Flashcards in Section 7 - Rates of Reaction & Energy Changes Deck (27):
What is the rate of reaction?
How quickly a reaction happens.
How is the rate of reaction calculated?
Rate of Reaction=a/t
a= amount of product formed or reactant used
Which three common techniques are used to measure the rate of reaction?
Precipitation, Change in Mass and The Volume of Gas Given Off.
How is the Precipitation method used to measure the rate of reaction? Describe how to perform the method. Any drawbacks?
This method can be used when two see-through solutions produce a precipitate that can cloud the solution when they react.
To perform the method you mix the two reactant solutions together and put the flask on a piece of paper with a mark on it.
To measure the rate of reaction, measure how long it takes for the mark to be obscured. The faster the mark disappears, the quicker the reaction.
One drawback is that the result is subjective and different people will not agree on exactly when the mark is obscured.
How is the Change in Mass method used to measure the rate of reaction? Describe how to perform the method. Any drawbacks?
This method can be used when a reaction produces a gas.
To perform the method the reaction must be done on a mass balance. as the reaction occurs, gas is released, decreasing the mass. To keep it accurate, a piece of cotton wool is often placed in the mouth of the flask, which prevents any solid, liquid or aqueous substance escaping.
The rate of reaction can be plotted on a graph as mass lost over time. The faster the mass drops, the quicker the reaction.
One drawback of the method is that a harmful gas- if produced- would be released directly into the air, so safety precautions must be taken.
How is the Volume of Gas Given Off method used to measure the rate of reaction? Describe how to perform the method. Any drawbacks?
This method can be used when a reaction produces a gas.
To perform this method, as soon as the reaction starts a rubber bung must be pressed into the mouth of the flask, leading into a gas syringe. As the reaction happens, the gas given off will fill the syringe.
The rate of reaction can be plotted on a graph as gas volume over time. The faster the volume rises, the faster the reaction.
One drawback of the method is that if the syringe is too small, the plunger may be blown out of the end of the syringe.
In a gas volume over time graph, what does the gradient show?
The steeper the gradient, the faster the reaction, if the line flattens out, the reaction has finished.
How does surface area affect the rate of reaction? Why does this happen, according to collision theory?
A larger surface area increases the rate of reaction because there are more solid particles exposed, this means there are more collisions, which increases the rate of reaction.
How does the concentration (or pressure) of solutions affect the rate of reaction? Why does this happen, according to collision theory?
A more concentrated solution (or an increased pressure) will increase the rate of reaction because there are more reactant particles in the same volume, which makes collisions more likely, which raises the rate of reactions.
How does temperature affect the rate of reaction? Why does this happen, according to collision theory?
Higher temperatures increase the rate of reaction because the particles will be moving faster, which leads to more collisions, and the particles will transfer more energy in collision, which means that there will be more successful collisions, both reasons raise the rate of reaction.
How does collision theory explain how chemical reactions occur?
Chemical reactions occur when particles collide and enough energy is transferred in the collision for a reaction to occur.
How is the rate of reaction explained by collision theory?
The rate of reaction depends on two factors, the collision frequency and the energy transferred in the collision. more collisions mean a higher rate of reaction and more collision energy means that there are more successful reactions, leading to a higher rate of reaction.
What are catalysts?
Chemicals that increase the rate of reaction without being used up or chemically changed in the reaction.
How do catalysts work?
They lower the activation energy required for a chemical reaction by offering an alternate reaction pathway that has a lower activation energy.
What are enzymes?
Biological catalysts that speed up cell reactions like respiration, photosynthesis and protein synthesis.
What is an exothermic reaction?
A chemical reaction which releases energy to the surroundings.
What is an endothermic reaction?
A chemical reaction which takes in energy from the surroundings.
What are reaction profiles?
graphs which display the energy levels of the reactants and products in a reaction.
Describe a reaction profile for an exothermic reaction.
The energy level raises from the reactants to the activation energy and then drops below the initial energy level of the reactants before levelling out at the energy level of the products.
Describe a reaction profile for an endothermic reaction.
The energy level raises from the reactants to the activation energy and then drops, stopping at the products' energy level somewhere above the initial energy of the reactants.
What is activation energy?
The minimum amount of energy needed for bonds to break and a reaction to start.
How is the activation energy displayed on a reaction profile?
As the difference in energy between the reactants and the highest point on the curve.
What type of process is bond-breaking? Why?
Endothermic, because energy must be supplied to break a bond.
What type of process is bond-forming? Why?
Exothermic, because energy is released when a bond forms.
How is overall energy change calculated? What is the calculation for overall energy change?
Every chemical bond has a particular bond energy that varies depending on the compound it occurs in. The overall energy change can be calculated using the following formula;
ΔE₀= Overall Energy Change
E₁= Energy required to break bonds
E₂= Energy released to form bonds
What does it mean if the overall energy change is positive?
The reaction is endothermic.