7) Rates of Reaction & Energy Changes Flashcards
What does collision theory explain?
How reactions happen and why they happen at different rates
What must collide successfully for chemical reactions to happen?
Particles
What is the activation energy?
The minimum amount of energy required for particles to collide successfully in order to cause a chemical reaction
How can we increase the rate of a chemical reaction?
Increase the number of collisions
Increase the energy of the reactant particles
What factors affect reaction rate?
Temperature
Concentration
Surface area
Pressure (for gases)
What can be used to calculate rate of reactions at specific times?
A graph
How do you calculate the rate of reaction on a graph?
Draw a line tangent to the curve
Work out the gradient of the tangent
rate of reaction = change in y / change in x
What shows a complete reaction on a graph?
When the line plateaus
What does the steepness of the gradient tell us about reaction rate?
Steeper gradient = faster rate of reaction
What different variables can be put against time on a rate of reacting graph?
Mass
Volume
Concentration
How does increasing temperature affect the rate of chemical reactions?
Higher temperature = increased collision rate
Why does increasing the temperature increase the rate of collisions?
Temperature increases the energy in the reactant particles
Greater proportion of particles have the activation energy needed
How does increasing the concentration of dissolved reactant affect the rate of chemical reactions?
Increased concentration = increased collision frequency
The concentration of what affects the rate of chemical reactions?
Concentration of dissolved reactant
How does increasing the surface area of solid reactants affect the rate of chemical reactions?
Increases collision frequency
How is the surface area of solid reactants often increased?
By breaking up solids into smaller lumps such as powders to increase the surface area to volume ratio
The surface area of what affects the rate of chemical reactions?
Surface area of solid reactants
What are catalysts?
Substances that increase a chemical’s reaction rate without reducing the quantity
What do catalysts do in chemical reactions?
Increase the rate of reaction by lowering the activation energy
What are examples of catalysts?
Iron is used in the process to make ammonia
Platinum and palladium are used in catalytic converters in cars
Enzymes catalyse reactions in biological systems
What catalyst is used in the process to make ammonia?
Iron
What catalysts are used in catalytic converters in cars?
Platinum
Palladium
Why aren’t catalysts found in chemical equations?
They are not used up in reactions so can be used indefinitely
What do catalysts often need to ensure they work?
Cleaning
Regenerating
What does the rate of a chemical reaction tell us?
How quickly reactants are converted into products
What are drawbacks of reaction rate in industry?
Expensive to generate conditions needed for very fast reactions
Safety concerns
What are reaction rates in industry a trade-off between?
Speed
Cost
Safety
What is the experiment to measure gas production (rate of reaction)?
1) Set up a beaker connected to a measuring cylinder with water in cylinder
2) Place known mass of marble chips in beaker
3) Pour known concentration and excess volume of acid onto chips
4) Start timer
5) Record volume of gas produced every 5 seconds
6) Plot graph of time against volume of gas
What is the independent variable when measuring gas production?
Mass of marble chips
What is the dependent variable when measuring gas production?
Volume of gas produced per 5 seconds
How do we see how surface area affects reaction rate?
Meaure how much gas is produced when marble chips of different surface areas react with hydrochloric acid
What is the experiment to observe a colour change (rate of reaction)?
1) Place a beaker on top of a piece of paper with an x in the middle
2) Place the solutions (except acid) in the beaker
3) Pour in acid with a known concentration and start clock
4) Stop clock when cross cannot be seen
5) Repeat for different concentrations of acid
6) Plot graph of time against concentration of acid
What is the independent variable when observing a colour change?
The acid with a known concentration
What is the dependent variable when observing a colour change?
Time taken for cross to no longer be seen
How do we see how concentration affects reaction rate?
Observing the colour change between sodium thiosulfate and hydrochloric acid
What reactions cause a change in heat energy to take place?
Neutralisation reactions
Precipiation reactions
Displacement reactions
Salt dissolving in water
What is a neutralisation reaction?
The reaction between an acid and a base
What is a precipitation reaction?
When two solutions react to make an insoluble salt
What is a displacement reaction?
When a more reactive metal takes the place of a less reactive metal
What happens when salt dissolves in water?
Ionic bonds in salt break and the ions become more mobile in solution
What is an exothermic reaction?
Energy reacting in chemicals is transferred to the surroundings
Increase in temperature
What are examples of exothermic reactions?
Combustion
Neutralisation
Oxidation
Hand warmers
How do hand warmers release energy?
Iron oxidation
What is an endothermic reaction?
Energy from the surroundings is transferred to the reacting chemicals
Decrease in temperature
What are examples of endothermic reactions?
Citric acid + baking soda
Thermal decomposition
Sports injury packs
How do sports injury backs absorb heat?
When squeezed forcefully ammonium nitrate and water mix resulting in instant cooling
What do bond energies measure?
The strength of chemical bonds
How do we see if a reaction is exothermic or endothermic?
Total bond energies of products - total bond energies of reactants
What is energy change measured in?
kJ mol^-1
How does the overall exothermic energy change of a reaction compare to 0?
overall energy change of reaction < 0
Negative energy change
How does the overall endothermic energy change of a reaction compare to 0?
overall energy change of reaction > 0
Positive energy change
What happens to bonds in all chemical reactions?
Existing bonds are broken in reactants
New bonds formed in products
What does the bond energy determine?
The amount of energy used up or released in bond breaking or making
What process is bond breaking?
Endothermic
Requires energy
What process is bond making?
Exothermic
Releases energy
What is the calculation for energy change?
energy change = total bond energies of reactants - total bond energies of products
What is on reaction profiles’ axis?
x-axis: Progess of reaction
y-axis: Energy level
What do reaction profiles tell us?
Amount of energy contained in reactants and products
Activation energy of a reaction
Overall energy change in a reaction
How do catalysts affect reaction profiles?
Lower activation energy
Which activation profile has reactants higher than products?
Exothermic reaction profiles
