C5.2 Controlling reactions

Question 1

Why do solids react the slowest?

Answer 1

Only the particles on the surface of a solid can react, so rate of reaction increases as surface area increases

Question 2

Why does rate of reaction increase with higher surface area?

Answer 2

The higher the surface area, the more particles there are on the surface on the solid which are available for reaction

Collisions are more likely and more frequent

Question 3

As the size of lumps decrease…

Answer 3

Surface area to volume ratio increases

Question 4

List some alternatives to lumps

Answer 4

Strips, chips, powder

Question 5

Why do iron bars not burn while fillings do?

Answer 5

Iron bars have a lower surface area to volume ratio so they react slower so only the particles on the surface can react. In filling there is higher surface area to volume ratio

Question 6

explain why you should keep temperature and concentration the same when investigating the effect of surface area on Rate of reaction?

Answer 6

Temperature and concentration are also a factor that affects the rate of reaction so it wouldn’t be a fair test

Question 7

Why is fine powder a hazard in factories?

Answer 7

Fine powder is very reactive because it has a high surface area to volume ratio. Therefor the smallest spark can set it off.

Question 8

Describe an experiment to test the effect of changing concentration on rate of reaction

Answer 8

First Put a magnesium ribbon into a conical flask which has a hole in in for a rubber tube. Attach the rubber tube to a gas syringe. Decide 3 concentrations of acid such as 1,1.5and 0.5 mol/dm3 and use measure out 50cm3 of all 3. Add the hydrochloric acid to the conical flask and attach a bung while starting a stopwatch. Take a reading of how much gas there is every 30 seconds until the reaction stops. Repeat this for the other 2 concentration with the same size of magnesium and volume of acid. The reaction which stopped the fastest would have the highest rate of reaction. You can calculate the rate by doing 1/reaction time or by drawing a graph.

Question 9

What are catalysts?

Answer 9

Catalysts are substances that speed up a reaction but DONT GET USED UP THEMSELVES

Question 10

How do catalysts speed up reactions?

Answer 10

Catalysts decrease the activation energy. This means that a greater proportion of reactants have the activation energy to react. The area of successful collisions also increases

Question 11

How are catalysts in chemistry similar to biological catalysts

Answer 11

They both catalyse specific reactions

Question 12

What is the effect of concentration on the rate of reaction?

Answer 12

As concentration increases, particles become more crowded so they collide more frequently

Question 13

Describe and explain the effect of changes in pressure of reacting gases on the rate of reaction

Answer 13

Rate of reaction increases as pressure increases

This is because particles become more crowded so they collide more often

Question 14

How do you change the concentration?

Answer 14

Add water to dilute

Question 15

Key practicals

Answer 15

Revise the PAG on rate of reactions on one note

Question 16

How does changing the temperature effect the rate of reaction

Answer 16

As temperature increases, the particles gain more kinetic energy so they move more quickly and collide more frequently. A greater proportion collisions are successful which means that more of reactants have the minimum activation energy required or more to react.

Question 17

What is a successful collision?

Answer 17

A successful collision is when the colliding particles have the minimum activation energy required to react

Question 18

How do you calculate rate of reaction?

Answer 18

1/reaction time = rate of reaction

Question 19

What is rate of reaction?

Answer 19

How quickly reactants are used up or products are formed

Question 20

List some methods of measuring the rate of reaction

Answer 20

If using sodium thiosulfate and different concentrations of hydrochloric acid use the black cross method where you measure the time it takes for a black cross under the beaker to disappear

Use a gas syringe attached to a conical flask with a small rubber tube

Use an upturned measuring cylinder full of water in a tub of water and measure the dropping levels of water in the cylinder to measure the gas

Alternatively use a burette if you want accurate results

Question 21

How do you calculate mean rate of reaction?

Answer 21

Draw a triangle underneath the graph and do rise/run

Question 22

How do you calculate instantaneous rate of reaction?

Answer 22

Draw a tangent and use a triangle