Name: _________________________ 

Teacher: ______________ 

EXAM DATE:  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Calculating the rate of reaction

Rate of reaction = Amount of reactant used OR product formed Time

The rate of reaction can be found in different ways:

Measuring the amount of reactants used. - If one of the products is a gas, measure the mass in grams (g) of

the reaction mixture before and after the reaction takes place and measure the time it takes for the reaction to happen.

- The mass of the mixture will decrease because the gas will escape.

- The units for the rate of reaction may be given as g/s (grams per second).

- The amount of a reactant can also be measured in moles (mol). - As the reaction takes place the reactant is used up, so the

amount of reactant remaining decreases. - The concentration of the reactant is calculated as the amount

(mol) divided by the volume of the reactant mixture (dm3). It is measured in the units of mol/dm3.

Measuring the amount of products formed. - If one of the products is a gas, measure the

total volume of has produced in cubic - centimetres (cm3) with a gas syringe and the

time it takes for the reaction to happen. - The units for the rate of reaction may then

be given as cm3/s (volume of gas per second. Measuring the time it takes for a reaction

mixture to become opaque or change colour. - Time how long it takes for the mixture to change colour. - Rate of reaction = 1 .

Time taken for solution to change colour

 

 

 

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Collision theory

Chemical reactions only occur when reacting particles collide with each other with sufficient energy.

The minimum amount of energy required to cause a reaction is called the activation energy.

There are four important factors that affect the rate of reaction: Temperature, concentration, surface area and catalysts

Temperature and the rate of reaction

Increasing the temperature increases the rate of reaction.

Adding more heat energy to the particles will cause them to move faster and with more energy (Heat energy kinetic energy).

The more (and faster) energy the particles are moving with, the more collisions there will be between the reactants. 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Concentration and the rate of reaction

Increasing the concentration increases the rate of reaction.

Having more particles of each reactant in the same volume of solution will cause there to be more collisions. This also works with increasing pressure.

Think of it like a big traffic jam. There’s a higher change that there will be a crash than if the roads were emptier.

Low 

concentrationHigh 

concentration

Surface area and the rate of reaction

Increasing the surface area increases the rate of reaction.

Having a large lump of a chemical will mean there are less particles of the reactant on show. This means there will be less successful collisions.

Grinding or chopping up a chemical will cause the particles on the inside to also be on show. This will increase the amount of collisions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Required practical – Investigating how changes in concentration affect the rates of reactions by methods involving the production of gas or a colour change.

This investigation uses the reaction between sodium thiosulfate and hydrochloric acid. Sample method:

1. Add the hydrochloric acid to a conical flask full of sodium thiosulfate and swirl to mix the reactants.

2. Start the timer and watch the cross through the flask (see diagram below)

3. When the cross is no longer visible, stop the timer.

4. Repeat the experiment using hydrochloric acid of a different concentration.

Considerations, mistakes and errors: - There should be a correlation

between the concentration of the acid and the time taken for the cross to ‘disappear’.

- The higher the concentration of the acid, the faster the rate of reaction, and the shorter the time for the cross to disappear.

Variables: - The independent variables is the concentration

of the acid. - The dependent variable is the time is takes of

the cross to disappear. - The control variables are the concentration

volume of sodium thiosulfate.

Hazards and risks: - Corrosive acid can cause

damage to eyes, so eye protection must be worn.

- Sulfur dioxide gas release can trigger an asthma attack, so the temperature must always be kept below 500C

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Plotting reaction rates

Graphs can be plotted to show the progress of a chemical reaction. There are three key things to remember:

- The steeper the line, the faster the reaction. - When one of the reactants is used up, the reaction stops (the line

becomes horizontal). - The same amount of products is formed from the same amount of

reactants, no matter how fast or slow the rate of reaction is. The rate of reaction at a particular time is given by graphs:

1. Draw a tangent to the curve at that time. 2. Find the gradient of that tangent. 3. The gradient is equal to the rate of reaction at that time.

Gradient = y axis value ÷ x axis value

Quick test!

