SC7: rates of reaction and energy changes

Question 1

what is the rate of reaction?

Answer 1

the speed at which a reaction takes place

Question 2

why does the rate of reaction vary?

Answer 2

-variables (eg: temp, pressure, SA, conc)
-the reaction itself

Question 3

collision theory

Answer 3

for a chemical reaction to happen:
-reactant particles must collide with each other
-the particles must have enough energy for them to react

Question 4

what is a collision that produces a reaction called?

Answer 4

successful collision

Question 5

activation energy

Answer 5

the minimum amount of energy needed for a collision to be successful

Question 6

gas syringe

Answer 6

equipment used to collect and measure the volume of gas produced

Question 7

what does the gradient of a line in a graph say about the rate of reaction?

Answer 7

-the steeper the line, the faster the rate of reaction
-fast reactions finish sooner (when the line becomes horizontal) than slow reactions

Question 8

rate of reaction equation

Answer 8

amount of product formed /time taken

Question 9

how does surface area affect the rate of reaction?

Answer 9

-for a given mass of a solid, large lumps have smaller surface area to volume ratios than smaller lumps or powders
-if a large lump is divided or ground into a powder the area of exposed surface increases
-more reactant particles are exposed at the surface
-the frequency of collisions between reactant particles increases, therefore the rate of reaction increases

Question 10

how does temperature affect the rate of reaction?

Answer 10

increasing the temperature increases the speed & energy of the reacting particles so that they collide more often and are more likely to have enough activation energy for a reaction to occur

Question 11

how does concentration affect the rate of reaction?

Answer 11

-there are more reactant particles moving together
-increasing the concentration of reactants in solutions increases the frequency of collisions as the particles are closer together, this means that rate of reaction increases

Question 12

how does pressure affect the rate of reaction?

Answer 12

increasing the pressure of the reactants leads to less space for the particles & more chance of collision

Question 13

catalyst

Answer 13

substance that speeds up the rate of a chemical reaction or provides an alternate pathway with less activation energy needed (without taking part in it)

Question 14

catalyst facts

Answer 14

-needed in small amounts
-unchanged at the end of reaction
-different reactions need different catalysts
-when a catalyst is added to a reaction, the same amount of product is formed but in a shorter time frame

Question 15

why are catalysts used in industry?

Answer 15

-save time by speeding up slow reactions
-companies can sell more & make more products
-can be reused as they don’t get used up in reactions
-reactions can happen at lower temps so less money is spent on heating

Question 16

production of gas practical steps

Answer 16

1. support a gas syringe with a stand, boss and clamp
2. using a measuring cylinder, add 50 cm³ of dilute hydrochloric acid to a conical flask
3. add 0.4 g of calcium carbonate to the flask. immediately connect the gas syringe and start a stop clock
4. for every 10 cm³ of gas produced, record the time
5. when the reaction is complete, clean the apparatus

(repeat steps 1 to 5 with different concentrations of hydrochloric acid)

Question 17

colour changes practical steps

Answer 17

1. using a measuring cylinder, add 50 cm³ of dilute sodium thiosulfate solution to a conical flask
2. place the conical flask on a piece of paper with a black cross drawn on it
3. using a different measuring cylinder, add 10 cm³ of dilute hydrochloric acid to the conical flask. immediately swirl the flask to mix its contents and start a stop clock
4. measure and record the temperature of the reaction mixture
5. look down through the reaction mixture. when the cross is no longer visible, record the time on the stop clock
6. measure and record the temperature of the reaction mixture, and clean the apparatus as directed by the teacher

(repeat steps 1 to 6 with different starting temperatures of sodium thiosulfate solution)

Question 18

exothermic

Answer 18

energy is given out

Question 19

endothermic

Answer 19

energy is taken in

Question 20

what type of process is breaking bonds and why?

Answer 20

endothermic as energy is transferred from the surroundings to stores of energy in chemical bonds to break bonds

Question 21

what type of process is forming bonds?

Answer 21

exothermic as energy is transferred from stores of energy in the bonds to the surroundings to form bonds

Question 22

a reaction is exothermic if…

Answer 22

more heat energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants

Question 23

a reaction is endothermic…

Answer 23

less heat energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants

Question 24

test for oxygen

Answer 24

glowing splint relights

Question 25

energy level diagram

Answer 25

chart showing the energy in the reactants and products, and the difference in energy between them

Question 26

reaction profile

Answer 26

-chart showing how the energy of reactants and products changes during a reaction
-it includes the activation energy shown as a hump in the line which: starts at the energy of the reactants
is equal to the difference in energy between the top of the ‘hump’ and the reactants

Question 27

how can the energy change in a reaction be calculated?

Answer 27

bond energies

Question 28

bond energies

Answer 28

the amount of energy needed to break one mole of a particular covalent bond

Question 29

to calculate an energy change for a reaction:

Answer 29

1) calculate energy taken in
2) calculate energy released
3) calculate energy change

Question 30

energy change

Answer 30

energy in - energy out

Question 31

negative energy change is…

Answer 31

exothermic

Question 32

positive energy change is…

Answer 32

endothermic