Measuring and calculating rates, reading rate graphs including drawing tangents, and explaining everything through collision theory.
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1.What is the rate of a chemical reaction?
How quickly reactants are used up, or how quickly products are formed.
2.State the equation for mean rate using a reactant.
mean rate = quantity of reactant used divided by time taken
3.State the equation for mean rate using a product.
mean rate = quantity of product formed divided by time taken
4.Give two units that rate of reaction can be measured in.
Grams per second, or cubic centimetres per second, depending on what is being measured.
5.Give two ways of following the rate of a reaction.
Measuring the volume of gas produced over time, or measuring the loss in mass as gas escapes.
6.What does collision theory say?
For a reaction to happen, particles must collide with each other and with enough energy.
7.What is a successful collision?
One where the colliding particles have at least the activation energy, so a reaction occurs.
8.What two things determine the rate of reaction under collision theory?
The frequency of collisions and the proportion of collisions that have enough energy.
9.What is activation energy?
The minimum energy particles must have when they collide in order to react.
10.What does the gradient of a rate graph represent?
The rate of reaction at that moment.
11.What does a steeper gradient mean?
A faster rate of reaction.
12.Why is a rate graph steepest at the start?
The concentration of reactants is highest, so collisions are most frequent.
13.Why does a rate graph level off?
The reaction has finished because one of the reactants has been completely used up.
14.How do you find the rate at a particular moment from a curve?
Draw a tangent to the curve at that point and calculate its gradient.
15.How is the gradient of a tangent calculated?
Change in the y value divided by change in the x value along the tangent.
16.A tangent rises 20 cm3 over 10 seconds. What is the rate?
20 divided by 10 = 2 cm3 per second.
17.Two experiments level off at the same volume but one is faster. What differs?
The rate is different, but the amount of limiting reactant is the same, so the same total volume of gas is produced.
18.What would a graph look like for a reaction with double the concentration?
A steeper initial gradient, levelling off at the same final value if the limiting reactant is unchanged.
19.Why does a reaction slow down as it goes on?
The reactants are being used up, so their concentration falls and collisions become less frequent.
20.60 cm3 of gas is produced in 30 seconds. Calculate the mean rate.
60 divided by 30 = 2 cm3 per second.
21.0.6 g of mass is lost over 120 seconds. Calculate the mean rate.
0.6 divided by 120 = 0.005 g per second.
22.Why is mean rate less useful than the rate at a given moment?
The rate changes throughout a reaction, so a mean hides how fast it was at the start and how it slowed.
23.Why does a reaction stop even when one reactant is left?
The other reactant is limiting, and once it is used up no more collisions between the two can occur.
24.What must be true of a collision for no reaction to occur?
The particles collided with less than the activation energy, so they simply bounce apart.
25.Which graph line represents the faster reaction: a steep one levelling early, or a shallow one levelling later?
The steep one that levels off earlier.
26.What is measured on the y axis of a gas production graph?
The volume of gas produced, usually in cubic centimetres.
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