Topic 5 - Chemical Reactions

Question 1

Describe the endothermic reaction of sodium chloride into sodium and chlorine.

Answer 1

NaCl –> Na + Cl

-Originally strong bonds between the sodium and chlorine, and energy is supplied to break the bonds.

Question 2

What happens during a chemical reaction?

Answer 2

Old bonds are broken and new ones form.

Question 3

Describe the exothermic reaction of magnesium and oxygen into magnesium oxide.

Answer 3

Mg+O—->MgO+energy released

-The magnesium and oxygen form a strong bond

Question 4

What is meant by the term ‘exothermic reaction’?

Answer 4

An exothermic reaction is one which overall gives out energy to the surroundings in the form of heat, shown by a rise in temperature.

Question 5

Give some examples of exothermic reactions.

Answer 5

Combustion (water and carbon dioxide form requiring energy)

Explosions

Question 6

What is meant by the term ‘endothermic reaction’?

Answer 6

An endothermic reaction is one which overall takes in energy from the surroundings in the form of heat, shown by a fall in temperature.

Question 7

Give some examples of an endothermic reaction.

Answer 7

• Photosynthesis (light energy from sun to convert carbon dioxide and water into glucose and oxygen)
• Ammonium nitrate (dissolves in water it takes in heat from surroundings)

Question 8

Which are more common, endothermic or exothermic reactions?

Answer 8

Exothermic

Question 9

Describe the energy levels throughout an exothermic reaction

Answer 9

• Reactants are at a higher energy than the products. -Difference in height represents energy given out in the reaction.
• The initial rise represents the activation energy - energy needed to break old bonds.

Question 10

Describe the energy levels throughout an endothermic reaction.

Answer 10

• Reactants are at a lower energy than the products.

- The difference in height represents energy taken in during a reaction.

Question 11

What are the four reactions where you could measure a change in temperature?

Answer 11

• Dissolving salts in water.
• Neutralisation reactions (acid & base reacting together).
• Displacement reactions (more reactive element taking the place of a less reactive element)
• Precipitation reactions (two solutions react to form an insoluble precipitate)

Question 12

Give an example of:
A) a slow reaction.
B) a moderate reaction.
C) a (very) fast reaction

Answer 12

A) rusting of iron
B) metal reacting with acid forming a gentle steam of bubbles
C) explosion

Question 13

What are the four factors that influence rate of reaction?

Answer 13

Temperature
Concentration/pressure (gas)
Catalyst
Surface area (solid)/size of particles

Question 14

Explain the reaction of hydrochloric acid with marble chips

Answer 14

• Used to show effect of increasing surface area of a solid.
• Measure volume of gas evolved with a gas syringe at regular intervals.
• Repeat with same volume of acid and mass of marble chips, just more crushed up (to increase surface area), including repeat with powdered marble.
• Experiment can also show how concentration of acid affects the rate of reaction. -Change concentration of hydrochloric acid each time and keep other factors the same.
• Can also find the rate of reaction by measuring total mass of the flake during the reaction.

Question 15

Explain the reaction of sodium thiosulfate and hydrochloric acid

Answer 15

• Both chemicals are clear solutions, and react together to form a yellow precipitate of sulphur.
• Tim how long it takes a black mark to disappear as the solution forms the cloudy sulphur.
• Repeat reaction for different temperatures (water bath to heat solutions to the right temperature before mixing).
• Must keep the depth of the liquid the same for each.
• Results will show the higher the higher the temperature, the quicker the reaction and therefore takes less time for the black mark to disappear.
• Negative: can not get graphs, just a set of readings.

Question 16

What is meant by the term ‘catalyst’?

Answer 16

A catalyst is a substance which changes the speed of a reaction, without being used up in the reaction.

Question 17

Explain how a catalyst works.

Answer 17

• Lowers the activation energy.
• Makes it easier for the reaction to happen & increases the rate of the reaction.
• Means a lower temperature can be used, without lowering the rate of reaction.

Question 18

Where is a catalytic converter found?

Answer 18

In the exhaust pipe of motor vehicles

Question 19

Explain how a catalytic converter works in vehicles.

Answer 19

• Vehicles can give off poisonous gases like carbon monoxide when petrol doesn’t burn properly.
• Catalytic converters increase the rate carbon monoxide & unburnt fuel in exhaust gases react with oxygen in the air to produce carbon dioxide and water.
• Has a large surface area & works best at high temperatures to increase the rate of reaction.

Question 20

What are the four things that affect the rate of chemical reactions?

Answer 20

• Temperature - higher temperature increases the rate of reaction & vice versa.
• Concentration/pressure - higher concentration/pressure increases the rate of reaction & vice versa.
• Surface area - larger surface area increases the rate of reaction & vice versa.
• Use of a catalyst - using a catalyst increases the rate of reaction.

Question 21

Explain how a higher temperature increases collisions and therefore rate of reaction.

Answer 21

When temperature increases, the particles move quicker.

If they move faster, they have more frequent collisions.

Question 22

Explain how a higher concentration/pressure increases the rate of reaction

Answer 22

• More particles of the reactant colliding with other molecules.
• Makes collisions between particles more likely.
• In a gas, increasing the pressure means that particles are more squashed up together so there are more frequent collisions.

Question 23

Explain how a larger surface area increases the rate of reaction.

Answer 23

• Has a larger surface area by being broken down into smaller pieces.
• Particles around it in the solution will have a larger surface area to react with, so there will be more frequent collisions.

Question 24

Explain how a catalyst increases the rate of a reaction.

Answer 24

Gives the particles reacting a surface to stick to.

Increases the number of successful collisions by lowering the activation energy.

Question 25

What ONE way can you get faster collision?

Answer 25

Faster collisions are ONLY caused by an increase in temperature.