calcium hydroxide reacts with hydrochloric acid in water,
dissolved calcium chloride and water are formed. This
reaction gives off heat.
to solve a problem like this:
1: Write the unbalanced equation by translating the written
names into chemical formulas
case, the formulas you need to know are those for calcium
hydroxide, hydrochloric acid, calcium chloride, and water.
When you translate these into their formulas, you should get the
+ HCl --> CaCl2 + H2O
forgotten how to write formulas, visit here
for more info about writing formulas for ionic compounds and here
for writing formulas for covalent compounds.
2: Balance the equation
to balance the equation to ensure that the chemical reaction
follows the law of conservation of mass, which says that you've
got to have the same number of atoms of each element on both sides
of the equation. I'll assume for the purposes of this
activitiy that you know how to balance equations. If you
don't, try visiting here
reaction, the equation, when balanced, looks like this:
+ 2 HCl --> CaCl2 + 2 H2O
3: Figure out the states of each of the chemicals in the
refers to the form in which you can find a chemical. The
states you need to worry about are solid, liquid, gas, and
aqueous. Solid, liquid, and gas are probably familiar to
you, and "aqueous" is just a fancy word for "dissolved
in water". The symbol for a solid is (s), liquid is
(l), gas is (g), and aqueous is (aq). You need to make sure
you write these in the parentheses, and that you write them right
after the formulas in the same place and size that you put the
subscripts in the formulas. Check out the example below to
see what I mean.
big question is this: How can you tell if something is a
solid, liquid, gas, or aqueous? Here are some guidelines
that might help you:
equation might tell you. For example, if something is
"dissolved in water", you know it's aqueous. If
something is a "powder", this indicates that it's a
solid. "Vapors" are gases.
chemicals are so common that you should be able to figure it
out. You should know that carbon dioxide is a gas because
you learned that you breathe it out of your lungs after you
breathe in oxygen. Likewise, you should have a pretty good
idea that water is generally a liquid, except at very low
temperatures (when it is solid ice) or very high temperatures (as
you're not explicity told otherwise, assume that ionic compounds
are solids. That's because ionic compounds have very high melting
and boiling points, so they usually are. If they're
dissolved in water or in some other form, the equation should
you're not explicity told otherwise, assume that covalent
compounds are liquids. This is actually not that great an
assumption because there are a lot of exceptions to this rule,
but it's better than nothing. Generally, covalent compounds
have fairly low melting and boiling points, and many organic
compounds are liquids at room temperature. Still, this is
just a vague rule of thumb, and won't always work.
metallic elements but mercury are solids. Mercury is a
nonmetallic elements are solids, except for the following:
Bromine is a liquid; The noble gases, chlorine, fluorine,
nitrogen, oxygen, and hydrogen are gases.
let's take a look at our equation: (In case you forgot what
it was, it was
+ 2 HCl --> CaCl2 + 2 H2O
hydroxide is an ionic compound, so we'll assume it's a solid.
acid is a covalent compound, so we'll assume it's a liquid.
equation tells us that calcium chloride is dissolved in water, so
is a liquid, because nothing about the statement told us that the
reaction took place at anything but room temperature.
all this stuff together, we get the following equation:
+ 2 HCl(l)
+ 2 H2O(l)
4: Sticking all the other relevant symbols in here somewhere
last thing we need to do is to stick a bunch of other symbols
around here to indicate other relevant things about the reaction.
These relevant things may include reaction conditions (things you
need to do to make the reaction take place) or indications about
whether the reaction is exothermic (gives off heat and feels hot)
or endothermic (absorbs heat and feels cold). Here's a list
of the symbols you may need to use:
What it means
reaction requires heat or added energy to occur. This
symbol is typically written over the arrow.
reaction takes place at the temperature indicated, in this
case 50 degrees Celsius. This symbol is written over the
unit "kPa" stands for kilopascals, which is a unit
of pressure. This symbol is also written above the arrow
and indicates the pressure at which the reaction should take
place. Other pressure units are "atmospheres",
"Torr", and "mm Hg".
other stuff around
whatever the instructions tell you to do. There are
actually quite a few other symbols which are commonly used,
but memorizing them all can be tough.
at the end
symbols tell you how much energy is absorbed or given off
during the reaction. If energy is absorbed, the reaction
is endothermic and DH
has a positive sign. If energy is given off, the
reaction is exothermic and DH
gas a negative sign. If a number is given here, this
indicates how exo or endothermic the reaction is. Common
units for DH
are "kJ/mol" or "kcal/mol".
reaction we were given, nothing much was said except that the
reaction gives off heat, meaning that it's an exothermic
reaction. As a result, the only symbol we really need is a
Since the amount of heat given off wasn't specified, all we can do
is say that DH
is negative. As a result, our equation looks like this:
+ 2 HCl(l)
+ 2 H2O(l)
that's all you need to do!