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Revise
Concentration: – At higher concentrations, the particles are crowded closer together – they collide more often, so there are more successful collisions. – Increasing the pressure of reacting gases also increases the frequency of collisions. REQUIRED PRACTICAL
Timer
Investigate 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. Set up the equipment as shown. 2. Add the hydrochloric acid to the flask and swirl to mix the reactants. 3. Start the timer. 4. Watch the cross through the flask. 5. When the cross is no longer visible stop the timer. 6. 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 variable is the concentration of the acid. • The dependent variable is the time it takes for the cross to ‘disappear’. • The control variables are the volume of acid and the concentration and volume of sodium thiosulfate.
Hazards and Risks • Corrosive acid can damage eyes, so eye protection must be used. • Sulfur dioxide gas can trigger an asthma attack, so the temperature must always be kept below 50°C.
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 product is formed from the same amount of reactants, regardless of rate.
HT
Paper with cross drawn on it
Key Point
Surface area: – Small pieces of a solid reactant have a large surface area in relation to their volume. – More particles are exposed and available for collisions, so there are more collisions and a faster reaction.
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Flask
Sodium thiosulfate
The rate of reaction at a particular time is given by graphs: – Draw a tangent to the curve at that time. – Find the gradient of the tangent. – The gradient is equal to the rate of reaction at that time.
Smaller pieces have a higher surface area to volume ratio than larger pieces.
gradient =
Amount of Product Formed or Amount of Reactant Used Up
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Add dilute hydrochloric acid
difference in the amount o off product formed / reactant used time
A B
Time The graph shows that reaction A is faster than reaction B.
Key Words Quick Test 1. Why does increasing 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. How does surface area affect the rate of reaction?
rate of reaction HT concentration activation energy HT tangent HT gradient
The Rate and Extent of Chemical Change: Revise
227841 AQA Higher Tier Combined Science All-in-One_P120_157.indd 125
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