Chemistry exam

Question 1

a student investigated the reaction between magnesium and hydrochloric acid. give two observations the student could make during the reaction

Answer 1

• effervescence

- exothermic

Question 2

the student investigated how the rate of this reaction changed when the concentration of hydrochloric acid was changed

Answer 2

firstly, using a concentration of 0.25mol/dm3 of HCl time how long it takes from when the magnesium is added to the HCl to the end of the reaction. This measures the rate of the reaction. To make the experiment a fair test use the same volume of hydrochloric acid. Repeat reaction twice with 0.1 and 0.5 mol/dm3 of hydrochloric acid

Question 3

name the product that made the reaction go cloudy

Answer 3

sulphur

Question 4

describe trends shown in the students results

Answer 4

as temperature increased relative rate of reaction also increased

Question 5

student investigated the effect of changing the concentration of sodium thiosulfate solution on therate of reaction.
suggest two variables the student would need to control to make sure that her results were valid

Answer 5

-the volume of sodium thiosulfate must stay the same

Question 6

from this investigation the student correctly concluded
‘As the concentration of sodium thiosulfate doubles, the rate of reaction doubles.’
explain the students conclusion in terms of particles

Answer 6

• if concentration increases there is a greater frequency of successful collisions.
• Particles are closer together

Question 7

explain what is meant by equilibrium

Answer 7

the forward reaction and backward reaction occur at the same rate

Question 8

how would increasing the temperature change the yield of ethanol at equilibrium?
give a reason for your answer

Answer 8

increasing the temp would decrease the yield of ethanol as the equilibrium favours the endothermic reaction

Question 9

how would increasing the pressure change the yield of ethanol at equilibrium?
give a reason for your answer

Answer 9

increasing the pressure would increase the yield of ethanol because the equilibrium favours the side of the reaction with less moles

Question 10

a catalyst is added to increase the rate of a reaction

explain how adding a catalyst increases the rate of a reaction

Answer 10

a catalyst increases the rate of a reaction because less energy is needed to complete the reaction. So it has a lower activation energy with a catalyst

Question 11

explain, as fully as you can, why the average rate between 30 and 50 seconds is different from the rate between 0 and 10 seconds

Answer 11

As the reaction continues the concentration increases

Question 12

a high pressure (between 50 and 100 atmospheres) is used in this process.
explain why the highest equilibrium yield of methanol is obtained at a high pressure

Answer 12

because as pressure increases the equilibrium moves to the side of the reaction with less moles

Question 13

the temp ised in this process is about 250C
it has been stated that, ‘the use of this temperature is a compromise between the equilibrium yield of product and rate of reaction’
explain this statement

Answer 13

-this is because with a low temperature there is a good yield but a slower rate of reaction

Question 14

which sub-atomic particles are present in the nucleus of an atom?

Answer 14

• protons

- neutrons

Question 15

there are two isotopes of the element chlorine:

describe, in terms of sub-atomic particles

Answer 15

similarity: the number of protons
difference: the number of neutrons