Unit 1-Rates Of Reactions

Question 1

What is meant by the rate of a chemical reaction?

Answer 1

The rate of a chemical reaction is a measure of how fast the reactants are being used up and how fast the products are being made.

Question 2

Name the four factors which speed up the rate of a reaction.

Answer 2

1. Increase temperature
2. Increase concentration of a reactant
3. Increase the surface area (decrease particle size)
4. Use a suitable catalyst

Question 3

What word is used to describe a biological catalyst?

Answer 3

Enzyme

Question 4

The rate can be determined by measuring what quantities?

Answer 4

changes in the mass of the reactants or products

changes in the volume of the reactants or products

changes in the concentration of the reactants or products

Question 5

When dilute hydrochloric acid reacts with magnesium metal, magnesium chloride and hydrogen gas are produced.

Describe a method to measure the rate of this reaction in the lab.

Answer 5

1. A balance can be used to measure the mass of the apparatus throughout the reaction. As hydrogen bubbles escape, the apparatus will lose mass.
2. The volume of gas could be collected in a gas syringe or over water (as long as it is insoluble in water).

Question 6

How is the average rate of reaction calculated?

Answer 6

Average rate = Change in measurable quantity ÷ change in time

Average rate = ∆ Quantity ÷ ∆ Time

Question 7

The unit that rate is measured in depends on the measured quantity.

State the units used when:

1. a change in mass is measured in grams and a change in time in seconds
2. a change in concentration is measured (in mol l^-1) and a change in time in minutes
3. a change in volume measured in cubic centimetres and a change in time in seconds

Answer 7

1. a change in mass is measured in grams and a change in time in seconds = grams per second ( g s^-1 or g / s )
2. a change in concentration is measured ( in mol l^-1 ) and a change in time in minutes = moles per litre per minute ( mol l^-1 min^-1 or mol l^-1 / min)
3. a change in volume measured in cubic centimetres and a change in time in seconds = cm³ per second ( cm³ s ^-1 or cm³ / s )

Question 8

If a reaction produced 120cm³ of gas in 15 seconds, what is the average rate of reaction?

Answer 8

Average rate = ∆ Quantity ÷ ∆ Time

= 120 ÷ 15

= 8 cm³ s^-1

Question 9

Explain what happens to the rate of reaction as the reaction progresses?

Answer 9

As the reaction progresses the rate of reaction decreases.

This is because as reactants are used up, there will be fewer collisions and the reaction slows down.

Question 10

Which graph shows the faster reaction?

From a rate graph, how can you tell when the reaction is finished?

Answer 10

Graph A (red) is the faster reaction as it has the steeper curve.

When the reaction is finished (the end-point) the graph goes flat as no more products are being produced. For graph B (Green) this is at 15 seconds from the start of the reaction.