1. Why does increasing the temperature increase the rate of reaction? _____________________________________________________ _____________________________________________________

2. What is the name given to the minimum amount of energy that reacting particles must have to react? _____________________________________________________

3. Find the rate of reaction for all 3 plots of data on the graph below.

Single chip = ________ Same mass of small chips = ____ Same mass of powder =_______

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

Catalysts

- A catalyst is a substance that increases the rate of a chemical reaction without being used in the process.

Catalysts are not included in the chemical equation for the reaction, simply because it is not chemically involved in the reaction. A catalyst:

- Reduces the amount of energy needed for successful collisions. - Makes more collisions successful. - Speeds up the reaction. - Provides a surface for the molecules to attach to, which increases

their chances of bumping into each other. Different reactions need different catalysts, e.g;

- The cracking of hydrocarbons (see unit C7) uses broken pottery. - The manufacture of ammonia uses iron.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reversible reactions

Some chemical reactions are reversible, they can go forwards or backwards. In a reversible reaction, the products can react to produce the original reactants. These reactions are represented as:

A(g) + B(g) ⇌ C(g) + D(g) This tells us that:

- A and B can react together to make C and D. - C and D can react together to make A and B.

For example: - Solid ammonium chloride decomposes when heated to produce ammonia

and hydrogen chloride gas (both colourless). - Ammonia react with hydrogen chloride gas to produce clouds of white

ammonium chloride. Ammonium Chloride ⇌ Ammonia + Hydrogen Chloride

NH4Cl(s) ⇌ NH3(g) + HCl(g) The direction of reversible reactions can be changed by changing the conditions of the reaction. If a reversible reaction is exothermic in one direction, it is endothermic in the other direction. The amount of energy given out in the exothermic direction is always the same as the amount of energy needed to be absorbed in the endothermic direction.

Closed systems

In a closed system, no reactants are added and no products are removed.

When a reversible reaction occurs in a closed system, an equilibrium is achieved when the rate of reaction is equal to the rate of the backward reaction. The products are formed at exactly the same rate at which the reactants are formed.

Again, the relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.

 

 

 

 

 

 

 

   

Changing reaction conditions

This is higher tier ONLY

Le Chatelier’s Principle states that if a system in equilibrium is subject to change in conditions, then the system shifts to resists the change. In an exothermic reaction:

- If the temperature is raised, the yield decreases. - If the temperature is lowered, the yield increases.

In an endothermic reaction: - If the temperature is raised, the yield increases. - If the temperature is lowered, the yield decreases.

In reactions involving gases: - An increase in pressure favours the reaction that produces the least

number of as molecules. - A decrease in pressure favours the reaction that produces the greater

number of gas molecules. If the concentration of a reactant or product is changed:

- The system is no longer in equilibrium. - The system adjusts until it can reach equilibrium once more.

If the concentration of one of the reactants is increased, the position of equilibrium shifts so that more products are formed until equilibrium is reached again. In contrast, if the concentration of one of the reactants is decreased, the position of equilibrium shifts so that more reactants are formed until equilibrium is reached again. These factors, together with reaction rates, determine the optimum conditions in industrial processes.

    

Practice exam questions

This is FOR ALL ABILITIES

1. Which of the following changes would increase the rate of a chemical change? Tick one box. (1 mark)

Adding a catalyst Reducing the temperature by 100C Increasing the size of particles Decreasing the concentration

Quick test!

4. What is a reversible reaction? _____________________________________________________ _____________________________________________________

5. When is a system in equilibrium? _____________________________________________________ _____________________________________________________

6. What is a closed system and why is it constantly in equilibrium? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________

This is higher tier ONLY

7. How can the effect of changing the conditions in a system that is in equilibrium be predicted? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Practice exam questions continued

This is FOR ALL ABILITIES

2. Collision theory can be used to explain the rate of a chemical reaction. a) What two things must happen for a chemical reaction to take

place? (2 marks) _________________________________________________ _________________________________________________

b) Explain, in terms of collision theory, why increasing the temperature increases the rate of a chemical change. (2 marks) _________________________________________________ _________________________________________________

