore the rate of reaction will increase.

Alternatively, If the particle size of a reactant increases, the surface area of the solid decreases.

This leads to less chance of successful collisions and a slower rate of reaction.

Question 31

Danger of high surface area

Answer 31

However, the effect of particle size can sometimes be dangerous.

If a powdered substance ignites in the presence of air, it can cause an explosion.

Explosions are caused by the large surface area of the particles and the very fast rate of combustion.

Here are some examples of materials that have been known to cause dust explosions:

flour dust in flour mills
metal dust and wood dust in factories
coal dust in coal mines.

Question 32

The effect of catalysts

Answer 32

A catalyst works by decreasing the activation energy, Ea , of a reaction.

The Ea is decreased by the fact that the catalyst provides an alternative pathway for a reaction to happen.

This alternative pathway has a lower activation energy, which allows more particles to achieve this minimum amount of energy for a successful collision.

As a catalyst does not get used up in the reaction, they can be reused or recycled.

Question 33

The effect of temperature

Answer 33

If the temperature is increased, the particles gain more energy and move faster.

This means that they collide more often.

If the temperature is decreased, the particles have less energy, move more slowly and collide less often.

Collision theory states that collisions only occur if the particles collide with enough energy to get the reaction going. This minimum energy is known as the activation energy and is written as Ea .

Question 34

Collision theory

Answer 34

To explain exactly what must happen for a ‘successful’ chemical reaction to occur between particles:
-The particles must collide.
-The particles must collide with the correct orientation (head-on).
-The particles must collide with sufficient kinetic energy. The minimum amount of energy required to create a reaction is known as the activation energy, Ea .

Question 35

The relationship between concentration and rate of reaction

Answer 35

There is a mathematical relationship between concentration and rate of reaction.

The two variables are said to be directly proportional.

This means that if the concentration doubles then the rate of reaction doubles.

This is because if the concentration is doubled, the number of particles in a given volume doubles, and therefore the frequency of successful collisions also doubles.

Question 36

Dynamic equilibrium

Answer 36

The rate of the forward reaction is equal to the rate of the reverse reaction and at this point there is no change in the proportion of reactants or products.

Question 37

Equilibrium

Answer 37

In chemistry, this is a system in which all the concentrations of the reactants and products are constant as reaction rates are exactly balanced.

Question 38

Reversible reaction

Answer 38

Reactions that can go in both directions i.e. reactants to products (forward reaction) then back from products to reactants (reverse reaction). These reactions are represented by the symbol ⇌.

Question 39

A reversible reaction is at equilibrium when:

Answer 39

Rhe rate of the forward reaction equals the rate of the backward reaction

The concentrations of reactants and products remain constant

The reaction occurs in a closed system. A closed system is when there is no exchange of substances with the surroundings. An example of a closed system is a beaker with a lid on it.

Question 40

Le Châtelier’s principle

Answer 40

When conditions change in a reversible reaction at chemical equilibrium, then the equilibrium moves in the direction that will oppose that change and bring the system back to equilibrium.

Question 41

Posistion of equilibrium

Answer 41

Where the equilibrium reaction lies. For instance more to the left-hand side or the right-hand side of a reversible reaction.

Question 42

The effect of changing concentration on equilibrium

Answer 42

If the concentration of the reactants increases, the equilibrium shifts to the right-hand side to remove the extra reactant that has been added. The effect is that there is an increase in yield of product.

If the concentration of the products increases, the equilibrium shifts to the left-hand side to remove the extra product that has been added. The effect is that there is a decrease in the yield of product.

Question 43

The effect of changing pressure on equilibrium

Answer 43

When reversible reactions occur in the gaseous state adjusting the pressure will affect the position of the equilibrium.

If the pressure is increased the equilibrium shifts to the side with fewer moles as this would reduce the overall pressure and restore the balance of equilibrium.

If the pressure was decreased, then the equilibrium will move to the side with more moles as this would increase the overall pressure, and therefore this would restore the balance of equilibrium.

Question 44

Haber process

Answer 44

The industrial manufacture of ammonia from atmospheric nitrogen and hydrogen from methane. The forward reaction is between nitrogen and hydrogen to form the desired product, ammonia.

N2(g) + 3H2(g) ⇌ 2NH3(g)

For the reaction between nitrogen and hydrogen, increasing the pressure would shift the position of equilibrium to the right-hand side as there are 2 moles of ammonia compared to the 4 moles of nitrogen and hydrogen.

This means that the equilibrium shifts towards the production of ammonia. The result would be an increased yield of ammonia.

Question 45

Typical condition for the Haber process

Answer 45

Typical conditions used in Haber process are 450 °C, 200 atm/20 000 kPA, and iron filings are used as a catalyst.
On an industrial scale, the reactants need to be sourced in vast quantities.
Nitrogen gas comes from the fractional distillation of liquid air, while hydrogen gas is sourced by reacting methane with steam.

Question 46

The effect of changing temperature on equilibrium

Answer 46

If the temperature is increased then the equilibrium would shift to the endothermic reaction (the reverse reaction) to counteract the increase in temperature. This means less product would form.

If the temperature is decreased, the reaction would shift to the right-hand side to restore equilibrium because the forward reaction is exothermic. This would result in a higher yield of products.

Question 47

Contact process

Answer 47

The industrial process for producing sulfuric acid.
2SO2(g) + O2(g) ⇌ 2SO3(g)

You may therefore expect the Contact process to be carried out at low temperatures to favour the production of sulfur trioxide. However, when the temperature is low, the rate of reactions is also low as the number of successful collisions will be reduced.

Question 48

Typical conditions for the Contact process

Answer 48

However, the industrial process uses sulfur dioxide is usually obtained from burning sulfur or roasting sulfide ores in oxygen (air).
The sulfur dioxide is reacted with oxygen in the presence of a vanadium(V) oxide catalyst at conditions of 450 °C, a pressure of 200 kPa/2 atm.

Question 49

The effect of changing catalyst on equilibrium

Answer 49

A catalyst speeds up both the forward and backwards reactions equally and so does not affect the position of equilibrium.

Question 50

The effect of changing conditions on the position of equilibrium.

Answer 50

Change in condition: Effect on equilibrium
Increase in concentration of reactant: Moves to the right-hand side
Increase in concentration of product: Moves to the left-hand side
Increase in pressure: Moves to the side with fewer gas moles
Decrease in pressure: Moves to the side with more gas moles
Increase in temperature: Moves to the endothermic reaction side
Decrease in temperature: Moves to the exothermic reaction side

Question 51

Why is the rate of reaction the highest at the start

Answer 51

Highest concentration of reactants

Question 52

Two features of equilibrium

Answer 52

Constant concentrations of reactants and the products
the rate of the forward reaction equals the rate of the backward reaction