Topic 7 - Rates of reaction and Energy Changes

Question 1

What is the formula to calculate the rate of reaction

Answer 1

amount of reactant used or amount of product formed / time

Question 2

How we measure rate of reaction with a precipitation reaction

Answer 2

-mix any two clear solutions to form a cloudy precipitate solution ontop of paper with a pencil cross on
-measure how long it takes for the cross to become invisible/ obscured
-faster = faster reaction
(is subjective however as people might not agree on when it disappears)

Question 3

How can we measure the rate of reaction with change in mass (where gas is given off)

Answer 3

• reaction carried out on mass balance (where gas is produced)
• carry out in conical flask on mass balance with cotton wool to only let gas out and no liquids or anything flying out in reaction
• you know its finished when mass doesnt change
• plot change in mass against time graph

(gas may be harmful so do in fume cupboard)

Question 4

How can we measure rate of reaction with volume of gas given off?

Answer 4

• use a gas syringe to measure volume of gas given off
• more gas given off in less time means faster rate
• you know its finished when no more gas is produced
• plot gas volume against time

-make sure you use the right size syringe as if the reaction is too vigorous then it might blow the plunger off the end.

Question 5

describe an experiment to see how surface area effects the rate

Answer 5

• using different sized marble chips in HCl
• measure the volume of gas poduced into gas syringe at regular time intervals and plot time against volume graph
• repeat with diiferent sized chips

Question 6

what would having finer particles have an effect on the rate of reaction?

Answer 6

The smaller the particles the faster the rate as theres a larger surface area for the reaction to take place on

Question 7

Describe an experiment to measure the rate of reaction with varying temperatures

Answer 7

• measure out fixed volumes of sodium thiosulfate and hydrochloric acid using a measuring cylinder
• use hot water bath to gently heat solutions to desired temperatures before mixing
• mix together and time how longer you can no longer see the cross on paper beneath flask
• repeat for several tempertures

Question 8

If the gradient on a graph is steeper what does it show

Answer 8

a faster rate of reaction

Question 9

What must particles do in order to react?

Answer 9

collide with each other

Question 10

What does the rate of reaction depend on

Answer 10

How often they collide (frequency)

The energy transferred in a collision (minimum energy is activation energy)

Question 11

The more collisions the …… the rate of reaction

Answer 11

higher

Question 12

What does increasing the temperature do to the rate of reaction? and why

Answer 12

It increases the rate of reaction because the particles have more energy and move faster and therefore have more successful collisions

Question 13

What does increasing the concentration (or pressure in gases) do to the rate of reaction? and why

Answer 13

If it is more concentrated then there are more particles in the same volume so it makes collisions more likely

In a gas the particles are more crowded in a smaller area so frequency of collisions will increase –so rate increases

Question 14

What does increasing the surface area do to the rate of reaction? and why

Answer 14

Smaller solid particles means a larger surface area as there is more space for the reaction to take place on so frequency of collisions increases

rate of reaction is faster for solids with a larger surface area to volume ratio

Question 15

What does a catalyst do to the rate of a reaction

Answer 15

• (wont change products)
• work by decreasing the activation energy needed for reaction by providing ALTERNATIVE REACTION PATHWAY so minimum amount of energy is needed for reaction
• catalysts are very particular though about which reactions they catalyse

Question 16

What do enzymes do

Answer 16

control living cell reactions

• they are biological catalysts
• used in respiration, photosynthesis, and protein synthesis
• yeast cell enzymes used in fermentation
• enzymes have a high specificity for the reaction they catalyse

Question 17

Define an exothermic reaction

Answer 17

a reaction which gives out energy to surroundings usually from heat and is shown by a rise in temperature of the surroundings

Question 18

Define an Endothermic reaction

Answer 18

a reaction which takes in heat from the surroundings in the form of heat and is shown by a fall in temperature of surroundings

Question 19

What is the activation energy

Answer 19

The minimum amount energy needed to start a reaction to make BONDS BREAK

Question 20

describe a reaction profile for an exothermic reaction

draw

Answer 20

• reactant line higher
• which then increases a bit before decreasing and making products
• product line is lower
• energy after is lower because energy is given out

Question 21

describe a reaction profile for an endothermic reaction

draw

Answer 21

• reaction line lower
• which then increases lots and then decreases a bit before making the products line
• product line is higher in terms of energy as energy taken in

Question 22

When bonds are broken its an …..reaction

And why?

Answer 22

endothermic

because greater energy is used to break bonds than energy released by forming them

Question 23

when bonds are formed its an….. reaction

And why?

Answer 23

exothermic

because energy released by forming bonds is greater than energy to break them

Question 24

what is the formula to work out bond energy calculations

Answer 24

Overall energy change = energy required to break bonds - energy required to make bonds

Question 25

If answer to bond energy calculation is negative its a ……reaction
WHY?

Answer 25

Exothermic (because more energy is released by forming bonds than breaking them)

Question 26

How can we measure temperature change?

Answer 26

• Put polystyrene cup in large beaker of cotton wool to insulate
• Add known volume of first reagent to cup
• measure initial temperature
• Add known mass/volume of second reagent and stir mixture
• Put lid on to stop energy lost by evaporation
• Recorde the maximum of minimum temperature (depending on whether it increases or decreases)
• measure temperature change
• for different results change reagents used

Question 27

Give examples of exothermic reactions

Answer 27

Neutralisation, Displacement, Precipitation all end in increase of temperature of surroundings as energy is given out

Question 28

Give an example of an endothermic reaction

Answer 28

Dissolving salts in water

- endothermic for ammonium chloride as mixture DECREASES in temperature because the ammonium chloride TAKES IN energy