3. In the ‘disappearing cross’ experiment, hydrochloric acid reacts with sodium thiosulfate. One of the products of the reaction is sulfur, which is insoluble. A student carries out the experiment using 1.00mol/dm3 acid. She adds the acid to the sodium thiosulfate and times how long it takes for the cross to ‘disappear’. She then repeats the experiment using 0.500mol/dm3.

c) Name the independent variable in this investigation. (1 mark) _________________________________________________

d) Temperature could affect the rate of reaction, so the same temperature must be used for each part of the investigation. Predict and explain how increasing the temperature would affect the time taken for the cross to disappear. (3 marks) ____________________________________________________________________________________________________________________________________________________________________________________________________

1. If a reversible reaction is carried out in a closed system, equilibrium can be reached.

e) Describe what a closed system is. (1 mark) _________________________________________________

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Practice exam questions continued

This is FOR ALL ABILITIES

f) Explain how you can tell that a system is in equilibrium (1 mark) __________________________________________________________________________________________________

1. When a reaction is carried out in a sealed container, it reaches equilibrium. Which of the following statements are true? (1 mark)

g) At equilibrium, all the reactants have reacted to form products

h) At equilibrium, the amount of products equal the amount of reactants.

i) At equilibrium, the rate of forward reaction is equal to the rate of the backward reaction

j) At equilibrium, both the forward and the backward reactions stop.

2. The word equation for the reaction between anhydrous cobalt chloride and water is below:

Anhydrous cobalt chloride + Water ⇌ Hydrated cobalt chloride (Blue) (Pink)

k) When the student added water to anhydrous cobalt chloride, what happened? (1 mark) _________________________________________________

l) A student measured the temperature rise when anhydrous cobalt chloride was added to water. The student’s results are shown in the table below: Trial 1 Trial 2 Trial 3Temperature rise in 0C

8.5 8.2 8.2

Calculate the mean temperature rise:

Temperature = ___________0C

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Practice exam questions continued

This is FOR HIGHER TIER ONLY

1. The position of equilibrium of a reaction is dependent on the conditions that the reaction is carried out under.

a) What does Le Chatelier’s principle say about the effect of changing the conditions of a reversible reaction at equilibrium? (1 mark) __________________________________________________________________________________________________

b) State two conditions you could change in order to alter the position of equilibrium of a reaction that happens in solution. (2 marks) ____________________________________________________________________________________________________________________________________________________________________________________________________

2. A mixture of iodie monochloride (ICl) and chlorine is sealed in a gas syringe. The gases react in a reversible reaction to form iodine trichloride (ICl3) and eventually reach an equilibrium. The equation for the reaction is: ICl(l) + Cl2(g) ⇌ ICl3(s)

c) Given that the forward reaction is exothermic, explain how the relative quantities of ICl and ICl3 would change is the mixture was heated, and all other conditions remained the same. (2 marks) ___________________________________________________________________________________________________________________________________________________

d) Explain how the relative quantities of ICl and ICl3 would change if the plunger were pushed into the syrine and all other conditions remained the same. (3 marks) ___________________________________________________________________________________________________________________________________________________

 

Practice exam questions continued

This is FOR HIGHER TIER ONLY

1. Yellow iron(III) ions and colourless thiocyanate ions react reversibly in solution to form dark red iron thiocyanate:

Fe3+(aq) + SCN ⇌ FeSCN2+

The following observations are made about this reaction: When a yellow solution containing Fe3+ ions and a colourless

solution containing SCN- ions are mixed, a pale red colour forms which initially grows darker but then stays constant.

When more Fe3+ ions are added to the solution it initially becomes more orangey in colour but then grows darker red than before the Fe3+ was added and remains like this.

If FeSCN2+ ions are added to the solution it initially becomes darker in colour but then becomes more orangey.

Explain what is happening in each of these observations:

Observation 1: _______________________________________________________________________________________________________________________________________________________________

Observation 2: _______________________________________________________________________________________________________________________________________________________________

Observation 3: _______________________________________________________________________________________________________________________________________________________________