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  • Created by: Lotto65
  • Created on: 15-05-18 14:02
What is a simple definition of rate of reaction?
The speed at which reactants are used up or products are formed
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Which two ways could the rate of the reaction between calcium carbonate and hydrochloric acid be measured?
Volume of gas given off every 10 seconds; mass lost every 10 seconds
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How is rate at a certain point calculated?
Gradient of the tangent of the line at that point
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How is average rate calculated?
Total change in mass/ volume of gas over time
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What are some issues with the calcium carbonate and hydrochloric acid experiment?
Some gas could escape before the bung is put on, different size calcium carbonate pieces
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What could cause a rate of reaction to be higher than expected?
Exothermic reactions
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Why do we use cotton wool for mass change rate experiments?
Allows gas to escape but not splashes of solution
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At what point do you use the time taken for the experiment in reaction equations?
When the graph first flattens
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What is the more reliable definition of rate of reaction?
The change in concentration of reactants or products per unit time
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How can rate be measured in the iodination of propane in the presence of an acid catalyst?
Measure colour change with colorimeter - difference in absorption of light
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What colour change occurs in the iodination of propanone?
Brown (iodine) to colourless
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What happens if particles collide with not enough energy?
Just bounce off each other
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What is activation energy there for?
To overcome repulsions
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Why is there a peak at the transition state on an energy level profile?
All atoms are joined by partial bonds leading to an increase in potential energy
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When explaining how factors affect the rate of a reaction, what should be included?
The concept of time
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Are all reactions affected by pressure?
Only those with gases
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What is proportional to temperature in Kelvin?
Average kinetic energy of particles
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Does the identity of a gas affect average kinetic energy?
No it is the same
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How does an increase in temperature change the kinetic energy?
Increases exponentially
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What two reasons are there for why an increase in temperature increases rate of reaction?
Increase in kinetic energy (more particles above activation energy) increases frequency of collisions but they also collide harder to increase chance of a collision leading to a reaction
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What are the axes on a Maxwell-Boltzmann distribution curve?
Energy on x axis and number of particles with a certain amount of energy on the y axis
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What does not change about a Maxwell-Boltzmann distribution curve as temperature changes?
Area under the curve
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How does a catalyst affect the rate of a reaction?
Increases the rate without itself being used up
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What is the equation for kinetic energy?
0.5 x m x v squared
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When is manganese (IV) oxide used as a catalyst?
Decomposition of hydrogen peroxide
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What changes on a Maxwell-Boltzmann distribution curve when a catalyst is added?
Activation energy value moves to the left (is lower)
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Does a catalyst change the enthalpy change on a potential energy profile?
No
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Where is activation energy on a potential energy profile?
The section from the reactants / or products in endothermic to the height of the transition state
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How must the rate equation be determined?
Experimental data
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What is the rate equation?
An experimentally determined equation that relates the rate of a reaction to the concentrations of substances in the reaction mixture
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In the rate equation, is k upper or lower case?
Lower case
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Under what conditions is the rate constant a constant?
For a specific reaction at a specific temperature
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If the rate equation is experimentally determined, what must be investigated in these experiments?
The effect of changing the concentration of reactants on the rate of reaction
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Is there a connection between the rate equation and stoichiometric equation?
No
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Why can we not determine the rate equation from the stoichiometric equation?
The reaction could occur in multiple steps
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The experimental data relates to what sort of rate?
Initial rate
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When writing the units of a rate equation, what is it good practice to do?
Put the positive power terms first
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How do you work out the order of a reaction with respect to a reactant when there is no constant concentration?
Add in another row, make one reactant constant and what the rate would be given the other reactant is already constant
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How can you determine the change in concentration for a zero order reaction over time?
The rate does not change as the concentration of the reactant decreases so the gradient is constant
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How can you determine the differences in gradients of the lines on a graph of concentrations of reactants and products in the equation 2A --> B?
The A line will decrease with a gradient twice as steep as the B gradient
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For a first order reaction, what is the shape of the graph of concentration over time for reactants?
Exponential decrease
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Does the time for the concentration of a first order reaction to drop by half change with concentration?
No
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What two graphs could you be tested on for the different orders of reaction?
Rate of reaction against concentration and concentration against time
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What sort of shape (not actual relationship) between rate of reaction and concentration for a second order reaction?
Exponential increase (actually steeper than that)
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How can you prove a reaction is second order not third order etc?
Plot rate against concentration and if it goes through the origin, it is second order
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How does concentration of reactant change with time in a second order reaction?
Initially high negative gradient that gradually decreases and flattens
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What is the general equation for calculating the units of k in a rate equation?
Concentration^(1-overall order)x time^-1
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What does it suggest if the rate equation is not equal to the stoichiometry equation?
The reaction occurs in multiple steps
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What is a reaction mechanism?
A series of steps that make up a more complex reaction. Each step involves a maximum of two molecules colliding
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What are laws for reaction mechanisms?
Only two molecules react at a time and it must be consistent with the overall reaction equation (cancel species on both sides)
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When can we derive the rate equation from a reaction mechanism/ stoichiometric equation?
When in each step of the mechanism, there are only 2 molecules colliding
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How can you determine the rate-determining step in a reaction mechanism?
Whichever step is slower and where the rate equation for that step is equal to the overall rate equation
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Which step in a reaction mechanism has a higher activation energy?
Rate-determining step
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Do intermediates appear in the rate equation?
No
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How do you determine which reaction mechanism is more appropriate for a reaction?
Experimental data
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What sort of catalyst forms intermediates which lowers the activation energy for a reaction?
Homogeneous catalysts
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Where can catalysts appear? Where can they not appear?
Can appear in rate equations but not in chemical equations
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What is the rate equation in SN1 reactions?
The initial hydrocarbon reactant alone
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What is molecularity?
The number of molecules that react in a particular step
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What is the rate equation in SN2 reactions?
Both reactants
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If both reactants are in the rate equation what does that suggest?
The reaction only occurs by one step
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What does the power of a reactant in the rate equation show?
The number of times it appears in the mechanism up to and including the rate-determining step
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As temperature increases, what happens to the rate constant?
Increases exponentially
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What does A represent in the Arrhenius equation?
The frequency factor taking into account the frequency of collisions with the orientation of collisions
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Is A a constant in the Arrhenius equation?
Yes
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On a graph, what is lnk, lnA, 1/T and -Ea/R?
lnk is the y axis, lnA is the y intercept, 1/T is the x axis and -Ea/R is the gradient of the line
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How do you calculate the information for the Arrhenius equation?
Run a series of experiments at different temperatures and measure k for each one
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What will the units of A be the same as?
The units of k
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What is more accurate, using an equation or a graph for the Arrhenius equation?
Equation because graph introduces uncertainties doing it by hand and the points chosen might not lie exactly on the line of best fit
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What is the rate of reaction and k like if the activation energy is large?
Rate of reaction slower and k smaller
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Why is A smaller for more complicated reactions?
More complicated molecules means many different orientations and ways they could collide
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What does a catalyst do to k?
Increases it
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Other cards in this set

Card 2

Front

Which two ways could the rate of the reaction between calcium carbonate and hydrochloric acid be measured?

Back

Volume of gas given off every 10 seconds; mass lost every 10 seconds

Card 3

Front

How is rate at a certain point calculated?

Back

Preview of the front of card 3

Card 4

Front

How is average rate calculated?

Back

Preview of the front of card 4

Card 5

Front

What are some issues with the calcium carbonate and hydrochloric acid experiment?

Back

Preview of the front of card 5
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