SC18 Rates of reaction

Question 1

SC18c
1) What is a catalyst?
2) How are catalysts useful?
3) What is the activation energy of a reaction?

Answer 1

1) Catalysts are substances that speed up chemical reactions without being permanently changed themselves, and without altering the products of the reaction.
2) Catalysts will increase the rate of reaction by lowering the activation energy.
3) The activation energy is the minimum amount of energy needed by colliding particles for a reaction to happen.

Question 2

SC18c
1) How do catalysts speed up chemical reactions?
2) What is an enzyme?
3) Name an example of an enzyme

Answer 2

1) A catalyst decreases the activation energy needed for the reaction to occur by providing an alternative reaction pathway that has a lower activation energy than the reaction without a catalyst.
2) Enzymes are large, complex protein molecules that act as catalysts in biological reactions.
3) The enzymes in yeast are used in the fermentation process to produce alcoholic drinks.

Question 3

1) What must occur for a reaction to take place?
2) Why do changes in the energy of particles affect rates of reaction?
3) Why do changes in the frequency of collisions between particles affect the rate of reaction?

Answer 3

1) For chemical reactions to occur, the reactant particles must collide together with enough energy to react.
2) An increase in the energy of particles means that the particles collide with more force. This increases the number of successful collisions, and therefore increases the rate of reaction. An increase in the energy of particles also means that the particles collide with more frequently. This increases the number of successful collisions, and therefore increases the rate of reaction.
3) An increase in the frequency of collisions increases the number of sucessful collisions, and therefore increases the rate of reaction.

Question 4

Explain why the following affect the rate of reaction:
- Changes in temperature
- Concentration
- Surface area
- Pressure

Answer 4

• Increasing the temperature increases the rate of reaction because the particles move faster which results in more frequent collisions. Higher temperatures also increases the energy of collisions which means more collisions will be successful.
• Increasing the concentration of solutions increases the rate of reaction as there are more reacting particles in the same volume so collisions occur more frequently.
• Increasing the surface area to volume ratio increases the rate of reaction as there is a larger area for collisions to occur on, so collisions happen more frequently.
• Increasing the pressure of gases increases the rate of reaction, because there are more particles in a given volume. This will increase the frequency of successful collisions between particles.

Question 5

1) Describe 4 ways of speeding up chemical reactions.
2) Why is only a small amount of a catalyst necessary in a reaction?

Answer 5

1) - Increasing the temperature
- Increasing the surface area to volume ratio
- Increasing the pressure of gases
- Increasing the concentation of solutions
2) Only a small amount of a catalyst is necessary because the enzyme does not get used up during the reaction.

Question 6

SC18a - Core Practical
1) What is the aim of the investigating reaction rates core practical?
2) What is the method of the investigating reaction rates core practical?

Answer 6

1) Investigating the effect of changing the conditions of a reaction on the rates of chemical reactions.
2) A. Set up the apparatus. Measure 40cm (cubed) of dilute hydrochloric acid into a conical flask.
C. Add 5g of small marble chips to the flask.
D. Immediately stopper the flask and start the stop clock.
E. Note the total volume of gas produced after every 30 seconds until the reaction has finished.
F. Repeat the experiment using 5g of larger marble chips.

Question 7

SC18a - Core Practical
1) What is the method for the observing a colour change practical?
2) What are safety considerations for the investigating reaction rates core practical?

Answer 7

1) A. Place 50cm (cubed) of sodium thiosulfate.
B. Measure out 5cm (cubed) of dilute hydrochloric acid into a test tube.
C. Clamp the conical flask in a water bath at a certain temperature. Place the test tube in a rack in the same water bath. Record this temperature.
D. After 5 minutes, remove the flask and place it on a piece of white paper marked with a cross.
E. Add the acid to the thiosulfate and start the clock.
2) For dilute hydrochloric acid, avoid contact with the skin and use safety goggles
The reaction between sodium thiosulfate and hydrochloric acid produces sulfur dioxide which is toxic if inhaled