Topic 5 - Energy Changes

Question 1

Exothermic reactions

Answer 1

Thermal energy is transferred from the chemicals to the surroundings

Question 2

Endothermic reactions

Answer 2

Thermal energy is transferred from surroundings to the chemicals

Question 3

What is activation energy

Answer 3

The minimum energy particles must have to react

Question 4

What is a cell

Answer 4

Two electrodes in an electrolyte used to generate electricity

Question 5

What is a battery

Answer 5

Two or more chemical cells connected

Question 6

What is an electrolyte

Answer 6

A liquid that conducts electricity

Question 7

Advantages of using cells and batteries

Answer 7

Provide very convenient portable source of electricity which can be used in many devices

Relatively cheap

Some are rechargeable so can be used many times

Question 8

Disadvantages of using cells and batteries

Answer 8

Many contain harmful or toxic chemicals and can harm the environment unless recycled

Question 9

What is a fuel cell

Answer 9

A chemical cell with a continuous supply of chemicals to fuel the cell

Question 10

Advantages of a fuel cell

Answer 10

The only waste product is water

Produce electricity continuously

Question 11

Disadvantages to fuel cells

Answer 11

Very expensive

Contain some toxic chemicals in the electrodes and electrolyte which could harm the environment

Use hydrogen which is a flammable gas and is difficult to transport and store

Production of hydrogen could cause pollution

Most hydrogen is made from fossil fuel methane, also making the greenhouse gas carbon dioxide in the process

Another method of making hydrogen is from the electrolysis of water which may use electricity generated from fossil fuels

Question 12

rate of reaction graph

Answer 12

steeper the slope the faster the reaction

Question 13

mean of rate reaction=

Answer 13

quantity of product formed / time taken

Question 14

collision theory

Answer 14

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

the rate of a chemical reaction is determined by the frequency of successful collisions

Question 15

required practical- the effect of concentration on reaction rate: thiosulfate cross

Answer 15

thiosulphate cross-
put 10cm of sodium thiosulfate solution into conical flask

place flask onto paper with cross on

swirl solution and start stopwatch

look over flask and stop timer once the cross can no longer be seen

repeat with lower concentrations

Question 16

problem with thiosulfate cross

Answer 16

people have different eyesights so some can see the cross for longer than others

Question 17

required practical- the effect of concentration on reaction rate : gas

Answer 17

use measuring cylinder to place 50cm of hydrochloric acid into conical flask

attach conical flask to bung and delivery tube

place delivery tube into container of water

then place upturned measuring cylinder also filled with water over delivery tube

add a 3cm strip of magnesium to the hydrochloric acid and start timer

reaction produces hydrogen gas which is trapped in measuring cylinder

every ten seconds measure the volume of hydrogen gas in the cylinder

continue until no more hydrogen is given off

repeat using different concentrations of hydrochloric acid

Question 18

effect of surface area on rate of reaction

Answer 18

increase in surface area means more collisions per second so the rate increases

Question 19

surface area:volume ratio

Answer 19

smaller sized blocks of solid reactant have a greater surface area to volume ratio than larger blocks

this means that they have more particles on the surface so there are more collisions per second

this increases the rate of reaction

Question 20

surface area effect on rate of reaction practical

Answer 20

place marble chips in hydrochloric acid with cotton wool at end of conical flask and place on scale

cotton wool allows carbon dioxide to escape and prevents acid to splash out

Question 21

activation energy

Answer 21

minimum amount of energy required for particles to react successfully

Question 22

effect of temperature on rate of reaction

Answer 22

increasing the temperature increases the energy of the particles so they move faster and there are more frequent collisions

also the increase in energy means more particles can overcome the activation energy barrier and collide successfully

Question 23

catalysts

Answer 23

increase the rate of chemical reactions but are not used up during the reaction

allow us to carry out reactions quickly without needing to increase the temperature which saves money

they can be reused over and over since they are not used up during reactions

Question 24

effect of catalysts on rate of reaction

Answer 24

increase the rate of reaction by providing an alternative pathway for the reaction that has a lower activation energy

this means less energy is needed to have a successful collision which increases the rate

Question 25

reversible reactions

Answer 25

if a reversible reaction is endothermic in one direction, it is exothermic in the opposite

Question 26

equilibrium

Answer 26

when both reactions happen at the same time

Question 27

le chateliers principle

Answer 27

if a system is at equilibrium and a change is made to the conditions, then the system responds to counteract the change

Question 28

concentration in reversible reactions

Answer 28

example:
if there is a decrease in the concentration of product then the more reactant will react to form the product until equilibrium is reached again

Question 29

temperature in reversible reaction

Answer 29

example:
if the forward reaction is exothermic and the backwards reaction is endothermic and we increase the temperature of the system, the equilibrium would shift to the left to reduce the temperature

that is because the reverse reaction is endothermic so energy is taken in causing the temperature to fall

Question 30

pressure in reversible reactions

Answer 30

if the pressure is increase on a reversible reaction at equilibrium, the position of the equilibrium shifts to the side with the smaller number of molecules

if the pressure is reduced then the position of the equilibrium shifts to the side with the larger number of molecules