SC7 - Periodic Table, Groups, Rates And Energy Change

Question 1

What are the physical properties of group 1 (alkali metals)?

Answer 1

• malleable
• conduct electricity
• relatively low melting points
• soft, easily cut
• very reactive

Question 2

Describe and explain the trend in reactivity in group 1.

Answer 2

The reactivity of alkali metals increases as you go down the group. This is because the elements near the bottom of the group form larger atoms, meaning the outer shell electrons are further away from the nucleus. This means that the atoms can more easily be ionised (due to weaker electrostatic forces) and so they are more reactive.

Question 3

Describe the structure of halogens (group 7).

Answer 3

All halogens exist as diatomic molecules (two atoms held together by a single covalent bond). Simple molecular

Question 4

What are some physical properties of halogens?

Answer 4

• low melting and boiling points
• coloured (colour gets darker as you go down the group)
• low density
• poor conductors of heat and electricity
• toxic and corrosive

Question 5

What colour are the first four halogens at room temperature?

Answer 5

Fluorine - yellow gas
Chlorine - green gas
Bromine - orange liquid
Iodine - purple solid

Question 6

Describe and explain the trend in reactivity of the halogens (group 7).

Answer 6

The reactivity of the halogens decreases as you go down the group. This is because the further down the element, the further away the outer shell electrons are from the nucleus, making the electrostatic forces weaker in the larger atoms. The larger atoms require more energy to gain the electron required to ionise them, so lower down halogens are less reactive.

Question 7

What is the test for chlorine?

Answer 7

Damp blue litmus paper turns red then bleaches.

Question 8

How can the reactivity trend of the halogens be tested?

Answer 8

Halogen displacement reactions. More reactive halogens displace less reactive halogens from compounds. This causes a colour change.
For example:

Potassium bromide + chlorine —> potassium chloride + bromine

Colour changes from yellow (chlorine is yellow in solution) to orange

Question 9

Physical properties of group 0 (noble gases)

Answer 9

• colourless
• very low melting and boiling points
• poor conductors of heat and electricity
• inert (don’t react)

Question 10

Why are noble gases inert?

Answer 10

They have full outer shells, so do not need to ionise to become stable. It is very difficult to get a noble gas to react.

Question 11

Uses of noble gases

Answer 11

Krypton is used in photography lighting (it produces a brilliant white light when electricity is passed through it). Argon is denser than air, so is used to fill the space above wine in wine barrels to stop oxygen reacting with the wine. Helium is used in weather balloons and airships. Neon is used in long-lasting illuminated signs.

Question 12

What is the rate of a chemical reaction?

Answer 12

The speed at which the reactants are turned into products.

Rate = amount of product formed / time
Or rate = amount of reactant used / time

Question 13

How can rate of reaction be measured?

Answer 13

By monitoring change in mass of products or reactants over time, rate can be calculated.

Question 14

What is the activation energy?

Answer 14

The minimum amount of energy needed for a reaction to occur.

Question 15

What is the effect of concentration on rate of reaction?

Answer 15

Increasing concentration increases rate of reaction because there are more reacting particles in the same volume so collisions occur more frequently.

Question 16

What is the effect of surface area on rate of reaction?

Answer 16

Increasing the surface area to volume ratio (by using smaller particles e.g. magnesium powder rather than ribbon) increases the rate of reaction. This is because there is more surface for the collisions to occur on, so collisions occur more frequently.

Question 17

What is the effect of temperature on rate of reaction?

Answer 17

Increasing the temperature increases the rate of reaction a lot because a higher temperature means the particles have more kinetic energy so they speed up, resulting in more frequent and more energetic collisions (so each collision is more likely to exceed the activation energy).

Question 18

What is the effect of pressure on rate of reaction?

Answer 18

Increasing the pressure of gases increases the rate of reaction because the reactant particles are closer together, so collisions occur more frequently.

Question 19

What are catalysts?

Answer 19

Substances that speed up chemical reactions without being used up themselves or altering the products of the reaction.

Question 20

What do catalysts do in relation to activation energy?

Answer 20

Catalysts provide an alternative reaction route, which requires a lower activation energy. They do not alter the overall energy change, but since less energy is needed to start the reaction, more reactant molecules have enough energy and so more collisions are successful, meaning the reaction is faster.

Question 21

What is an exothermic reaction?

Answer 21

A reaction in which energy is transferred from the stores of energy in chemical bonds to the surroundings.

Question 22

What is an endothermic reaction?

Answer 22

A reaction in which energy is transferred from the surroundings to stores of energy in chemical bonds.

Question 23

Which types of reaction are always exothermic?

Answer 23

• neutralisation
• displacement
• combustion

Question 24

What are the temperature changes as bonds are made or broken?

Answer 24

Making
EXothermic
Breaking
ENDothermic

Making bonds is exothermic as energy is transferred to the surroundings as bonds form. Breaking bonds is endothermic as energy is transferred to the reactants to break their bonds.

Question 25

What makes a reaction exothermic or endothermic?

Answer 25

All reactions involve both making and breaking bonds, so both give out and take in energy at some point. If more energy is given out when the bonds in the products are formed than was taken in when the bonds in the reactants were broken, then the reaction is exothermic. If more energy was taken in to break the bonds in the reactants than was given out to make the bonds in the products, then the reaction is endothermic.

Question 26

How do you do bond energy calculations?

Answer 26

You will be given a table saying the bond energies for each type of bonds you will need to know for the question. Count the number of each bond there is (remember to multiply out the number of moles - draw it out of it helps). Multiply the number of bonds by the bind energy for that bond. Add up the total energy taken in (bonds broken in reactants - LHS) and the total energy released (bonds made in products - RHS). Do energy in - energy out (LHS - RHS) to calculate the energy change.

Question 27

Core practical - investigating reaction rates

Answer 27

Method 1:
- set up a conical flask with a bung and delivery tube in it. Fill a measuring cylinder with water and place in a water trough, over the delivery tube
- measure 40 cm3 of dilute hydrochloric acid into the conical flask
- add 5g of small marble chips to the flask
- immediately stopper the flask and start the stop clock
- note the total volume of gas produced after every 30 seconds until the reaction has finished
- repeat the experiment using 5g of larger marble chips

Method 2:
- place 50 cm3 of sodium thiosulphate solution into a 300cm3 conical flask
- measure out 5cm3 of dilute hydrochloric acid in a test tube
- clamp the conical flask in place in a water bath at a certain temperature. Place the Test Tube in a rack in the same water bath. Record the temperature
- after 5 minutes, remove the flask and place on a piece of white paper marked with a cross.
- add the acid to the thiosulphate and start the stop clock
- looking down from above, stop the stop clock when the cross disappears
- note this time and take the final temperature of the mixture
- repeat at 3 or 4 other temperatures, between 20 and 50 degrees C