Rates of Reaction

Question 1

What is meant by the rate of a chemical reaction?

Answer 1

How fast the reactants are changed into products

Question 2

How to know if a reaction is fast from a graph?

Answer 2

Steep line and flat in the least time

Question 3

What 2 things does the rate if a chemical reaction depend on?

Answer 3

• Collision frequency of reacting particles - how often they collide, more = faster reaction
• Energy transferred during a collision - p. have to collide with enough energy for a collision to be successful

Question 4

Factors that affect rate of reaction

Answer 4

• Temperature
• SA
• Catalyst
• Concentration of a solution/pressure of a gas

Question 5

What happens when you increase the temperature?

Answer 5

• P. moves faster
• Collide more frequently
• Faster they move, more energy they have, so more collisions will have enough energy to make reaction happen

Question 6

What happens when you increase conc/pressure?

Answer 6

• Conc = more particles in the same volume of water
• Pressure -same no of p. but in a smaller space
• Makes collisions more frequent

Question 7

What happens when you increase SA?

Answer 7

• Breaking solid into smaller pieces increases SA:V ratio
• Means that for the same volume, p will have more area to work on - more collisions

Question 8

What happens when you add a catalyst?

Answer 8

• Diff c. needed for diff reactions
• They all decrease activation energy by providing an alternative reaction pathway with a low AE

Question 9

Formula for rate of reaction

Answer 9

Amount of reactant used or amount of product formed / time (s)

Question 10

Disappearing cross reaction

Answer 10

sodium thiosulfate solution + hydrochloric acid –> sulfur (solid)

Question 11

What does sulfur do to the solution

Answer 11

Makes it go cloudy (turbidity)

Question 12

Disapearing Cross Reaction Method

Answer 12

1. Measuring cylinder 10cm3 sodium thiosulfate solution into conical flask
2. Put conical flask onto a printed black cross
3. Add 10 cm3 Hcl to conical flask
4. Swirl solution and start stopwatch
5. Stop clock when you cant see the cross (will turn cloudy)
6. Repeat with low conc of sodium thiosulfate solution
7. Calc mean values of each conc

Question 13

Prob with disappearing cross experiment

Answer 13

• Diff ppl have diff eyesights - some may see cross for longer - may not get same results
• not reproducible
• HOWEVER, cross is same size so prob may not be too great

Question 14

Unit to measure gas

Answer 14

cm3

Question 15

Disadvantage of recording visual changes

Answer 15

Results are subjective -diff ppl might not agree over the extact point the mark disappears
You cant plot a rate of reaction graph from results

Question 16

How can you measure the speed of a reaction that produces a gas?

Answer 16

Mass balance

Question 17

Advantage of measuring change in mass with a gas given off

Answer 17

Most accurate out of 3 methods cuz balance is accurate

Question 18

Disadvantage of measuring change in mass with a gas given off

Answer 18

Might release gas straight into the room

Question 19

How to measure volume of gas given off?

Answer 19

Gas syringe

Question 20

Disadvantage of measuring volume of gas given off

Answer 20

If reaction is too vigorous, you can easily blow the plunger out of the end of the syringe

Question 21

advantage of measuring volume of gas given off

Answer 21

Gas syringes are accurate to nearest cm3
You can take measurements at regular intervals and plot RAR graph using this method

Question 22

3 ways to measure rate of reaction

Answer 22

• Volume of gas given off
• Change in mass (with gas given off)
• Precipiation and colour change

Question 23

What do you put on the y and x axis on a RAR graph?

Answer 23

x-axis = time
y-axis = amount of reactant used up

Question 24

How to find mean rate for whole reaction?

Answer 24

Overall change in y-value divided by total time taken for reaction