Chemistry topic 6 Flashcards
(22 cards)
What does the rate of reaction mean?
How fast the reactants are changed into products. Slowest reactions include rusting iron, chemical weathering (acid rain) etc.
Moderate speed reactions include metal magnesium, reacting with an acid to produce a gentle stream of bubbles.
Burning is fast, but explosions or even faster, release lots of gas (over in a fraction of a second).
How do graphs show the rate of reaction?
Speed of reaction is found by recording the amount of product formed or amount of reactant used up over time.
Steeper the line, faster the rate. Overtime line becomes less steep as reactants are used up.
Quickest reactions have steepest lines and become flat in least time.
Why do particles need to collide with enough energy to react? (Collision theory)
It’s called the collision theory;
Dependant on;
Collision frequency of reacting particles - more collisions there are, faster the reaction.
Energy transferred during a collision, particles have to collide with enough energy for collision to be successful.
Minimum amount of energy particles need to react is called the activation energy, particles need this much energy to break bonds in the reactants and start the reaction.
What are the factors that affect the rate of reaction? (4)
1) temperature
2) concentration of a solution or pressure of gas
3) surface area
4) presence of a catalyst
How does an increased amount of collisions increase the rate of reaction?
All four methods of increasing the rate of reaction can be explained in terms of increasing the number of successful collisions between reacting particles.
How does increase temperature increase the rate of reaction?
When temp is increased particles move faster.
If they’re moving faster, they will collide more frequently.
The faster they move, the more energy they have, so more of the collisions will have enough energy to make the reaction happen.
How does increasing the concentration or pressure increase the rate of reaction?
If a solution is more concentrated, more small particles are knocking about in the same volume of solvent.
Similarly, when the pressure of gas is increased, it means the same number of particles occupies a smaller space.
This makes collisions between reactant particles more frequent.
How does increasing the surface area increase the rate of reaction?
If one of the reactants is solid, breaking it into smaller pieces will increase its surface area to volume ratio.
This means for the same volume of solid, the particles moving around it will have more area to work on so collisions will be more frequent.
How does using a catalyst increase the rate of reaction?
A catalyst speed up the rate of reaction without being used up itself, meaning it’s not part of the overall reaction.
Different catalysts are needed for different reactions, but they all work by decreasing the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy.
Enzymes are biological catalyst, meaning they catalyse reactions in living things.
What are the three different ways to measure the rate of a reaction?
Precipitation and colour change
Change in mass
Volume of gas given off
What is the equation for the rate of reaction?
Amount of reactant used or amount of product formed divided by the amount of time taken.
What are the two practical investigations which describe how concentration has an effect on the rate of reaction?
Magnesium and hydrochloric acid react to produce hydrogen gas.
Sodium thiosulfate and hydrochloric acid produce a cloudy precipitate.
How do you calculate the mean reaction rate from a graph?
A rate of reaction graph shows amount of product formed or amount of reactant used up on the Y axis and time on the X axis.
To find the mean rates for whole reaction, work out the overall change in the Y value and divide this by the total time taken for the reaction.
You can also use the graph to find the mean rate of reaction between any two points in time.
How do you find the reaction rate at a particular point?
Find the gradient of the car at that point. Draw a tangent to the curve, a straight line that touches a curve at one point and doesn’t cross it, then workout the gradient of the tangent.
What is the equation for a reversible reaction?
A + B (reaction symbol) C + D
Means the reaction goes both ways
How to reversible reactions reach equilibrium?
As reactants react their concentrations fall, forward, reaction for slow down. As more products are made their concentrations rise, backward reaction will speed up.
After a while, the forward reaction will be going at exactly the same rate as the backward, one - system is at equilibrium.
At equilibrium, both reaction still happen, but there is no overall effect, meaning of concentrations of both reactants and products have reached a balance and won’t change.
Equilibrium is only reached if reversible reaction takes place in a closed system, meaning none of the reactants or products can escape.
How can the position of the equilibrium be on the right or left? (of equation)
When a reactions reached equilibrium, it doesn’t mean amounts of reactants and products are equal.
if equilibrium lies to the right, the conc of products is greater than the reactants.
If equilibrium lies to left, the conk of reactants is greater than products.
Its position depends on the following conditions;
Temperature, pressure or concentration.
Why can a reversible reaction be endothermic and exothermic?
If reaction is endothermic in one Direction, it will be exothermic in the other.
Energy transferred from surroundings by endothermic reaction is equal to energy transferred to the surroundings by EXO thermic reaction.
E.g. thermal decomposition of hydrated copper sulphate;
If you hate blue hydrated, copper sulphate, crystals, it drives the water off and leaves white powder, (Endo thermic). If you add a couple of drops of water to the powder, you get the blue crystals again (exothermic).
What is Le Chatelier’s principle?
The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change.
Can be used to predict the effect of any changes you make to a reaction system.
How can you use Le Chatelier’s principle to predict the effect of temperature on a reaction?
All reactions exothermic in One Direction, endothermic in the other.
If you decrease the temp, equilibrium will move in the exothermic direction to produce more heat. Meaning you’ll get more products for the exothermic reaction and fewer for the endothermic.
If you raise the temp, the equilibrium will move in the Endo thermic direction to try and decrease it. You’ll get more products for endothermic and fewer for exothermic now.
How can you use the le Chatelier principle to predict the effect of pressure on a reaction?
Changing pressure only affects an equilibrium involving gases.
If pressure is increased equilibrium tries to reduce it - move in the direction where there are fewer molecules of gas.
If pressure is decreased equilibrium, tries to increase it - move in the direction where are there are more molecules of gas.
You can use the balance symbol equation for the reaction to see which side has more molecules of gas.
How can you use Le Chatelier’s principle to predict the effect of concentration in a reaction?
If you change the concentration of either reactants or products, the system will no longer be at equilibrium.
This means it will respond to try and bring itself back to equilibrium.
If you increase the concentration of the reactants, the system tries to decrease it by making more products.
If you decrease the concentration of the reactants, the system tries to increase it by reducing the amount of reactants.
