Chem 32 ch. 6
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What is a physical change?
A process tht does not alter ther chemical formulas of the starting materials.
What is a chemical change / chemical reaction?
A process in which the chemical formulas of the starting materials and final products differ.
Chemical changes are often called chemical reactions.
What is a physical property of a substance?
Any atribute of a substance that can be measured or described withouth changing the chemical formula of the substance.
Whatis a chemical property?
A property that involves a chemical reaction...?
What is the law of mass conservation?
In any change ( like chemical reaction ) the mass of the final products equals the mass of the starting materials.
In any change the number of atoms of each element remains constant.
What are reactants in a chemical formula?
What are products in a chemical formula?
Reactants are to the left of the arrow ( Na + Cl ---> ), products are to the right of the arrow ---> NaCl ).
Which is correct and why?
Mg + 2Cl = MgCl2
Mg + Cl2 = MgCl2
In this case MgCl2 is correct. Cl will covalently bond with itself to form Cl2 in most cases so it will be in Cl2 form as a reactant rather than Cl by itself.
What is an exothermic reaction?
A reaction that produces heat.
What is an endothermic reaction?
A reaction that absorbs heat.
What is nutritive value?
How much energy is gained by eating an amount of food.
How much energy is gained from eating 1g of; carbohydrate, protein, fat?
- 1g carbohydrate = 4 kcal
- 1g protein = 4 kcal
- 1g fat = 9 kcal
What is a combustion reaction?
The reaction of a chemical compound with oxygen ( in the form of O2 ) that results in fire.
Combustion reactions release a lot of head, produce a lot of energy.
What do compounds made from carbon, hydrogen, and oxygen have in common?
They react with O2, producing CO2 and H2O.
What is precipitation?
The formation of an insoluble compound by two or more soluble compounds. It has to start with just aqueous and end up with at least on solid.
Two liquidy things get together and end up producing a solidy thing.
A precipitation reaction occurs when solutions that contain ions are mixed and one of the possible combination of ions does not dissolve in water.
What is a net ionic equation?
A chemical reaction equation that only shows the substances that change during a reaction.
- Instead of:
- CaCl2 + Na2CO3 ---> CaCO3 + (?)
- Ca2+(aq) + CO32-(aq) ---> CaCo3(s)
What are spectator ions?
Ions that are present in a reaction mixture but do not form a bond or change into some other chemical during the reaction. They do not appear in a net ionic equation.
What is reaction rate?
The speed at which a reaction occurs.
A reaction that consume its reactants quickly has a high reaction rate.
Concentration of the reactants
Contact area between reactants ( how much of the reactants are touch each other )
Availability of a catalist
Nature of reaction
all influence reaction rate.
How does concentration of reactant affect reaction rate?
The higher the concentration of a reactant the faster the reaction happens.
Reactions happen only when atoms of reactants collide with one another, if there is a high concentration of reactants there are more atoms colliding with each other.
How does the contact area between reactants affect reaction rate?
More contact area makes for a faster reaction.
If a "chunk" of a solid is used only it's surface can react. If the chunk is ground into a powder then there's lots and lots of surface area to react and the reaction will happen faster.
How does temperature affect reaction rate?
Higher temperatures make for faster reactions.
As temperature increases, the atoms of a substance move faster. If they move faster they colide with other atoms more frequently and harder.
The less energy a reaction needs, the faster the reaction happens.
How does the presence of a catalyst affect reaction rate?
A catalyst is a chemical that speeds up a reaction but is not consumed by the reaction.
How does the nature of the reaction affect the reaction rate?
For a reaction between chemicals / elements to occur atoms / molecules of the elements must colide with one another. Not only that but they must collide with one another fast enough and hard enough to break covalent bonds held by each so they can then bond with each other.
H2 and I2 are elements that have two atoms of each bound to themselves ( 2 H bound together, 2 I bound together ). These molecules have to hit each other hard enough to break the H-H bonds and I-I bonds so the individual H and I atoms can bond with each other.
Increasing the temperature of the reaction can cause the molecules to move fast enough to facilitate this.
What are two ways catalysts work?
The less common way catalysts speed up a reation is by lining up the reactants in such a way that they will collide with each other at the right orientation.
The more common way they speed up a reaction is by decreasing the amount of energy required to facilitate a reaction and thus cause the reaction to happen more quickly.
The catalyst will decrease the amount of energy required for a reaction to take place, and will speed up the reaction, but it will not change how much energy results from the reaction.
What is a reversible reaction?
A reaction that can occur in either direction and will actually occur in both directions simutaenously.
Chemical equations for reversible reactions are written with a double arrow ( arrow pointing in opposite directions ) to show that the reaction can happen each way.
What is an equilibrium mixture?
A stable, unchanging mixture of reactants and products.
In such a mixture, the forward and reverse reactions occur at the same rate. The forward reaction produces products at the same rate as the reverse reaction breaks them back down into the original reactants.
When a reaction has reached such a point of equilibrium that the concentrations of reactants and products no longer change the reaction has reached chemical equilibrium.
What is chemical equilibrium?
When a equilibrium mixture has reached a state where it is so balanced that the concentration of reactants and products no longer change, the reaction has reached chemical equilibrium.
The amounts of reactants and products may not be equal to each other but their ratio to each other is the same.
As the reaction starts there are only the two ( or more ) reactants.
They combine to create new products and leave fewer reactants behind, the forward reaction begins to slow down.
The new products then start to break down back into the original reactants, this reverse reaction speeds up as more products become available.
This back and forth happens until both forward and reverse reactions occur at the same rate.
You cannot know whether a reaction is reversable from the chemical equation ( unless it tells you it is by using the double-arrow ). You also cannot know what the concentrations of the reactants and products of the reaction will be in an equilibrium mixture.
What is Le Chatelier's Principle?
Le Chatelier's Principle says that, in an equilibrium mixture if you add more reactant the reaction is no longer at equilibrium and will move forward until it is. Or, if you add product to the mixture it will no longer be at equilibrium and will go in reverse until it is.
Or, one of the reactants or products can be removed from the reaction, in which case the reaction will make more of that reactant or product until the reaction is in equilibrium again.
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