# AQA Topic 6 - The Rate and Extent of Chemical Change 9-1

• Created by: Grace
• Created on: 17-04-18 21:15
What is a rate of a chemical reaction?
How fast the reactants are changed into products
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On a graph of rate of reaction what does a steeper line indicate?
The steeper the line the faster the reaction
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On a graph of rate of reaction what does it mean when the line begins to plateau?
The reactants are being used up
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What is the collision theory? (1)
That the rate of reaction depends on: 1. The collision frequency of reacting particles i.e the more collisions the faster the reaction is, this means if you double the frequency you double the rate of reaction.
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What is the collision theory? (2)
2. The energy transferred during a collision, particles have to collide with enough energy for the collision to be successful
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What four things does the rate of reaction depend upon?
1. Temperature 2. Concentration of a solution OR pressure of a gas 3. Surface area 4. Presence of a catalyst
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What can all four methods of increasing the rate of reaction be explained by?
They can be explained in terms of increasing the number of successful collisions between the reacting particles
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Why does increasing the temperature increase the rate?
Energy is given to the particles when temperature is increased which means the particles are going to collide more frequently. The faster the particles move the more energy they have so more collisions will have enough energy to react
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Why does increasing the concentration OR pressure increase the rate?
If a solution is more concentrated then there are more particles in the same solvent, similarly when the pressure of a gas is increased as the same number of particles occupy a smaller space which makes more frequent collisions
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Why does increasing the surface area increase the rate?
If one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area:volume ratio, this means for the same volume of the solid the particles around have more area to work on so there will frequent collisions
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Why does using a catalyst increase the rate?
A catalyst decreases the activation energy needed for the reaction to occur so particles with less energy can collide and react
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What is the rate of reaction equation?
Rate of Reaction = Amount of reactant used OR amount of product formed ÷ Time
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Name three different ways of measuring the rate of reaction
1. Precipitation and colour change 2. Change in mass 3. Volume of gas
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What is a precipitate?
An insoluble solid that emerges from a liquid solution
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When can you conduct a 'precipitation and colour change' experiment?
You can record the visual change in a reaction if the initial solution is transparent and product is a precipitate which clouds the solution before coming opaque
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Describe the 'precipitation and colour change' experiment for rate of reaction (1)
Observe a mark through the solution and measure the time it takes to disappear, the faster the mark = faster rate of reaction. If the reactants are coloured and the products are colourless (or vice versa).
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Describe the 'precipitation and colour change' experiment (2)
ctd... Then measure the time it takes for the solution to lose or gain its colour
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What is a disadvantage of the 'precipitation and colour change' experiment?
You can't plot a rate of reaction graph from the results
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Describe the 'change in mass' experiment for rate of reaction
As a gas is released the mass disappearing is measured on the balance, the quicker the reading on the balance drops = faster reaction. By taking measurements at regular intervals you can plot a rate of reaction graph
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What is an advantage and disadvantage of the 'change in mass' experiment?
ADVANTAGE - Most accurate method out of the three. DISADVANTAGE - Releases gas straight into room
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Describe the 'volume of a gas' experiment for rate of reaction
Use a gas syringe to measure the volume of gas given off, the more gas given off during a given time interval = faster reaction
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Describe an experiment to investigate the effect of concentration on the rate of reaction
Add a set volume of dilute HCl acid to a conical flask and place on a mass balance, add Mg ribbon and plug the flask with cotton wool. Start stopwatch and record the mass on a balance. Repeat with a more conc acid solution and plot results
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Describe an experiment betwixt sodium thiosulfate and HCl to produce a cloudy precipitate
Add a set vol of dilute sodium thiosulfate to a conical flask and place on a piece of paper w/ a black cross. Add dilute HCl and start the stopwatch. Record the time it takes for the cross to disappear and the precipitate to form.
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How do you find the reaction rate of a particular point on a graph?
Draw a tangent at the point where you want to know the rate and draw a triangle. Find the gradient by change in y divided by change in x to find rate of reaction at that point
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What is a reversible reaction?
A reaction where the reactants from products which react to give the reactants back
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What is a dynamic equilibrium?
When both reactions are still happening but theres no overall effect which means the concentrations of reactants and products have reached a balance and won't change
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What must be in place for a reversible reaction to reach equilibrium?
That the reaction takes place in a 'closed system' so no reactants or products can escape and nothing else can get in
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It doesn't mean the amounts of products and reactants are equal but the forward and reverse reactions are still going at the same right as one another
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What does it mean when the position of equilibrium is on the right?
That the concentration of products is greater than that of the reactants
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What does it mean when the position of equilibrium is on the left?
That the concentration of reactants is greater than that of the products
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What conditions does the position of equilibrium depend on bar the reaction itself?
1. Temperature 2. Pressure (for gases) 3. Concentration of the reactants and products
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If the forwards reaction in a reversible reaction is endothermic what is the backwards reaction? Why?
Exothermic because the energy transferred from the surroundings by the endo reaction is equal to the energy transferred to the surroundings during the exothermic reaction
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Complete the sentence: Reversible Reactions try to _______ changes
Counteract
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What is Le Chatelier's principle?
That if you change the conditions of a reversible reaction at equilibrium the system will try to counteract that change which can help predict the effect of any changes you make to a reaction system
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Give an example and explanation of Le Chatelier's principle working (1)
Because all reactions are exothermic in one direction and endothermic in the other if you decrease the temperature the equilibrium will move in the exothermic direction to produce more heat which means you'll get more products for the exo reaction
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Give an example and explanation of Le Chatelier's principle working (2)
ctd.. and fewer products for the endo reaction. However if you raise the temperature the equilibrium will move in the endo direction to try and decrease it which means you now get more products for endo reaction and less for exo
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What happens if you increase the pressure in a reversible reaction?
The equilibrium tries o reduce it and moves in the direction where there a re fewer molecules of gas
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What happens if you change the concentration of either the reactants or products? What does it do to reverse this?
The system will no longer be at equilibrium so the system responds by bringing itself back to equilibrium i.e if you increase the conc of the reactants the system will decrease it by making more products vice versa
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## Other cards in this set

### Card 2

#### Front

On a graph of rate of reaction what does a steeper line indicate?

#### Back

The steeper the line the faster the reaction

### Card 3

#### Front

On a graph of rate of reaction what does it mean when the line begins to plateau?

### Card 4

#### Front

What is the collision theory? (1)

### Card 5

#### Front

What is the collision theory? (2)